[1] - Terminal
Made from metal it is at the opposite end to the electrodes.

[2] - Insulator
Houses the centre electrode and terminal and insulates them from the shell. Made of a highly purified alumina powder, which gives exceptional dielectric strength and thermal conductivity at high temperatures, as well as greater physical strength.

Shell
Made from metal, it includes a hexagon shape at one point to allow tightening with a socket and a threaded area at the electrode end for securing it to the cylinder head.

[3] - Centre Seal
Mixture of powder that bonds the centre electrode and insulator together for a gas-tight seal. Proper heat is maintained and hot combustion gases are excluded.

[4] - Centre Electrode
In standard plugs it is melted gas-tight into the insulator. It protrudes slightly from the insulator creating a specific size gap with the ground electrode. The diameter is important and determines certain characteristics.

[5] - Ground Electrode
In standard plugs it reaches out from the threaded end of the plug and curves over to create a specific gap size with the centre electrode.