Lesson3/Learning Event 2
FIGURE 24. TYPICAL SPARK PLUG CONSTRUCTION AND
HEAT RANGE DESCRIPTIONS.
The spark plug (the remaining part of the secondary circuit) consists of a metal shell, a porcelain
insulator with an electrode extending through it, and a ground electrode which is attached to the
metal shell. The shell has external threads to allow it to be screwed into a threaded hole leading to
the combustion chamber. The insulated and the grounded electrodes are separated by an air gap
(also called a spark gap) of 0.025 to 0.040 of an inch. In operation, the high-voltage current
produced in the secondary winding of the coil will arc across the spark plug air gap to ignite the
Spark plugs used in waterproof systems are much like the ones we have just discussed, except this
type plug will be completely surrounded by a metal shell to which the nut on the spark plug cable is
threaded. This shell is for both shielding and waterproofing the spark plug.
Spark plugs are usually classified in two ways: first, as to the diameter of the threaded hole into
which they are screwed (10 mm, 14 mm, 18 mm, and so forth), and second, according to the heat
range of the plug.