A spark plug is used in every type of gasoline engine. Many people change their own spark plugs. Spark plugs handle the ignition spark that combusts or ignites the engines fuel and moves the piston. Spark plugs have to endure very high temperatures from combustion. Spark plugs also have to handle the spark erosion from the ignition system.

There are many popular brands of spark plugs like champion, ngk, bosch, denso and autolite. Spark plugs can be gapped widely to help burn the mixture in the cylinder, especially if the mixture is lean. Spark plug gap refers to the distance between the little metal piece that hangs below the plug and the center electrode. A lean fuel mixture can not ignite as easily. A wide spark gap is used to jump across more of the chamber mixture and can help induce combustion.

The plug gap can be too wide, especially if the ignition system is weak. This is why race engines use very high power CDI (capacitive discharge ignition) like MSD to ensure good spark. The engine needs more power to jump a wider gap or the high compression of a performance engine. There are many types of spark plugs such as platinum and iridium for longevity because they resist spark erosion (wearing away of metal) better than steel.

Spark plugs have heat ranges that either transfer more heat or contain more. This is accomplished by the design and transferring of heat through the base of the plug. Heat range is either cooled or lowered for use with high compression, turbo or supercharged four stroke engines. This is so that the hot plug won't pre-ignite the mixture causing engine damage.

Many two stroke engines or oil consuming four strokes will have a problem with fouling plugs. If fouling occurs the heat range may need to be increased to keep the oil burned off the plug. Oil on a spark plug will cause it the engine to foul, misfire and run rough. Champion plugs use higher numbers on their plugs to mean that the temperature will be higher. NGK's use the opposite range, higher number means a colder plug.

Most two stroke engines use NGK plugs for their excellent anti-fouling capabilities. Some plugs will also be a p type or projected insulated electrode. Depending on the engine chamber design, this can help to place the actual spark closer to the center of the cylinder head chamber.

Spark plugs are located in the engine's head, under the spark plug wires. The plug wires are easy to spot due to their larger size and location close to the top of the engine. They also come from the engines distributor to the plug. Remove the wire from each plug, and don't forget to place the wire in the same spot that it was first hooked to when finished! Most spark plugs can be removed with either a 13/16" or 5/8" socket. Inspect plugs to see if the electrode (center piece) and the grounding tab still has defined edges or are rounded, eroded and worn out. If worn replace them, this will help fuel mileage and performance.

Although the spark plug gap is different for different engines, most are gapped between .030" and .045". The spark plug gap spec should be located under the hood or on a sticker on recreational vehicles. Use a feeler gauge to measure the plug gap, then tap the electrode closed or gently pry gap wider with a screw driver until the distance is reached. Tighten the spark plug snugly as the sealing washer will compress a little the first time. Always check the small top metal cap on a plug to see that it's screwed on tight.