Always change your spark plugs with the engine cold. Grabbing the plug wire by the boot, carefully pull the spark plug wire from the end of the spark plug. Do not pull the wire itself. If the boot sticks, twist the boot left and right and pull the plug wire off. I would recommend changing the plugs one at a time to avoid mixing up the spark plug wires.

If you have it, use compressed air to blow any dirt away from the spark plug area. Otherwise, clean off the old plug and the area around it with a rag or small brush. This will help prevent any foreign material from falling into the cylinder when the plug is removed.

Remove the plug by turning it counterclockwise with a spark plug socket and ratchet. Once you crack it loose, spin it out about three or four turns. Then remove the socket and remove it completely by hand. If you can't reach it, slip the 5/16" vacuum line over the spark plug and turn it out with that.

If you haven't done so yet, gap the new plug with a spark plug gap gauge (The proper gap can be found on the engine specifications decal under the hood). Slip the correct thickness wire or feeler between the inner and outer electrodes at the tip of the plug. A flat gauge is good; the wire type is better. When the plugs are properly gapped, the wire or feeler should slide between the electrodes with a slight drag. If the gap is incorrect, gently bend the outer electrode slightly until the correct gap is achieved. Make sure that the outer electrode is centered directly over the inner electrode. If it's not, align the two by gently bending the outer electrode.

Take a good look at the cylinder head threads. They should be in good condition, clean, and free of dirt and debris. This new spark plug should freely screw into the cylinder head by hand. Any binding of the plug is an indication that there's a problem. Remove the plug and inspect the threads.

Insert the plug into the spark plug hole by hand and turn it clockwise until it's snug. I always attach a short piece of 5/16" rubber hose to the top of the plug and use it as an extension to install the new plugs

After installing the plug by hand as far as it will go, firmly tighten it with a spark plug wrench or socket. It's a good idea to use a torque wrench, if one is available, to ensure that the plug's properly seated. Be very careful; do not over tighten the spark plugs. Remember, you'll get an accurate torque reading only if the spark plug and cylinder head threads are clean and dry.

Reattach the plug wire to the new plug. Use a twisting motion on the boot until it's firmly seated on the top of the plug. You will feel and hear a click as the wire clamps onto the spark plug.

• REPLACING THE DRIVE BELT / SERPENTINE BELT

There are two types of belts: Drive Belts (sometimes called V-Belts) and Serpentine Belts (sometimes called Multi-V Belts). A single serpentine belt will typically drive all the various accessories on the engine, while a drive belt will usually only drive one or two. Most vehicles that use drive belts have more than one belt. Occasionally, vehicles equipped with serpentine belts will have a seperate belt which runs the air conditioning compressor alone.

Regardless of what type of belt your vehicle uses, be sure to check them often and replace them if they are worn. Belts need to be replaced when the inside of the belt is badly cracked, glazed, frayed, or when the rubber begins to harden.

Regardless of whether you are replacing a drive belt or a serpentine belt, the idea is the same; gain access to the belt, loosen the tension off the belt, remove the belt, and reinstall in the reverse order.

This sometimes involves the removal of radiator fans, shrouds, and other belts (which may be in the way). Start by disconnecting the battery to prevent nay electrical shocks.

Next, remove any components which will prevent the belt from being removed. Have a good look to be sure you have the necessary space to remove the old belt and re-route the new belt. Before you remove the belt/belts, check the engine compartment for a belt routing diagram and verify that it matches the vehicle.

If no diagram is shown in the engine compartment you may need to have one printed off for you at your local parts store, or draw your own. If the belt is not re-routed in the correct fashion, various accessories may not be turning in the proper direction which will cause a whole bunch of problems.

If the car is equipped with drive belts (V-Belts) you will need to loosen the adjusters or accessory mounting bolt that keeps it tight. This will often be the top mounting bolt on the alternator or power steering assembly.

• REPLACING THE ENGINE MOUNTS

The engine end and transmission end engine mounts can be replaced separately, if one is bad and the other is good, I would recommend replacing both. Generally if one is bad, the other is not too far behind. If the front or rear engine mount is bad, you need to replace both. If you have a forward facing engine, you need to replace both the left and right engine mounts.

Place a hydraulic floor jack under the engine. If you have to place it under the oil pan, make sure you put a block of wood between the jack and oil pan. Spraying the engine mount nuts and bolts will make removing them easier. Most of the time you just removing the nuts and bolts and raising the engine a bit is all that's needed to remove the old engine mounts.

If you have to raise the engine a significant amount, then you may stress and break another engine mount. Check the other mounts and loosen if they look like they will be damaged.

A lot of engine mounts have studs that go through the mounting holes on the frame. Make sure that it is aligned precisely with the hole before you lower the engine. On some engines you will have to work from the top and the bottom of the engine to get all the nuts and bolts started.

Sometimes you will need a helper to move the engine around with the pry bar to get a bolt started. If the mounting hole the stud goes through is slotted, let the engine down and tighten the nuts finger tight. Start and run the engine at idle for a couple of minutes to let the engine settle down and then tighten the nuts to specification.

"Dog bone" and shock absorber type struts:

Some engines use a shock absorber type mount for vertical control. It is usually mounted to the top side of the engine and the bottom bolts to the cross member. Checking this type of engine mount is the same as a suspension shock absorber. Disconnect one end and push it in and out. If it is loose, rough, leaks or has no resistance, it needs to be replaced.

Many engines use a "Dog Bone" strut or two. They are usually mounted at the top of the engine, on the right side, and connects to the radiator support. Sometimes there is one mounted on the front, belt end, of the engine to help out the front engine mount. Sometimes you will find one on the left side, between the engine and firewall, at the bottom center of the engine.

There are rubber bushings in the ends of the dog bone. If they are cracked, broken or distorted, replace the whole dog bone. The bushings can't be replaced separately. When you take a dog bone out, the engine will shift forward so you, or a helper, will need to shift the engine back to get the dog bone in and the bolts started.

Some higher quality dog bone struts have electronic hydraulic controls. These dog bones have hydraulic chambers, almost the same as the struts and shocks for the suspension, so vibration dampening is done hydraulically instead of cushioning by rubber bushings. There is really no difference in feel, it's said that the hydraulic dog bones last a lot longer.

They look a bit complicated, but they are easy to test. Start the engine and turn the A/C on. With the engine idling, disconnect the connector at the solenoid valve. If the engine starts shaking, or the shaking increases, it's working.

• REPAIRING AN OIL LEAK

Start by determining what type of fluid is leaking. Most of the time you can determine this by the color of the fluid. Dribble a few drops of the mystery fluid onto a white sheet of paper. Engine oil is normally black. Automatic transmission fluid will be red as will be power steering fluid. But these fluids may turn brown or black and difficult to tell from engine oil. Windshield washer fluid will be blue in color and anti-freeze can be red, green, brown, orange or gold. It depends on the vehicle or the anti-freeze brand. Feel the fluid; if it's very oily feeling it's a lubricant. If it has a slightly oily feel, it's probably anti-freeze.

If it is oil, but you can't really determine the color, the next thing to do is to start checking dipsticks. I usually start at the power steering pump reservoir. If it has little or no fluid, then the oil is probably power steering fluid. Then check the transmission fluid. If it's low, check the transmission cooler lines going to and from the radiator. If the transmission fluid is at it's proper level, that means it's engine oil.

If the oil is black and there is a puddle directly under the engine, you have an engine oil leak for sure. Engine oil leaks are by far, the most common type of leak. Now you have the daunting task of locating the leak. A slow leak, like from a valve cover gasket, will travel down along the manifolds, wires and vacuum lines and gets blown everywhere.

The first thing to do is get a light and look at the engine. You may get lucky and spot the source of the leak right off the bat. Don't start replacing anything unless you are absolutely sure of the location of the oil leak. Most of the gaskets in today's engines are very difficult to get to and are very expensive. It would be an expensive mistake to spend a lot of time and money to replace a gasket and still have the leak.

Spotting an oil leak is difficult at best and almost impossible when the engine is covered in oil. So the first thing I do is give the engine a through wash with engine cleaner. I use Gunk Engine Cleaner but any good cleaner will work. I follow the directions on the can, getting the engine hot, spraying on the cleaner paying special attention to heavy deposits and letting it sit for about 15 minutes. Since I have a lift in the shop I can get underneath the engine and car as well.

Once the engine is nice, clean and dry I have the customer pick up the car in the morning and bring it back in the afternoon. If the leak doesn't appear, I have the customer drive it another day, and another if necessary. About 70% of the time I can spot the leak and fix it on the first shot

There are a couple of other ways of locating an oil leak. The method of choice among professionals is a florescent dye and an ultraviolet (UV) light. This dye will work with all types of fluids, oil, transmission fluid, fuel, coolant, and A/C refrigerant, and makes spotting a leak pretty easy. When seen under the UV light, the dye glows a bright greenish/yellow that can't be mistaken for anything else.

By aiming the light, a small dye mark will show you the location of the oil leak. I have dye kits in my shop that have special yellow glasses that make the dye stand out even more.

Dye kits usually come with two bottles of dye, one for anti-freeze and one for oil. Dyes for A/C refrigerants are very specific for the type of A/C refrigerant and require the proper equipment to add to the system. They are not included with a general-purpose kit and should be done by a professional.

When you use the dye, follow the directions that come with your particular kit. In general you mix a ½ ounce of the dye and mix it with about a ½ quart of oil and pour it into the engine. Don't put the dye directly into the engine. It will get caught up in the oil filler area and take much longer to mix with the oil.

