Proper gapping of the spark plug is necessary to get maximum spark energy, lowest RFI release as well as what is best for the longevity of the secondary ignition components (coil, cap, rotor, wires, plugs).

    When checking plug gaps, the correct way is to use only wire gauges, though many of us are using the slider style gapping tools. These flat or feeler gauge style gauges do not accurately measure true width of spark plug gap.

    When increasing the gap size for our high performance applications utilizing advanced ignition systems such as Mallory, Accel, Jacobs, Crane and Holley ... it is important never to go more than plus or minus .008". This is to maintain parallel surfaces between ground and the center electrodes.

    Something many do not know, is that with Higher Compression ratios and Superchargers as well as Nitrous, in many cases smaller spark plug gaps must be used as well as the use of a much hotter ignition system (see above). These higher cylinder pressures require more energy to jump the spark plug gap.

    The rule of thumb on plug gaps is to open them up in .002" increments at a time. When the car (race vehicle) begins to lose power or slow down then go back .001-.002" and this in most cases is the optimum gap.

    Remembering that the Ignition Unit, plug brand as well as heat range, cap and/or rotor styles and in many cases fuel type or brand will change the optimum spark plug gap settings.

    Lastly, never use the porcelain insulator as a fulcrum point when setting these gaps, this can cause damage to the spark plug.

    How to Gap a Spark Plug

    The information below is provided as a general guide only. Please refer to your owner’s manual or the spark plug manufacturer for further information.

• To check the gap of a spark plug use a feeler gauge and choose the correct wire to match the stated plug gap.
• Run the wire between the electrodes, it should pass through fairly easily, just catching as it passes.
• Adjust the gap if required
• To widen gap - use the correct part of the gauge to hook on the ground electrode and gently tug.
• To narrow gap - press the ground electrode against a soft, clean surface very gently.
• Re-check gap until it is correct.

    Opening The Plug Gap

    On weaker or stock ignitions, opening up the gap can increase the spark kernel size, thereby creating a more efficient burn. The problem lies in that any added gap creates more strain on the other ignition parts.

• Coils may not have enough stored energy to fire, or in the least case, not enough energy to cross the gap, creating a miss-fire.

• Plug wires will break down due to the added resistance as the spark tries to reach ground.

• Rotor and Cap, as well as points (if you still have an interest in prehistoric ignitions), and the carbon bushing in the center of the distributor cap will show early failures.

    All of this is because the greater the gap and the higher the voltage requirement to jump the gap. Do not forget the gap between the rotor arm to the distributor cap too. A high performance rotor is a bit longer at the tip, allowing less spark loss or chance of spark scatter in the cap as the spark attempts to jump the plug gap.

    As many of us know that race, it is also possible to slow down a car if the gap is too big. I will get into this later when I describe proper spark plug gaps.