Spark Plugs

A typical spark plug consists of a metal shell surrounding a ceramic insulator. A metal electrode extends downward through the center of the insulator and protrudes a small distance. Located at the end of the plug and attached to the side of the outer metal shell is the side electrode. The side electrode bends in at a 90°angle so that its tip is even with, and parallel to, the tip of the center electrode. The distance between these two electrodes (measured in thousandths of an inch) is called the spark plug gap. The spark plug in no way produces a spark but merely provides a gap across which the current can arc. The coil produces anywhere from 20,000 to 40,000 volts which travels to the distributor where it is distributed through the spark plug wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode, and, in so doing, ignites the air/fuel mixture in the combustion chamber.

Spark plugs ignite the air and fuel mixture in the cylinder as the piston reaches the top of the compression stroke. The controlled explosion that results forces the piston down, turning the crankshaft and the rest of the drive train.

The average life of a spark plug is dependent on a number of factors: the mechanical condition of the engine; the type of engine; the type of fuel; driving conditions; and the driver.

Spark plugs should be checked frequently (approximately 5,000 miles) depending on use. All the recommendations are based on the ambient conditions as well as driving conditions. If you drive at high speeds constantly, the plugs will probably not need as much attention to those used for constant stop-and-start driving.

The electrode end of the plug (the end with the threads) is a good indicator of the internal condition of your engine. If a spark plug has fouled and caused the engine to misfire, the problem will have to be found and corrected. Often, reading the spark plugs will lead you to the cause of the problem. Spark plug conditions and probable causes are shown in the accompanying photos. It is a good idea to pull the plugs once in a while just to get an idea of the internal condition of your engine.

NOTE: A small amount of light tan colored deposits on the electrode end of the spark plug is quite normal. These plugs need not be replaced unless they are severely worn.

The gap between the center electrode and the side or ground electrode can be expected to increase not more than 0.001 in. every 1,000 miles under normal conditions.

When a spark plug is functioning normally or, more accurately, when the plug is installed in an engine that is functioning properly, the plugs can be taken out, cleaned, regapped, and reinstalled in the engine without doing the engine any harm.

CAUTION
The spark plugs used on the rotary engine are not adjustable. Attempting to adjust the air gap will damage the porcelain!

When, and if, a plug fouls and begins to misfire, you will have to investigate, correct the cause of the fouling, and either clean or replace the plug.