Put some newspapers under the engine and let it run. When you see some drips on the paper, look at them with the UV light. If they glow, you can start searching for the leak. Shut off the engine and starting from the bottom of the engine. Turn on your UV light and follow the trail of dye. The oil may go round and about but follow it up to the highest point and you will spot the source of the leak.

Most times it will be a valve cover gasket and tightening the bolts will end the leak. Or it may be a distributor "O" ring or leaking Oil Pressure Sending Unit. But sometimes it will be from a gasket that is totally gone and will need to be replaced.

Oil also may be seeping past a worn crankshaft or camshaft seal. The rubber lip that seals to the rotating shaft will eventually wear to the point at which the tension in the garter spring won't keep oil from leaking. This type of seal will only leak when the engine is running--and when it does oil will spray everywhere from the spinning shaft.

There is another way to locate an engine leak that is almost as effective as using a trace dye. And you probably already have it in your medicine cabinet. It's foot powder. What you do is get the engine as clean as you possibly can and let it completely dry.

Now spray the general area of the leak with the foot powder and work your way up as far as possible. The powder will stick to the engine and cover the metal in a white film. Don't be stingy with the stuff either.

Now drive the car fairly slow on clean, dry roads so you don't mess up the powder. Hopefully the oil will take a fairly direct route down. If it does, you will see a single black path down the side of the engine. You may need to do this higher up on the engine to locate leaks from the intake manifold or head gaskets. If the oil leak is too bad you will have to go with the trace dye to determine where the oil leak is.

Remove the air intact duct between the air filter housing and the throttle body.

Disconnect all the hoses and unplug any sensor wiring connectors. Make a note of which hose goes where. A good thing to do to avoid confusing a hose connection or wiring connector is to put a piece of masking tape on the hose or connector, another on the hose neck or sensor, and mark each with the same letter.

Now, loosen any clamps and work the duct off the throttle body and air cleaner housing and set it aside. When all the wiring and hoses are disconnected, you should not try to start the engine. Even if it should happen start and run, it will throw all kinds of DTC's and turn the Check Engine light on. Then you'll have to clear all the codes and make extra work you don't need. In addition, the computer may have to relearn some idle and drivability settings, which will leave you with a poorly running engine for a while until it learns all over again.

Now take a good look inside the throttle body with a flashlight. Move the throttle linkage and open the throttle plate so you can get a good look inside. You'll see a black coating of dirt, gum, varnish and oil on the throttle plate and throttle body throat. This is what disrupts and blocks the airflow when the throttle is only open slightly or closed.

Most of it is the oil and combustion gases sucked up by the positive crankcase ventilation system and pulled forward by normal engine operation. Some of it is dirt that gets paste the air filter or cracks in the air filter housing or air intake duct. It would be a very good idea to check for cracks in the air intake duct, especially in the folds where the duct flexes. Also make sure the air cleaner housing is on tightly and secured properly.

One thing you have to watch out for is that some throttle body bores have a coating to help reduce this buildup. But even with the protective coatings protecting the throttle body bores they can be affected over time. There are a few ways to clean out the throttle body and plate. The easiest is with carburetor cleaner and a very soft bristled toothbrush.

There are three reasons why you have to be careful with choosing the type of solvent and how it's applied.

If there is a protective coating on the throttle body bore, which includes most Ford products, a hard brushing and a strong solvent will take it right off. Once it's off then you will need to clean it out more often. There will be a warning label on the Fords to let you know the throttle body is coated.

There may be a temperature or some other sensor sticking out in the throttle body area that hard brushing or a strong solvent could damage. Also it may destroy a sensor "O" ring or seal.

The shaft of the throttle plate where it goes through the throttle body is sealed to prevent unmetered air from entering the engine. This unmetered air will adversely affect the air/fuel mixture. Again, hard brushing and a strong solvent may damage these seals.

Before you do anything you will want to start the car up and let it get to operating temperature so the dirt in the throttle body will come loose. I highly recommend eye protection be worn through this process. You'll be working with spray chemicals that can cause severe eye damage and there are people out there who love you.

Now, what can we use to clean the throttle body? Most carburetor and choke cleaners are pretty strong. The same is true of cleaners that are sprayed into the air intake for combustion chamber cleaning. They have to be strong in order to be able to clean things without mechanical means such as scrubbing with a brush. Sprays cannot clean all areas, particularly the back of the throttle plate.

In addition it is almost impossible to see which areas have been cleaned and which haven't.

A safe solvent to use is a tune-up or injector cleaner mixed 4:1 or 5:1 with gasoline. What ever you don't use you can pour into the gas tank. If you can find Throttle Body Cleaner you can use that as well. Just hold onto that little straw to keep it from flying into your engine. If you can't get a good spraying angle, this is what I did. Cut the red straw in half and put a piece of small vacuum line between the halves. Stick one half of the straw into the can and use the other end to point the spray anywhere you need to while holding the can upright.

Start by cleaning the outside of the throttle plate and bore. Scrub it with your soft bristled toothbrush and get it good and clean. Next hold open the throttle plate by holding the linkage with a piece of wire. Now you can clean the backside of the throttle plate, the throttle body wall and some of the intake manifold. You just want to brake up this gunk for now. Be very careful working around any sensors and the throttle shaft seals. When you're done, clean the garbage out with solvent and clean rags.

• PERFORMING A TUNE-UP

A tune-up is one job that's changed a great deal over the course of automotive history. The outdated term is still widely used by many people to describe a service procedure that's supposed to make an engine run better.

There's no absolute definition of what exactly a tune-up should include, but most would agree that it involves replacing the spark plugs and performing other adjustments to maintain or restore like-new engine performance. The problem is there's not much that can adjusted under the hood on many late model vehicles. Ignition timing is fixed and controlled by the engine computer, as is idle speed and the fuel mixture. You can still check base timing (maybe), idle speed and various emission functions to make sure everything is functioning within factory specs and are functioning properly. But there really isn't much of anything left to "tune." Yet motorists still want tune-ups and believe tune-ups are an important and necessary service.

What most motorists really need when they ask for a tune-up, though, is something other than preventive maintenance. Unless a motorist is a rare bird who is actually following the scheduled maintenance recommendations in his vehicle owner's manual, he's probably asking for a tune-up because he's experiencing some kind of driveability problem. His vehicle might be getting hard to start, not getting the fuel mileage it once did, hesitating or stalling, knocking or not running with the same zip and power as before. Or, his vehicle may have failed an emissions test. So what he probably needs is an engine performance analysis - and maybe a new set of spark plugs, too.

A simple maintenance type tune-up (a new set of plugs) may make an engine easier to start, improve fuel economy, lower emissions, restore lost pep and power, and so on provided engine performance deteriorated because of worn or fouled spark plugs. But if the problem lies elsewhere, a new set of plugs alone won't do the trick. A "tune-up" under these circumstances would be a waste of time and money. The first thing you should do, therefore, when someone asks about a tune-up is to find out why he thinks he wants one. If he gives any reason other than scheduled maintenance, he has a performance problem that will require additional testing to identify the cause (or causes) of the problem. Only after the performance problem has been diagnosed should any parts be replaced.

What to check:

Any tune-up today should start with a battery of performance checks to baseline or confirm the engine's overall condition. These should include:

Battery voltage (very important with all of today's onboard electronics).

Charging voltage

Power balance or dynamic compression (to identify any mechanical problems such as leaky exhaust valves, worn rings, bad head gasket, bad cam, etc. that could adversely affect compression and engine performance)

Engine vacuum (to detect air leaks as well as exhaust restrictions)

Operation of the fuel feedback control loop (to confirm that the system goes into closed loop operation when the engine warms up)

Scan for fault codes (to verify no fault codes are present, or to retrieve any codes that may be present so they can be diagnosed and eliminated)

Check exhaust emissions (this should be a must in any area that has an emissions testing program to confirm the vehicle's ability to meet the applicable clean air standards, and to detect gross fuel, ignition or emission problems that require attention)

Verify idle speed (should be checked even if computer controlled to detect possible ISC motor problems)

Idle mixture (older carbureted engines only, but injector dwell can be checked on newer vehicles to confirm proper feedback fuel control)

Check ignition timing -- if possible (should be checked even if it is not adjustable to detect possible computer or sensor problems)

Operation of the EGR valve

In addition to these performance checks, hoses and belts should be visually inspected. All fluids (oil, coolant, automatic transmission fluid, power steering fluid and brake fluid) should also be inspected to make sure all are at the proper level, and that the appearance and condition of each is acceptable. There should be no sludge in the oil, the ATF should not smell like burnt toast, the coolant should have the proper concentration of antifreeze and not be full of rust or sediment, the brake fluid should be clear and not full of muck, etc.

What to replace

If the tune-up checks find no major faults, the following items should be replaced for preventive maintenance:

Spark plugs (gapped to the correct specs, of course). Consider long life plugs on applications where plug accessibility is difficult or where longer service life may be beneficial

Rotor and/or distributor cap (if required)

Fuel filter; Air filter; PCV valve and breather filter

Other parts on an "as needed" basis (spark plug wires, belts, hoses, fluids, etc.)

Check and adjust (if required on older vehicles) ignition timing, idle speed and idle mixture

O2 sensor(s)

Spark plugs need to be changed periodically because the electrodes wear every time a plug fires. When high voltage current jumps from one electrode to another, it wears away a little metal from both electrodes. After 45,000 miles of operation, the plug has fired 60 to 80 million times and wear has increased the distance between the electrodes. At the same time, the nice sharp edges on the center electrode have become rounded and dull. All this increases the voltage required to jump the gap. If the ignition system can't deliver, the plug may begin to misfire under load. Accumulated deposits on the plug tip may also be interfering with reliable ignition. So by the time the average plug has seen 45,000 miles, it's getting close to the end of its service life.

