Normal service is assumed to be a mixture of idling, slow speed, and highway driving. Occasional highway driving is necessary for good spark plug performance because of the increased combustion heat. This heat burns 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. Flash-over 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. Corona is visible evidence of a high-tension field, and has no effect on ignition performance. Usually corona can only be detected in darkness. Corona 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 the shell and the insulator.

Spark plugs must operate within certain temperature limits if they are to provide the performance and service expected. Expected normal engine service determines spark plug selection. Refer to Maintenance Items in Maintenance and Lubrication.

Do not mix different numbered spark plugs. The plugs are designed for specific applications. Replace spark plugs only with the recommended catalog number.

Some applications may use a different numbering method from that shown in the table. Some applications may only use the eight digit Service Parts number.