Spark Plug: Description and Operation

CAUTION: This engine has aluminum cylinder heads. Allow the engine to cool before removing spark plugs, or the spark plug threads in the cylinder head may be damaged.

DESCRIPTION
This engine uses tapered seat resistor type spark plugs without gaskets. Always replace plugs with the same spark plug as listed.

OPERATION
Normal service is assumed to be a mixture of idling, slow speed, and high speed driving. Occasional or intermittent highway driving is needed for good spark plug performance because it gives increased combustion heat that bums away any excess deposits of carbon or oxides that have built up from frequent idling or continual stop-and-go driving. Spark plugs are protected by an insulating boot made of special heat-resistant material which covers the spark plug terminal and extends downward over a portion of the plug insulator.
These boots prevent flash-over which causes engine mis-firing. Do not mistake corona discharge for flash-over or a shorted insulator. Corona is a steady blue light appearing around the insulator, just above the shell crimp. It is the visible evidence of a high-tension field, and has no effect on ignition performance. Usually it can be detected only in darkness. This discharge may repel dust particles, leaving a clear ring on the insulator just above the shell. This ring is sometimes mistakenly regarded as evidence that combustion gases have blown out between shell and insulator.

Spark plugs must operate within certain temperature limits if they are to provide the performance and service life expected. The spark plug selected for an engine is based on the normal service which the engine was designed.