Long-life plugs, on the other hand, don't wear as much as standard plugs. The electrodes are made of tough platinum or gold-palladium alloys that resist erosion. Such plugs may go 100,000 miles under optimum conditions (no fouling). Of course, no plug will last anywhere near its potential lifespan if an engine is burning oil, experiencing abnormal combustion such as detonation or preignition, or has a fouling problem. Oxygen sensor:

Though most motorists don't even know what an oxygen sensor is, let alone that their engine may have one or more of these devices, the fact remains that sluggish O2 sensors cause a lot of driveability problems. A recent EPA study found that 70% of all vehicles that fail an I/M 240 emissions test need a new O2 sensor.

To prevent such woes, the O2 sensor can be replaced for preventive maintenance during a tune-up. Unheated 1 or 2 wire wire O2 sensors on 1976 through early 1990s applications should be replaced for preventative maintenance every 30,000 to 50,000 miles. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. And on OBD II equipped vehicles (all '96 and newer), the recommended replacement interval is 100,000 miles.

The O2 sensor is the master switch in the fuel control feedback loop. The sensor monitors the amount of unburned oxygen in the exhaust and produces a voltage signal that varies from about 0.1 volts (lean) to 0.9 volts (rich). The computer uses the O2 sensor's signal to constantly fine tune and flip-flop the fuel mixture so the catalytic converter can do its job and clean the exhaust. If the O2 sensor circuit opens, shorts or goes out of range, it usually sets a fault code and illuminates the Check Engine or Malfunction Indicator Lamp. But many an O2 sensor that is badly degraded will continue to function well enough not to set a fault code but not well enough to prevent an increase in emissions and fuel consumption. So the absence of a fault code or warning lamp doesn't mean the O2 sensor is doing its job.

Deterioration of the O2 sensor can be caused by a variety of substances that find their way into the exhaust (such as lead, silicone, sulfur, even oil ash) as well as environmental factors such as water, splash from road salt, oil and dirt.

A sluggish sensor may not allow the computer to flip-flop the fuel mixture fast enough to keep emissions within acceptable limits. A dead sensor will causes the system to go back into open loop with a fixed, rich fuel mixture. Fuel consumption and emissions go up, and the converter may suffer damage if it overheats.

The best way to check O2 sensor performance is with a digital oscilloscope. A good sensor should produce an oscillating waveform that flip-flops from near minimum (0.1 to 0.2v) to near maximum (0.8 to 0.9v). O2 sensors in feedback carburetor applications have the slowest flip-flop rate (about once per second at 2500 rpm), those in throttle body injection systems are somewhat faster (2 to 3 times per second at 2500 rpm), while multiport injected applications are the fastest (5 to 7 times per second at 2500 rpm).

When the mixture is made artificially rich by injecting some propane into the intake manifold, the sensor should respond almost immediately (within 100 milliseconds) and go to the maximum (0.9v) reading. Likewise, making the mixture artificially lean by opening a vacuum line should cause the sensor's output to drop immediately to the minimum (0.1v) reading.

Something else that should be part of a tune-up today is cleaning the fuel injectors and intake system. The need for injector cleaning isn't as great as it once was thanks to improved fuel additives and redesigned injectors. But in areas that have gone to reformulated gasoline, injector clogging is on the rise again.

Fuel varnish deposits that form in injectors restrict the amount of fuel that's delivered with every squirt, which has a leaning effect on the air/fuel mixture. The result can be lean misfire and a general deterioration in engine performance and responsiveness. Deposits can also build up on the backs of intake valves, causing cold hesitation problems in many engines.

The cure is to clean the injectors and valves. Cleaning should be recommended for any engine that is suffering a performance complaint or has more than 50,000 miles on the odometer. Cleaning the throttle body can also help eliminate idle and stalling problems that plague many of today's engines.

100,000 mile tune-up intervals:

Some would say the auto makers' move to 100,000 mile "tune-up" intervals on many new vehicles will finally kill the tune-up as we know it today. Maybe, but what the car makers are really talking about are 100,000 mile spark plug change intervals -- which does not include the need for other maintenance such as oil and filter changes or other repairs that might be needed during the life of the vehicle.

If the average motorist fails to grasp the true meaning of today's 100,000 mile tune-up and thinks he can get away with gas-and-go driving for 100,000 miles without spending a dime on maintenance or repairs, he'll find out the hard way that lack of proper maintenance can be very costly. Today's vehicles don't require as much maintenance as they used to because things such as idle speed and mixture adjustments, timing adjustments, etc. have been eliminated. So too has the need for chassis lubrication thanks to "sealed-for-life" ball joints and tie rod ends. Many OEM parts are also being built to much higher standards of durability.

Even so, regular oil and filter changes are still necessary to maintain proper engine lubrication. Most experts still recommend changing the oil and filter 3,000 miles or three to six months. The oil change interval can be stretched out to reduce maintenance costs if a vehicle is driven under ideal conditions (no extremely hot or cold weather, no short trip, stop-and-go driving, no excessive idling, no extremely dusty road conditions, no trailer towing, no turbocharging). But the average driver is more often than not a "severe service" driver so should follow the 3,000 mile change interval.

Today's 100,000 mile tune-up interval also skirts around the issue of fuel and air filter replacement, too. A number of new cars and trucks now have "lifetime" fuel filters, most of which are located inside the fuel tank with the electric fuel pump. Such a filter might go 100,000 miles. Then again, it might not. A couple of tanks of bad gas or some corrosion caused by accumulated moisture can cut short the life of any filter, even a so-called lifetime filter. Sooner or later even a lifetime fuel filter will have to be replaced.

Does it make sense to replace a lifetime in-tank fuel filter for preventative maintenance? Maybe - if one considers what it costs to have a vehicle towed because of a plugged fuel filter.

As for air filters, the service life depends more on environmental factors rather than time or mileage. If a vehicle is driven on gravel roads, filter life may only be a few months or few thousand miles.

Back in the 1970s and 1980s, the average life cycle of an engine was about five to seven years. After 60,000 to 80,000 miles of everyday driving, most engines would develop an oil consumption problem and begin to experience other signs of wear (loss of compression, loss of power, increased emissions, lower oil pressure, internal noise, etc.). Carburetors were partly to blame for the wear because rich fuel mixtures wash the lubricating oil off the cylinder walls and dilute the oil in the crankcase. These older engines were also built much "looser" (wider tolerances) than most of today’s engines, which also increased blowby. Consequently, the rings, bearings and valve guides all experienced accelerated wear.

Today, the situation is much different. The average service life of a 1990s vintage engine is about 10 to 12 years. Fuel injection has all but eliminated the fuel wash down problem, and much tighter tolerances have greatly reduced blowby and oil dilution in the crankcase. So fewer engines are being rebuilt today as a result.

Improvements in engine technology have extended engine life and reduced the need for engine service. Even so, the current "technology trough" will eventually pass and the numbers of engines being replaced and rebuilt will once again rise. The number of five- to 10-year-old light trucks on the road, for example, has jumped from 18 million in 1985 to nearly 60 million today. Many of these will need engine work before long.

Repair options

When an engine needs major repairs, you’re faced with an important choice: you can replace the engine with a new, remanufactured or used engine, or you can repair or rebuild the original engine.

Replacing an engine with a brand new one is usually too expensive for most people’s budgets, so the choices come down to a remanufactured engine (or short block), a used engine (and the risks that go with it), or overhauling or repairing the engine yourself. A used engine is a temporary fix at best, and only buys the current owner a little more time. Sooner or later, most used engines experience problems of their own and have to be replaced or rebuilt, too.

Remanufactured engines are a popular option these days because they’re readily available at competitive prices, which has caused a decline in the number of engines being custom rebuilt ("repowered") by repair facilities and machine shops. A quality remanufactured engine can provide good value for the investment, and most come with a 90-day to one-year warranty. Even so, there are still valid reasons for doing your own engine work.

Engine rebuilding

Rebuilding an engine can cost less than replacing it. Assuming the original engine is rebuildable (wear is not excessive and there is no serious damage), and the amount of machine work required to restore it is minimal, you may realize 20% to 50% or more savings doing a rebuild versus replacing the engine. Most of the savings comes from the labor you put into tearing down the engine and then reassembling it after any necessary machine work has been done.

The tools required to rebuild an engine are minimal: normal hand tools, some feeler gauges, a torque wrench, a ring expander and ring compressor. Any machine work that’s needed can be farmed out to a local machine shop.

If the cylinders are worn, they’ll have to be bored or honed to accept oversize pistons and rings. If not, you can run a glaze breaker down the bores and do the work yourself. If you don’t have valve and seat refacing equipment, you’ll have to send that out, too. Worn guides can be reamed out, replaced or relined in-house with a few special tools. But jobs such as head resurfacing, line boring, crank refinishing, etc., will have to be farmed out. Find a reputable local machine shop that you can use for this type of work.

Another reason for doing your own engine work is to control the quality of parts and work that goes into the engine. This is something you can’t control when you buy an engine from an outside source. It may be top quality, or it may not. But you don’t want to find out "the hard way." The truth is, some remanufacturers reuse a much higher percentage of parts than others do, obviously for cost savings purposes.

