Spark Plug: Description and Operation

Resistor type, tapered seat spark plugs are used. No gasket is used on these tapered seat plugs.

Normal service is assumed to be a mixture of idling, slow speed, and high-speed driving. Occasional intermittent high-speed driving is needed for good spark plug performance. It gives increased combustion heat, burning away carbon or oxides that have built up from frequent idling, or continual stop-and-go driving.

Spark plugs are protected by insulating boots made of special heat-resistant materials, which cover the spark plug terminal and extend downward over a portion of the plug insulator. These boots prevent flash-over, which causes engine misfiring. 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 with less than the recommended amount of service can sometimes be cleaned and regapped, then retumed to service. However, if there is any doubt about the serviceability of a plug, replace it. Plugs with cracked or broken insulators should always be replaced.