The spark plug consists of a plated metal shell with a ceramic insulator and an electrode extending through the center of the insulator. Threads on the metal shell allow it to be screwed into the cylinder head and a short earth electrode attached to one side is bent in towards the center electrode.
The electrodes are of special alloy wire with a set recommended gap between them. The spark bridges this gap to ignite the air-fuel mixture in the combustion chamber.
Spark plugs are identified by three different features.
These are:
Spark plugs should operate between average temperatures of 750 to 1500 degrees Fahrenheit or 400 to 800 degrees Celsius. This is referred to as the heat range.
The temperature that a spark plug will reach depends on the distance the heat must travel from the insulator to reach the outer shell of the plug and enter the cylinder head and the water jacket.
If the heat path is long, the plug will retain more heat and therefore will run at a higher temperature than one with a short heat path.