You can also save by buying the parts you need in an engine kit rather than individually. A kit gives you everything you need in one box and reduces the chance of mismatching parts. The parts in a kit usually include bearings, rings, pistons, timing chain and gear set, valve seals, gaskets, oil pump, camshaft, lifters and other miscellaneous parts.

You can usually get OEM or better quality parts in most kits, which may be better than the parts found in some remanufactured engines.

One aftermarket supplier of engine kits now offers a 100,000 warranty (including labor) on all of the parts in its premium engine kits - which is a better deal than you’ll find on almost any replacement engine, new or remanufactured.

Crankshaft bearings

New bearings are almost always necessary when rebuilding an engine. When you remove the old bearings, inspect them for unusual wear or damage such as scoring, wiping, dirt or debris embedded in the surface of the bearings, pitting or flaking. Anything other than normal wear may indicate an underlying problem that needs to be corrected before the new bearings are installed.

Dirt contamination often causes premature bearing failure. The underlying cause may have been a missing air filter, air leaks into the crankcase (missing oil filler cap, PCV valve, etc.), or not changing the oil and filter often enough.

If the engine has a "spun" bearing, it’s likely the bearings were starved for oil - possibly as a result of a failed or badly worn oil pump, an obstruction in the oil pump’s pickup screen, or too low an oil level in the crankcase (leaky gaskets or seals).

Excessive heat can be another cause of bearing failure. Bearings are primarily cooled by oil flow between the bearing and journal. Anything that disrupts or reduces the flow of oil not only raises bearing temperatures but also increases the risk of scoring or wiping the bearing. Conditions that can reduce oil flow and cause the bearings to run hot include a worn oil pump, restricted oil pickup screen, internal oil leaks, a low oil level in the crankcase, aerated oil (oil level too high), fuel-diluted oil from excessive blowby or coolant-contaminated oil from internal coolant leaks.

Misalignment is another condition that may indicate the need for additional work. If the center main bearings are worn more than the ones toward either end of the crankshaft, the crankshaft may be bent or the main bores may be out of alignment. The straightness of the crank can be checked by placing it on V-blocks, positioning a dial indicator on the center journal and watching the indicator as the crank is turned one complete revolution. If runout exceeds limits, the crank must be straightened or replaced.

Main bore alignment can be checked by inserting a bar about .001 inch smaller in diameter than the main bores through the block with the main caps installed and torqued. If the bar doesn’t turn easily, the block needs to be align bored. Alignment can also be checked with a straight edge and feeler gauge. A deviation of more than .0015" in any bore calls for align boring. Line boring must also be done if a main cap is replaced.

The concentricity of the main bores is also important, and should usually be within .0015". If not, reboring will be necessary to install bearings with oversized outside diameters.

Connecting rods with elongated big end bores can cause similar problems. If the rod bearings show a diagonal or uneven wear pattern, it usually means the rod is twisted. Rods with elongated crank journal bores or twist must be reconditioned or replaced.

Uneven bearing wear may also be seen if the crankshaft journals are not true. To check the roundness of the crank journals, measure each journal’s diameter at either bottom or top dead center and again at 90 degrees either way. Rod journals typically experience the most wear at top dead center.

Comparing diameters at the two different positions should reveal any out-of-roundness if it exists. Though the traditional rule of thumb says up to .001" of journal variation is acceptable, many of today’s import engines can’t tolerate more than .0002" to .0005" of out-of-roundness (always refer to the specs).

To check for taper wear on the crankshaft journals (one end worn more than the other), barrel wear (ends worn more than the center) or hourglass wear (center worn more than the ends), measure the journal diameter at the center and both ends. Again, the generally accepted limit for taper wear has usually been up to .001", but nowadays it ranges from .0003" to .0005" for journals 2" or larger in diameter.

The journal diameter itself should be within .001" of its original dimensions, or within .001" of standard regrind dimensions for proper oil clearances with a replacement bearing. If a journal has been previously reground, there’s usually a machinist’s mark stamped by the journal. A 10, 20 or 30 would indicate the crank has already been ground to undersize, and that further regrinding may be out of the question depending on how badly the crank is worn.

When you’re ready to install new bearings, make sure you have the correct size (standard size for a standard crank, or oversized bearings for an undersize crank), that you’ve checked the installed bearing clearances, that the bearings are prelubed to protect them against a dry start, that the oil holes and tangs on the bearings are all properly located, and that the rod and main cap bolts are torqued to specifications.

Another component that should also be replaced along with the bearings is the oil pump. Oil pumps wear with age, and may cause a loss of oil pressure that can be very damaging to the bearings.

Piston rings

Low compression and oil burning are usually a sign of worn rings and/or cylinders. Replacing the piston rings can restore compression if the cylinders do not exceed service specifications. But if the cylinders are worn or damaged, reboring the cylinders to oversize will be necessary to restore proper clearances and compression.

Replacement rings come in various materials and sizes. Most compression rings are cast iron, though many import engines have steel rings. Rings may be plain faced, chrome-plated, inlaid with molybdenum ("moly") or nitrided for added durability. Replacement rings should generally be the same types as the original.

Ring sizes can be confusing because ring thickness and width may change from one model year to the next. You may have to refer to the VIN number to determine the correct rings for the engine. Oversize rings and pistons of the corresponding size will obviously be needed if the cylinders need to be bored or honed to oversize.

Some shops "plateau" the cylinders after honing. This can be done various ways, but one way to do this yourself is to give each cylinder a few strokes with a flexible brush-type "Flex-Hone" in a drill. This helps remove surface debris and knocks the sharp peaks off the ridges left in the bores by honing.

Cylinders must always be cleaned before new rings and pistons are installed. This means scrubbing the bores with warm soapy water and a brush to remove all traces of honing residue and metal.

Always use a ring expander to install new rings on pistons, and a ring compressor to install the piston assemblies in the block. Cylinder walls must also be lubed to protect the rings and pistons against scuffing when the engine is first started.

Camshaft

Camshaft wear in high mileage engines is a common problem, so inspect the cam carefully to see if it is worn or bent. If the engine needs a new cam, you can install a stock replacement cam or a performance cam. Performance cams provide increased lift and duration for more power. If you opt for a hotter cam, be sure to follow the camshaft supplier’s application recommendations for lift and duration. A common mistake is "overcamming" a street engine. Too much lift and duration will move the engine’s power curve too far up the rpm scale, and may require other extensive modifications such as larger valves, stiffer valve springs, performance manifolds and modifications to the carburetor or fuel injection system to optimize performance.

When a cam is being replaced, new lifters and valve springs should also be installed. Reusing old lifters with a new cam can damage the cam lobes.

Roller lifters can generally be reused, but not on a camshaft designed for flat bottom (actually slightly convex) lifters (and vice versa).

In addition to a new cam, the engine may also need a new timing belt or chain and gear set. The recommended replacement interval for timing belts on most older engines (those made before 1993) is 60,000 miles. The replacement interval for many newer belts has been increased to 100,000 miles. Timing chains have no specified replacement interval, but do stretch with age. This has an adverse effect on valve timing as well as ignition timing, so the chain and gears should be replaced if wear exceeds specifications.

Valve work

The valve components that may have to be replaced will depend on the age and condition of the engine. New exhaust valves are often needed because they run much hotter than intake valves and often "burn" or fail because of erosion and heat cracking. Exhaust valves also stretch with age, which increases the danger of valve breakage. For this reason, you might want to replace all the exhaust valves with new ones regardless of their condition.

Intake valves can generally be reused unless bent or worn. Replacement is required if stem wear exceeds specifications.

If guides are worn, which they usually are, the engine can suck a lot of oil. Evidence of this is usually heavy black carbon deposits on the backs of the intake valves and heavy carbon deposits on the pistons and in the combustion chambers. Minor guide wear can be reduced somewhat by knurling. If integral guides are worn, they may be drilled out to accept thin wall bronze or cast iron guide liners, or reamed to oversize (which requires new valves with oversize stems). Worn guides in aluminum heads can also be lined, or reamed to oversize or pressed out and replaced with new guides.

Unless valve seats are cracked or badly worn, they can usually be reconditioned by cutting or grinding. Damaged or badly worn seats in aluminum heads will have to be replaced. Bad seats in cast iron heads can sometimes be repaired by machining out the old seat to accept an insert.

Head gaskets

The number one mistake to avoid when replacing a blown head gasket is to simply install a new gasket without checking or repairing anything else. In many instances, a blown gasket is not the real problem but a symptom of some other underlying condition such as a hot spot, overheating or detonation. If the underlying problem is not identified and corrected, the new gasket will likely suffer the same fate as its predecessor.

Always inspect the cylinder head for cracks or other problems when it is removed, especially if the engine overheated. Aluminum overhead cam heads are much more likely to warp and crack than cast iron heads when an engine gets too hot. If an OHC won’t turn once the followers have been removed, the head is probably warped and will have to be straightened and/or align bored.

Cracks are not always visible to the naked eye. Porosity leaks in aluminum heads may not show up unless the cooling system is under pressure. To minimize the risk of a repeat gasket failure, cast iron heads should be Magnafluxed (magnetic crack detection) to check for cracks. Penetrating dye will reveal cracks in aluminum. Pressure testing is also an excellent method of detecting internal cracks and porosity leaks in both cast iron and aluminum.

The cylinder head and block should also be checked for flatness before the new head gasket is installed. Flatness specs vary depending on the application, but on most pushrod engines with cast iron heads, up to .003" (0.076 mm) out-of-flat lengthwise in V6 heads, .004" (0.102 mm) in four cylinder or V8 heads, and .006" (0.152 mm) in straight six cylinder heads is considered acceptable. Most aluminum heads, on the other hand, should have no more than .002" (.05 mm) out-of-flat in any direction.

