Metal case
The metal case removes heat from the insulator and passes it on to the cylinder head. It also provides structural strength to cope with the torque force applied when tightening the plug into place. The case also acts as the ground for the sparks passing through the center electrode to the side electrode.
Insulator
The insulator covering the outside of the metal case is usually made from an aluminum oxide ceramic with a high tolerance to heat and electrical voltage. The ribbing design, the exact composition of the insulator material, and length of the insulator, can all partly determine the heat range of the plug.
Seals
The spark plug seals the combustion chamber of the engine when installed, so the seals make sure that it is kept airtight. The seal is usually a hollow metal washer which at the correct torque setting is partially crushed between the flat surface of the head and the plug just above the threads. These seals should be replaced when plugs are removed and replaced.
Terminal
A terminal at the outer end of the spark plug connects it electrically to the ignition system. Some ignition leads clip on the plug. In other types they may be held in place by a threaded nut.
Side, or ground electrode
The nickel steel side electrode runs very hot and is welded to the side of the metal case. Some spark plug designs have multiple side electrodes that do not overlap the center electrode.
Center electrode
The center electrode is the hottest part of the plug, and it is from here that the electrons that cause the spark are emitted. The electrons stream from the edges of the flat electrode, and as these edges erode and become less sharp, the spark weakens. It used to be necessary to maintain spark plugs by sandblasting corroded surfaces and filing the electrodes sharp again, but as low-erosion materials have improved this is not necessary. Spark plugs now usually last a lot longer and are replaced rather than refurbished.
Spark plug gap
The same plug can sometimes be used in different engines with different gap settings, so the gap should be checked with an accurate feeler gauge and if necessary adjusted according to the specifications in the workshop manual when they are first fitted. The gap is critical to engine performance. A narrow gap may give a spark to weak to ignite the air-fuel mixture effectively. A gap too wide may fail to produce any spark at all. Either way, the wrong gap will reduce engine power and fuel efficiency.
Heat range
The operating temperature of a spark plug refers to the temperature at the sparking tip of the plug inside a running engine. If the tip is too hot, pre-ignition can occur. If it is too cold, deposits may form on the insulator causing spark energy loss. 'Hot' rated spark plugs are better insulators, keeping more of the engine heat in the tip of the plug. A 'cold' rated plug conducts more of the heat out of the spark plug tip. The heat range of a spark plug is affected by a wide range of characteristics, including its design profile and the types of material used in its manufacture.
