Lesson3/Learning Event 1
The heat range, or operating temperature of a spark plug, is determined by the length of the
insulator nose. The spark plug, when in operation, is exposed to the heat of the burning fuel. For
the plug to cool, the heat will have to pass up through the insulator nose to the shell of the plug
and from there to the cylinder head or engine block. The farther the heat has to go to get to the
shell of the plug, the hotter the spark plug will operate. This means a spark plug with a short
insulator nose will operate cooler than one with a long insulator nose.
The correct spark plug for any given engine can vary considerably. A hot plug will work better in
an engine that is burning oil or in one that is operated at low speeds or short distances. A colder
plug may be needed in the same engine if it is operated at high speed over long distances or under
Examine the insulator nose to determine if a spark plug of the correct heat range is being used. If
the deposits on the nose are a light grey or brown ash, the heat range is correct. If the nose is
covered with a black, gummy carbon, the plug is too cold. A blistered nose, or one with the
porcelain chipped off, is probably too hot.
Before we go any farther, let us see what happens when all the things we have discussed so far in
this lesson are in operation. Let us start with the number 1 piston going up on the compression
stroke. At this time, the air-fuel mixture in the cylinder is being squeezed into a small area of the
combustion chamber. With the ignition switch and the breaker points closed, a magnetic field is
building up in the ignition coil.
When the piston reaches near top dead center on the compression stroke (the exact point will
depend on engine design and engine speed), ignition of the fuel should take place. This is made
possible because the distributor is turning in time with the engine.