Aluminum OHC heads should be checked for flatness in two places: across the face of the head with a straight edge, and down the OHC cam bores with a straightedge or bar.

If an OHC aluminum head requires resurfacing, the amount of metal that can be safely removed is usually quite limited. If a head has been resurfaced and the installed height is too short, cam timing can be adversely affected. Too much compression may also create detonation problems. To compensate, a copper or steel shim may be used with the head gasket to raise the head and restore proper head height (if available). Otherwise, the head may have to be replaced.

Surface finish is also very important. As a rule, most push rod engines with cast iron heads can handle a surface finish of anything between 54 to 113 microinches RA (60 to 125 RMS). But many aluminum OHC heads require a smoother finish to seal properly. Many late model Japanese engines have "multi-layered steel" (MLS) head gaskets that require a very smooth finish of 7 to 15 RA. Such heads should not be resurfaced unless the head is warped or the surface is damaged.

• VALVE GUIDE REPAIRS

One thing you can almost always count on when rebuilding a cylinder head is worn valve guides. The guides experience a lot of wear because of the constant friction between the guide and stem. To make matters worse, positive valve seals on late model engines prevent the guides from receiving much lubrication. Side forces on the valve stem caused by changes in valvetrain geometry or by direct acting overhead cams further contribute to guide wear.

When the guides are worn or there is too much clearance between the guide and valve stem, the engine will use oil. This applies to both intake and exhaust guides. Though oil consumption can be more of a problem on the intake side because of constant exposure to engine vacuum, oil can also be pulled down the exhaust guides by suction in the exhaust port. The flow of exhaust past the exhaust guide creates a venturi effect that can pull oil down the guide.

Oil in the exhaust system of late model vehicles with catalytic converters may cause the converter to overheat and suffer damage. On the intake side, oil drawn into the engine past worn intake guides can foul spark plugs, cause the engine to emit higher than normal unburned hydrocarbon (HC) emissions, and contribute to a rapid buildup of carbon deposits on the backs of the intake valves and in the combustion chamber. Carbon deposits in the combustion chamber can raise compression to the point where detonation occurs under load. Deposits on the backs of the intake valves in engines equipped with multipoint fuel injection can cause hesitation and idle problems because the deposits interfere with proper fuel delivery.

Inadequate valve cooling and premature valve failure is another problem that can be caused by worn guides or ones with excessive clearance. About 75% of the heat from a typical valve is conducted to the seat, and the remaining 25% goes up the stem and out through the guide. On late mode engines with three-angle narrow seats, the amount of heat transfer that takes place through the stem is even higher because less heat can be dissipated through the seat. So if the guide is worn, the valve may run hot and burn.

Worn guides can also pass air. "Unmetered" air drawn into the intake ports past the guides creates an effect similar to worn throttle shafts on a carburetor. The extra air reduces intake vacuum and upsets the air/fuel calibration of the engine at idle. The result may be a lean misfire problem and rough idle.

Worn guides can also contribute to valve breakage. The guides support and center the valves as they open and close. A worn guide will allow the valve to wobble slightly as it opens, which cause it to drift off-center with respect to the seat. This can cause the head of the valve to flex slightly each time it closes (much like a valve with a nonconcentric seat). After so many cycles, the metal fatigues and the head of the valve breaks off from the stem.

Generally speaking, the intake valve stem-to guide clearance for most passenger cars ranges from .001 to .003 in., and .002 to .004 in. for exhaust guides (which generally require .0005 to .001 in. more clearance than the intakes for thermal expansion). Diesel engines as a rule have looser specs on both intake and exhaust guides than gasoline engines, and heads with sodium-filled exhaust valves usually require an extra .001 in. of clearance to handle the additional heat conducted up through the valve stems.

Checking guide wear

To check guide wear, some machinists insert a valve stem into a guide and "feel" for looseness by wobbling the valve. Others may use a valve seat pilot tool to check the guides. Though either technique will reveal badly worn guides, neither is a very accurate method of gauging guide clearances or wear.

The best way to check guide wear is with a gauge set designed for this purpose. A gauge set will give you precise measurements and can be used to measure any portion of the guide. To check guide wear (as well as taper) using a telescoping or split ball gauge, measure the guide ID at both ends and in the middle. Subtract the middle reading from the ends to determine taper wear. Compare the smallest ID measurement (usually in the middle of the guide) to the factory specs to determine total wear.

Valve stems should also be measured to check for wear and sizing. Nominal sizes vary quite a bit depending on the application, and there’s no way of knowing if the valve has been replaced previously with one of a different size without measuring.

Many late model engines have tapered valve stems. Taper stem valves are ground with the stem diameter smaller at the head end of the valve. This is done to create a larger clearance at the head where the temperatures are highest. This reduces the change of galling with unleaded fuel and narrow three-angle valve seats. When measuring a tapered stem, check the outside diameter about an inch in for each end.

Guide repair options

A variety of repair options are available for worn valve guides. Many professional engine rebuilders either install thin wall bronze liners, or ream the guides to oversize and install new or rechromed valves with oversized stems. Replacing guides is another option with aluminum heads as well as some cast iron heads, as is knurling.

Knurling

Though still used by some small shops, most professional engine rebuilders see knurling as a short term "quick fix" that doesn’t hold up as well as guide liners, new guides or valves with oversized stems. Knurling should only be considered as a guide repair option if guide wear is minimal (less than .006 in.). And even then, it may not provide satisfactory results.

Knurling typically decreases the inside diameter of the guide where it needs it the least, namely in the center where there is the least wear rather than at the ends where the wear is usually greatest. When the knurling tool is run through the guide, it leaves behind a spiral groove. The groove acts like a furrow and raises the metal on either side. This reduces the inside diameter of the guide so a reamer can then be used to resize the guide back to (or close to) its original dimensions. The grooves also help to retain and seal oil better than a smooth bore guide. This allows somewhat tighter guide-to-stem clearances (as close as .0007 in.). But the bearing surface area created by knurling is not that great, so it won’t last as long as a guide that offers greater bearing area.

Liners

Boring out the original guides and installing thin wall bronze liners to restore proper clearances is not only a fast and economical guide repair option, it also provides the benefits of a phosphor/bronze guide surface (better lubricity, scuff resistance and wear characteristics than cast iron).

Though liners are most often used to repair integral guides in cast iron heads, they are also a very effective way to repair replaceable guides in cast iron or aluminum heads—which saves time and eliminates the risks associated with driving out the old guides and pressing in new ones.

Liners also save the cost of having to replace the valves. If the original valves are not worn, standard sized liners can be used to restore the inside diameter dimensions of the guides. If the valves are worn, the stems can be turned down .0050 in. to accommodate liners with slightly undersized inside diameters.

Installation of guide liners is a simple process:

First, the old guides have to be bored out to accept the liners. The guides should be bored dry with no lubricant, using steady consistent pressure.

Once the guides have been bored out, they should be blown out and checked with a go-no go gauge to make sure they’re the proper size.

The liners should then be pressed in from the top side of the head using an air hammer and K-Line’s Auto Installer tool. The liners go in with the tapered side facing the guide hole. The liners are then driven in flush with the top of the guide boss.

Next, the liners are sized. Any of three different techniques may be used: roller burnishing (use with lubrication), broaching (driving a calibrated ball through the liner with an air hammer), or using K-Line’s ball broach tool in an air hammer.

Sizing the liners is a critical step because it accomplishes two things: it provides the proper clearances between valve stem and liner for proper lubrication and oil control, and it locks the liner in place so it will transfer heat efficiently to the surrounding metal for proper valve cooling. Bronze actually conducts heat more efficiently than cast iron, but requires a tight fit and metal-to-metal contact with the surrounding guide for good heat transfer. If the liner isn’t sized properly, it may cause the valve to run hot—or worse yet, come loose.

After the liners have been sized, turn the head over an trim the liner to length. The liner should be cut flush with the guide boss in the port. This step is not necessary if precut liners are being used that have the correct length for the application.

The final step is to Flex Hone the liner—after any seat work that’s necessary has been completed. The Flex honing step removes any burrs left from trimming the liner to length, and leaves a nice crosshatch finish that improves oil retention. One pass in and out is all that’s recommended to hone the liner. A flexible nylon brush should then be passed through the liner to clean the hole.

Though the just described procedure sounds more complicated than it really is, a typical four cylinder or V8 can be relined in six to seven minutes with practice. Also, the majority of the detailed steps listed regarding cleanliness and accuracy in the guide area are requirements no matter which method of guide repair the rebuilder chooses.

Oversized valves

Another popular guide repair option is reaming the guides to oversize and installing new valves with oversized stems or used valves with oversize chromed plated stems. Those who prefer this technique say it’s a fast and easy way to restore guides because all you have to do is ream the guide to oversize and drop in a new valve. In many instances, the exhaust valves have to be replaced anyway because of wear or burning so the added cost is not that great a factor. New valves also eliminate the need to regrind the old valve stems and the sizing hassles that go with reusing reground valves or stock valves that come in so many different nominal sizes.

About 30% of production engine rebuilders are installing new oversized valves. This compares to about 30% that are using guide liners, and 30% that are grinding or rechroming valves.

The cost of replacing the valves on popular engines is not that bad because the valves are relatively inexpensive. But on less popular engines, they can be rather pricey.

Honing guides

When guides are reconditioned by reaming to oversize (or knurling), the passage of a reamer through the guide fractures the metal leaving microscopic pullouts, tears and a relatively rough surface. This less than ideal bearing surface will not wear as well as one that’s been honed. So honing is usually recommended to smooth the guide bore by knocking the peaks off the ridges left by the reamer. This produces a superior bearing surface that will retain oil better and last longer than an unhoned guide.

Even new guides can benefit from honing. New guides are often rather rough. Honing provides a more uniform surface finish which will reduce stem and guide wear, and generally extend the life of the guides. This includes cast iron guides as well as bronze guides.

Guide replacement

On aluminum or cast iron heads with nonintegral guides, worn guides are often replaced. Pressing out the old guides and installing new ones can be difficult with some aluminum heads where the interference fit is considerable. Cracking the head or galling the guide hole is always a risk. One recommendation here is to preheat the heads in an oven prior to guide removal and to lubricate dry liners before driving them out. The head should also be preheated before the new guides are installed. Chilling the replacement guides can reduce the amount of interference during installation. Lubricant also helps prevent galling. With tapered guides, care must be taken to install them from the right side. Most wet guides are tapered, and also require sealer to prevent leaks.

• REPLACING VALVE SEATS

Replacing valve seats is one of the basic jobs that’s often necessary when rebuilding aluminum or cast iron heads with cracked, damaged or badly worn seats. But there’s a lot more to replacing a valve seat than prying out the old one and driving in a new one. If the head is cast iron with integral seats, the head has to be machined to replace the seat (sometimes called installing a "false" seat). And if the head is aluminum, the seat counterbore may have to be machined to accept an oversize seat if the bore is loose, deformed or damaged. Either way, a machinist has to figure the amount of interference that’s required for the new seat before cutting the head on a seat-and-guide machine. He also has to decide what type of seat to install. Replacing a seat, therefore, involves a number of decisions and steps, all of which affect the outcome of the repair job.

There are differing opinions about the right way and wrong way to replace valve seats, particularly with respect to the amount of interference fit that’s required to retain seats in aluminum heads. A common fear expressed by many engine rebuilders is concern over the possibility of seats falling out, particularly in aluminum heads where the difference in coefficients of thermal expansion between the head and seats can cause seats to loosen if the head overheats. Consequently, engine rebuilders expressed differing views on whether or not locking compound and/or peening or staking should be used as "insurance" when installing seats in aluminum heads.

One point everyone does seem to agree upon is that valve seats play a critical role in the longevity of the valves. The seats draw heat away from the valves and conduct it into the cylinder head. This provides most of the cooling that the valves receive and is absolutely critical with exhaust valves. Anything that interferes with the seat’s ability to cool the valves (such as a loose fit or deposits between the seat and its counterbore) can lead to premature valve failure and expensive comebacks.

The seat alloy and hardness must also be matched to the application and compatible with the type of valves that are installed in the engine. Again, we found differences of opinion regarding the selection and use of various seat materials.

To better understand the issues behind the differing opinions regarding valve seat replacement, let’s start with the seats themselves and why they fail.

Why seats fail

Nonintegral valve seats can fail for a number of reasons. Most of the seats that end up being replaced are replaced because they’re either cracked or too worn to be reground or remachined. Seats can crack from thermal stress (engine overheating usually), thermal shock (a sudden and rapid change in operating temperature), or mechanical stress (detonation, excessive valve lash that results in severe pounding, etc.).

A small amount of valve recession results from normal high mileage wear, but it can also occur when unleaded gasoline or a "dry" fuel such as propane or natural gas is used in an engine that isn’t equipped with hard seats. Recession takes place when the seats get hot and microscopic welds form between the valve face and seat. Every time the valve opens, tiny chunks of metal are torn away and blown out the exhaust. Over time, the seat is gradually eaten away and the valve slowly sinks deeper and deeper into the head. Eventually the lash in the valvetrain closes up and prevents the valve from seating. This causes the valve to overheat and burn. Compression is lost and the engine is diagnosed as having a "bad valve." The seat also has to be replaced, but it many instances it may not be recognized as the underlying cause of the valve failure.

As a rule, a seat should be replaced if the specified installed valve height cannot be achieved without excessive grinding of the valve stem tip (less than .030 in.), or if the specified installed spring height can’t be achieved using a .060 in. spring shim. This applies to integral valve seats as well as nonintegral seats. The only other alternative to replacing the seat is to install an aftermarket valve that has an oversized head (.030 in.). This type of valve rides higher on the seat to compensate for excessive seat wear or machining, and can eliminate the need to replace the seat.

A seat may also have to be replaced if it’s loose or if the cylinder head is cracked and requires welding in the combustion chamber area (the seats should be removed prior to welding).

One way to check a seat for looseness is to hold your finger on one side of the seat while tapping the other side with a hammer. If you feel movement, the seat is loose and should come out (so it doesn’t fall out later!).

The seats in an aluminum head may also loosen or fall out when the head is being cleaned in a bake oven or preheated in an oven for straightening. The same thing can happen to the guides. Whether or not this occurs depends on the amount of interference fit between the seats and head. The less the interference, the more likely the seats are to loosen and fall out when the head is baked. If you don’t want the seats to fall out, turn the head upside down or stake the seats prior to baking.

Seat removal

A variety of techniques are being used by engine rebuilders to extract nonintegral valve seats from cylinder heads:

Some are using their bake ovens or an open flame rotisserie thermal cleaning system to clean their heads and loosen the seats in one step. With a bake oven, the heads are loaded with the seats facing down and heated to 450 degrees F. If the seats don’t fall out of their own accord, they can be easily removed while the head is still hot.

Some have success using a simple pry bar to pop the seats loose (if there’s enough of an edge under the seat for the bar to grab). But using a pry bar runs the risk of damaging the counterbore.

Seats can also be removed if the underside of the seats are accessible through the valve ports by using a long punch to knock them out. But again, care must be taken not to damage the counterbore.

Cast iron seats in aluminum heads are also being removed by using a die grinder to cut through the seat. This relieves pressure and allows the seat to be easily removed. The danger with this technique, however, is grinding all the way through the seat and into the head. One slip can create a gouge that can be expensive to fix.

Another technique that’s sometimes used to remove soft cast iron seats in aluminum heads is to cut them out. A cutter that’s slightly smaller than the outside diameter of the seat is used to machine away most of the seat. If the thin shell that’s left doesn’t break loose and spin with the cutter (which can chew up the counterbore if you’re not careful!), it can be easily pried out. This technique doesn’t work very well on hard seats, though, because the seats are about as hard as the cutter.

To remove hard seats, some rebuilders arc weld a bead all the way around on the seat. As the bead cools, it shrinks and loosens the seat.

Another trick that’s sometimes used to remove a hard seat is to insert a valve that’s somewhat smaller than the seat in the head and then weld the valve to the seat. The valve stem can then be used like a driver to push out the seat.

Once a seat has been removed from a cylinder head, a determination must be made as to whether or not the counterbore needs to be machined to accept an oversized seat. If the original seat was loose, if the counterbore is flared more than .001 in. (wider at the top than the bottom), or if the difference between the counterbore’s inside diameter (ID) and a standard seat’s outside diameter (OD) isn’t enough to provide the desired interference fit, then machining will be necessary.

Seats are available in various oversizes. But the amount of metal that can be safely removed from most aluminum cylinder heads is minimal, so the less the amount of machining that’s required the better. Cutting a seat counterbore too large or too deep may weaken the head, cut into the water jacket or cut into the adjacent seat.

The amount of interference required to lock a seat in place depends on the diameter of the seat (the larger the seat, the greater the interference that’s required), the type of head (aluminum or cast iron), the application (hotter running applications typically require more interference to keep the seats from falling out), and in some cases the type of material used in the seat itself (hard seats can’t take as much interference as softer seats).

For cast iron heads, recommendations range from .003 to .006 in. for valve seats up to 2 inches in diameter. Some rebuilders and seat suppliers say more interference is needed because of the difference in the coefficients of thermal expansion between the head and seats. Aluminum expands several two to three times as much as cast iron when it gets hot, so recommendations range from .004 to as much as .0085 in. interference for valves up to 2 inches in diameter. Others say seats in aluminum heads actually require less interference than those in cast iron.

A common problem in rebuilt aluminum heads is improperly machined seat counterbores. The bore should have a smooth finish so the seat will fit tightly and won’t broach or shave the head metal as it is being driven in.

A good finish requires sharp tools and plenty of cutting speed. Cut at 600 rpm. Do not use the same tools on aluminum that have been used on cast iron.

When replacing a seat, measure the OD and depth of the original seat and then go .020 in. over, allowing for a .005 in. interference fit. If you go with too large an oversize, you’ll end up removing too much metal and weakening the head.

Choosing the right replacement seat

The original equipment manufacturers use a variety of seat materials, including cast iron, iron alloys, nickel alloys, cobalt alloys (stellite) and powdered metal (which generally contain no chrome or nickel, only vanadium and iron). Most OE seats in passenger car aluminum heads are a high grade of cast iron or powder metal. The better (more expensive) materials are usually found in high output and turbocharged engines, with hard seats and stellite being used mostly in diesels and industrial engines.

When replacing a seat, you should use one that’s at least as good as the original if not better. Hard seats are a must for high temperature, high load and dry fuel (propane or natural gas applications). In fact, most seat suppliers have special alloys specifically designed for dry fuel applications. But hard seats aren’t required for light duty passenger car applications. Even so, many aftermarket seats are made of premium grade alloys or heat treated iron to provide improved longevity and performance.

Seat and valve materials must be compatible with one another as well as suited for the application. A hard valve generally requires a hard seat and vice versa. A stellite faced valve in an industrial engine, for example, would require a stellite seat. A titanium racing valve, on the other hand (which is relatively soft), would require a soft cast iron or beryllium-copper seat.

Material compatibility is very important, especially with fuels such as propane and natural gas.

Most seat failures are the result of abnormal engine operation (overheating, detonation, wrong air/fuel mixture, etc.) or because someone chose the wrong type of replacement seat.

Seat installation

Once the counterbore in the head has been machined for the desired interference fit and a replacement has been selected, the next step is to install the seat.

As mentioned previously, the hole must be clean and have a smooth surface finish. The seat should be placed with the radius or chamfer side down and lubricated (ATF works fine) prior to being pressed or driven in with a piloted driver (recommended to prevent cocking).

If the replacement seat has a sharp edge, it should be chamfered or rounded so it won’t scrape any metal off the head as it is being driven into position. If metal gets under the seat, it will create a gap that forms a heat barrier. This, in turn, will interfere with the seat’s ability to cool the valve and premature valve failure will likely result.

Preheating the head and/or chilling the seats with dry ice or carbon dioxide (don’t use Freon because it damages the ozone) will make installation easier and lessen the danger of broaching the counterbore as the seat is being installed.

If you choose to peen or stake the seats after they’ve been installed as added insurance to prevent them from falling out (which shouldn’t be necessary if the seats have the correct interference and were properly installed), several engine rebuilders we interviewed recommended rolling or peening rather than staking. Their reason? Staking creates stress points and potential hot spots.

• PREVENTING REPEAT HEAD GASKET FAILURES

When a head gasket is installed between the cylinder head and engine block, tightening the head bolts compresses the gasket slightly allowing the soft facing material on the gasket to conform to the small irregularities on the head and block deck surfaces. This allows the gasket to "cold seal" so it won’t leak coolant until the engine is started.

The head gasket’s ability to achieve a positive cold seal as well as to maintain a long-lasting leak-free seal depends on several things: it’s own ability to retain torque over time (which depends on the design of the gasket and the materials used in its construction), surface finish and the clamping force applied by the head bolts.

Some head gaskets remain resilient and retain torque better than others, so they do not require retorquing. Others, though, can lose as much as 50 to 60% of their original torque after only 100 hours of service!

But even the best head gasket won’t maintain a tight seal if the head bolts have not been properly torqued. The amount of torque that’s applied to the bolts as well as the order in which the bolts are tightened determine how the clamping force is distributed across the surface of the gasket. If one area of the gasket is under high clamping force while another area is not, it may allow the gasket to leak at the weakly clamped point. So the head bolts must all be tightened in a specified sequence and equally torqued to a specified value to assure the best possible seal.

Another consequence of failing to torque the head bolts properly can be head warpage. Uneven loading created by unevenly tightened head bolts can distort the head. Over a period of time, this may cause the head to take a permanent set. So any head that has not been properly torqued should be checked for flatness prior to installing a new head gasket.

Flatness checks

Before the head goes back on the block, the flatness of both the head and block should be checked to make sure both are flat enough to provide a good seal for the new gasket. Warpage on either surface, deep scratches, corrosion, pitting, gouges, excessive roughness or waviness can all reduce a gasket’s ability to seal and allow combustion gases and/or coolant to leak.

Place the straight edge on the face of the cylinder head or block and then use a feeler gauge to check any gaps between the straight edge and the surface. If the clearance between the straight edge and surface exceeds the following maximum limits, the head or block is not flat enough to hold a good seal and should be resurfaced:

Out-of-flat lengthwise should not be more than .003 in. (0.076 mm) in a V6 head, .004 in. (0.102 mm) in a four cylinder or V8 head, or .006 in. (0.152 mm) in a straight six head. The maximum allowable limit for out-of-flat sideways in any head is .002 in. (.05 mm) -- with no sudden irregularities that exceed .001 in in any direction.

If you’re checking flatness on a late model Japanese engine that has a multi-layer steel (MLS) head gasket, both sealing surfaces must be even flatter: no more than .002 in. (.05 mm) of total distortion (that is, block plus the head combined) in any direction. Engines that use this type of gasket include 1990 & up Honda Accord 1.8L, 1990 & up Honda 1.5L, 2.2L & 2.3L, 1988 to ‘92 Mazda 3.0L V6, 1990 & up Mazda SOHC & DOHC 1.8L, and 1992 & up Mazda 1.8L.

Any head that fails to meet these specs needs to be straightened and/or resurfaced.

Milling a head to restore flatness has its limits because milling affects the installed height of the head. On overhead cam (OHC) engines, this can alter the OHC cam drive geometry enough to retard cam timing which can adversely effect emissions, performance and driveability. Milling also reduces the volume of the combustion chamber which increases compression and the risk of engine damaging detonation. It also reduces the clearance between the valves and pistons, which on some engines is pretty close already.

To minimize changes in head height, the amount of metal that’s removed when resurfacing should always be kept to a bare minimum. In other words, your machine shop should not remove any more metal than is absolutely necessary to restore proper flatness and surface finish.

A head can usually be "cleaned up" by removing only a couple thousandths of metal—unless it is warped or damaged, in which case the amount of metal that has to be removed will depend on how badly the head is warped or the depth of the surface depressions or damage. In cases where a head is cracked and has been repaired by welding or pinning, it may be necessary to take off a considerable amount of metal to restore the surface.

Aluminum heads should always be straightened prior to resurfacing. This will substantially reduce the amount of metal that has to be removed from the head to restore flatness.

If a head cannot be restored without exceeding the resurfacing limit specified by the vehicle manufacturer, it may be possible to save the head by installing a head gasket shim. A .020 inch thick shim can be used to restore proper head height, compression and OHC valve timing. Shims are designed to be be used with standard head gaskets, and should be coated with a tacky sealer on the block side before the head gasket is installed over it.

Surface flaws

The surfaces of both the head and block should be carefully inspected for pitting, corrosion, metal erosion (common on high mileage aluminum heads around combustion chambers), gouges and cracks. Any flaw which creates a cavity, low spot, valley, depression or ridge on the surface of the metal creates a potential leak path. Pay particular attention to the areas between the cylinders on the block, between the combustion chambers on the head, and where the combustion armor of the head gasket seats around the cylinders on both surfaces as these are the most highly stressed sealing areas. Any surface flaws that are found should be eliminated by resurfacing the head or block.

Resurfacing

For years, most aftermarket gasket manufacturers have said surface finishes with a roughness average (RA) of anywhere from 55 to 110 microinches (60 to 125 RMS) are acceptable. The preferred range they have recommended is from 80 to 100 RA. Even so, as long as the surface finish on the head and block end up somewhere between the minimum smoothness and maximum roughness numbers, there shouldn’t be any cold sealing or durability problems with the head gasket.

But like everything else, these numbers have been changing. These recommendations were primarily for older cast iron heads on cast iron blocks. As castings have become lighter and less rigid, the need for smoother, flatter surfaces has become more important. Consequently, some aftermarket gasket manufacturers now recommend a surface finish of 30 to 110 RA for cast iron head and block combinations, with a preferred range of 60 to 100 RA for best results.

For aluminum heads, the numbers are even lower. The typical recommendation today for an aluminum head on an OHC bimetal engine is a surface finish of 30 to 60 RA, with the preferred range being from 50 to 60 RA—unless, of course, it’s one of the Japanese engines already mentioned with the MLS steel head gaskets which requires an even smoother finish (typically 20 to 30 RA).

It’s not difficult for the OEM’s to achieve this type of mirror-like finish when they manufacture a brand new engine on an assembly line. But not every aftermarket machine shop has the proper equipment to reproduce this kind of finish. So some experts say heads that mate to MLS gaskets should not be resurfaced unless absolutely necessary. For this reason, you should use extra care when removing the old head gasket so you don’t scratch or change the surface finish.

Installing the head bolts

Make sure all head bolts are in perfect condition with clean, undamaged threads. Dirty or damaged threads can give false torque readings as well as decrease a bolt’s clamping force by as much as 50%. Wire brush all bolt threads, carefully inspect each one, and replace any that are nicked, deformed or worn.

Check the holes, too. Dirty or deformed hole threads in the engine block can reduce clamping force the same as dirty or damaged threads on the bolts. Run a bottoming tap down each bolt hole in the block. The tops of the holes should also be chamfered so the uppermost threads won’t pull above the deck surface when the bolts are tightened. Finally, clean all holes to remove any debris.

Lightly lubricate all head bolts that screw into blind holes. Apply 30W engine oil to the threads as well as the underside of the bolt head. For head bolts that extend into a coolant jacket, coat the threads with a flexible sealer (failure to do so may result in coolant leakage).

If the bolts are the "torque-to-yield" (TTY) type, you should probably use new ones rather than take a chance on reusing old bolts that have stretched. TTY bolts are usually longer and narrower than ordinary head bolts, and are designed to stretch slightly when tightened to provide more consistent clamping force. But reusing them increases the risk of breakage. A stretched bolt may also not hold the same torque load as before, which may cause a loss of clamping force resulting in head gasket leakage.

Check bolt lengths. Make sure you have the correct length bolts for the application and for each hole location (some holes require longer or shorter bolts than others).

Bolts should also be measured or compared to one another to check for stretch. Any bolt found to be stretched must be replaced because (1) it may be dangerously weak, (2) it won’t hold torque properly, and (3) it may bottom out when installed in a blind hole.

When installing head bolts in aluminum cylinder heads, hardened steel washers must be used under the bolt heads to prevent galling of the soft aluminum and to help distribute the load. Make sure the washers are positioned with their rounded or chamfered side up, and that there is no debris or burrs under the washers.

Resurfacing a cylinder head decreases its overall height, so be sure to check bolt lengths to make sure they won’t bottom out in blind holes. If a bolt bottoms out, it will apply little or no clamping force on the head which may allow the gasket to leak.

If a head has been milled and one or more head bolts may be dangerously close to bottoming out, the problem can be corrected by either using hardened steel washers under the bolts to raise them up, or by using a head gasket shim in conjunction with the new head gasket to restore proper head height.

Always use the specified tightening sequence and recommended head bolt torque values for the engine you’re working on—and make sure you have the latest specs because the torque specs on some engines have been revised since they were first published. If your reference material is more than a few years old, it may be out of date. A good source any changes that may have occurred would be an online service such as All-Data that allows you to search factory TSBs (technical service bulletins). Another source for engine bulletins is the Automotive Engine Rebuilders Association (AERA).

Also, keep in mind that the listed torque valves are for head bolts that have been lightly lubricated with 30W engine oil only, not for dry bolts or ones that have been coated with anything else (assembly lube, graphite, silicone, grease, etc.).

Use an accurate torque wrench to tighten standard head bolts in 3 to 5 incremental steps following the recommended sequence and torque specs for the application. Tightening the bolts down gradually creates an even clamping force on the gasket and reduces head distortion. It’s a good idea to double check the final torque readings on each head bolt to make sure none have been missed and that the bolts are retaining torque normally.

If a bolt is not coming up to normal torque or is not holding a reading, it means trouble. Either the bolt is stretching or the threads are pulling out of the block.

With TTY head bolts, a "Torque-To-Angle Indicator" should be used in conjunction with a torque wrench to achieve proper bolt loading.

Finally, if a head gasket is the type that requires retorquing, run the engine until it reaches normal operating temperature (usually 10 to 15 minutes), then shut it off. Retighten each head bolt in the same sequence as before while the engine is still warm. If the engine has an aluminum cylinder head or block, however, wait to retorque the head bolts until the engine has cooled back down to room temperature.

• RESURFACING CYLINDER HEADS

Head resurfacing is usually required when rebuilding an engine or reconditioning a cylinder head. It’s an extremely important job because the surface finish that’s put on the head (and engine block) affects not only the head gasket’s ability to cold seal fluids and combustion gases, but also its long term durability.

For years, most aftermarket gasket manufacturers have said surface finishes with a roughness average (RA) of anywhere from 55 to 110 microinches (60 to 125 RMS) are acceptable. The preferred range they have recommended is from 80 to 100 RA. Even so, as long as the surface finish on the head and block end up somewhere between the minimum smoothness and maximum roughness numbers, there shouldn’t be any cold sealing or durability problems with the head gasket (assuming everything is assembled correctly and the head bolts are torqued in the proper sequence and to the specified torque, too).

But like everything else, these numbers have been changing. These recommendations were primarily for older cast iron heads on cast iron blocks. As castings have become lighter and less rigid, the need for smoother, flatter surfaces has become more important. Consequently, some aftermarket gasket manufacturers now recommend a surface finish of 30 to 110 RA for cast iron head and block combinations, with a preferred range of 60 to 100 RA for best results.

Aluminum heads

For aluminum heads, the numbers are even lower. The typical recommendation today for an aluminum head on an OHC bimetal engine is a surface finish of 30 to 60 RA, with the preferred range being from 50 to 60 RA.

Smoothness has become a major issue with bimetal engines because the difference in thermal expansion rates between an aluminum head and cast iron block creates a tremendous amount of sideways shearing force and scrubbing action on the head gasket. If the surface finish is too rough (more than about 60 RA), the metal will bite into the gasket and pull it sideways as the head expands and contracts. The cumulative effect over time can cause a delaminating effect in the gasket, literally tearing it apart causing it to leak and fail.

Even lower numbers may be required for certain engine applications. General Motor’s, for example, specifies a surface finish of 27 to 47 RA for its 2.3L Quad Four engine when the OEM replacement gasket is used. Some aftermarket gaskets can handle a rougher finish on these engines, but it all depends on the design of the gasket. Even so, smoother is definitely better on these engines.

Ford specifies an unusually smooth surface finish for its 4.6L V8 engine. This engine, like a growing number of late model Japanese engines, uses a multilayer steel (MLS) head gasket. This type of laminated steel gasket is extremely durable because the multiple layers of metal (each of which is coated with a thin layer of rubber) prevents the gasket from losing torque over time. The design also reduces the amount of torque that’s required on the head bolts to seal the gasket, which in turn reduces cylinder bore distortion for better combustion sealing and reduced blowby. The recommended surface finish for the OEM gasket on the 4.6L V8 is 8 to 15 RA.

To appreciate just how smooth this is, compare the surface finish requirements for the head and block on the Ford 4.6L V8 to those for a typical cylinder bore. Honing with #220 grit stones typically leaves a finish in the 38 to 45 RA range. Honing with #280 grit stones will generally produce a finish of 18 to 25 RA. Honing with #400 grit stones, a finish of 10 to 15 RA can be achieved, and honing with #600 grit stones can push the numbers down to 5 to 10 RA. A pane of window glass, by comparison, measures about 3 to 4 RA.

Honing cylinder bores is obviously an entirely different process than resurfacing cylinder heads or blocks. For one thing, the walls of a cylinder bore must have a certain amount of crosshatch to retain oil so the bores can’t be too smooth. But neither can they be too rough because a rough finish can wear the rings excessively as they seat in.

The ideal finish for a cylinder bore is a "plateau" surface where the sharp peaks have been knocked off but the valleys remain. This provides plenty of bearing area to support the rings but also leaves the crosshatch for proper ring lubrication. More importantly, it virtually eliminates ring wear during the initial break-in process because the surface already has the profile of a broken-in cylinder. This kind of surface is produced by going in with a final honing step that uses a fine grit stone (#600) or a flexible abrasive.

Although an unusually smooth finish may be required for the Ford 4.6L V8 and certain late model Japanese engines that have MLS head gaskets, smoother is generally better for all engines because it improves cold sealability.

One thing you don’t want on the surface of the head or block is scratches. Every scratch is a potential leak path along which fluids and pressure can migrate. If a scratch is deep enough, coolant may find its way into the crankcase or cylinders before the engine is fired up. Or, combustion gases may force their way past the gasket into the cooling jacket or an adjacent cylinder eventually causing the gasket to burn out and fail. Either way, it’s bad news. So the best way to avoid cold sealing and durability problems is to take the proper steps when refinishing the head and block to ensure the surface finish is within the recommended limits of the gasket manufacturer and/or original equipment manufacturer.

Can the surface be too smooth?

Can a head or block surface be too smooth? After all, the smoother the surface the better the initial cold seal of the gasket and the less likely you are to have problems with coolant and combustion leaks.

Though most gasket manufacturers do not specify a minimum smoothness spec for aluminum heads that have MLS head gaskets, they do recommend a minimum of 30 RA for engines with aluminum or cast iron heads and a nonasbestos or graphite head gasket. The reason for doing so is because soft-faced head gaskets require a certain amount of lateral support from the head and block.

When the head is bolted to the block, the metal on both sides bites into the gasket to help hold it in place. You don’t want too much bite when the head is aluminum and the block is cast iron because of the sideways shearing forces that result from the expansion and contraction of the aluminum head. Yet a certain amount of support is necessary to keep the combustion gases in the cylinders from distorting the gasket and blowing past it. This is especially critical in the areas with narrow lands and between the head bolts where there is nothing to keep the gasket in place but the gasket itself. In high output or heavy-duty applications where combustion pressures exerts even greater force against the head gasket, a surface finish that’s below the minimum smoothness spec might lead to premature gasket failure.

Judging smoothness

Most engine rebuilders have no idea how smooth a surface finish they’re actually putting on the heads and blocks they resurface. As long as the head gasket seals initially and doesn’t fail during the warranty period, they assume they’re doing everything right—and maybe they are. Then again, maybe they aren’t. They may not be seeing the long term consequences of their actions because the head gasket doesn’t fail until the engine is out of warranty.

In many instances, a premature head gasket failure because of leakage or burn through can be traced back to the surface finish that was put on the head and block. Of course, there may be other factors involved, too, like engine overheating, detonation, etc., or mistakes that were made during engine assembly such as not torquing the head bolts properly. But if a head gasket fails, there’s always a reason why. Paying closer attention to the details of resurfacing, therefore, can help eliminate this as a potential source of trouble.

Most people can’t look at a finish and tell if it’s 20 RA or 60 RA. So the only way to know if a resurfaced head or block is within the proper range is to measure it. Judging surface finishes by appearance alone, or even feel, is not a very accurate means of controlling quality. Most people can’t tell the difference between an acceptable finish and one that isn’t unless it is really bad. Even then, their judgment may be shaded by outdated notions of what’s "good enough" and what isn’t. If they’re resurfacing aluminum heads the same way they’ve always done cast iron heads, chances are the surface is too rough.

One way to judge surface finishes is with an inexpensive comparator gauge. Available from at least one aftermarket gasket manufacturer as well as various tool suppliers, a comparator gauge has sample patches etched on a metal plate that indicate the different surface finish ranges. By placing the comparator gauge next to a resurfaced head and visually comparing and feeling the sample patches on the gauge to the head you can get an approximation of whether or not you’re in the correct range. But it’s not very exact, and it’s often hard to tell just how close you actually are to a given range of numbers.

A far better method of judging the quality of your work is to