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Spark Plug Insulator Height- Shorty Spark Plugs

Bulletin 1014

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Engine applications that require short or special insulator height


This bulletin explains ISO and JIS as it relates to spark plug insulator height


ISO (International Organization for Standardization) and JIS (Japanese Industrial Standards) are both organizations aimed at improving quality and standardization of products. When discussing spark plugs, one standard concerns the measurement of the insulator ceramic height as measured from the top of the terminal nut to the sealing flange. In the case of spark plugs with a sealing washer, the washer is not included in this measurement. While these standardized heights can vary based on individual engine applications, they are critical in OE designs such as a COP or coil-on-plug engine configuration.  All spark plug manufactures must make the spark plug the same height or the ignition coil will not make a secure secondary connection.


There are many discussions concerning shorty or “header plugs” in high performance engine application markets.  Most spark plug manufactures manufacture spark plugs to OE specifications in either ISO or JIS standards only. A shorty or header clearance spark plug is only required for a high performance or modified engine application where clearance is an issue between the spark plug wire boot and exhaust header pipe. In some instances, when extra clearance is necessary, a spark plug with the same characteristics but in a shorter insulator height can be substituted to achieve the needed header to spark plug wire boot clearance. Using a compact height design spark plug is also an option, however, it requires a special resistor cap to accommodate for the spark plugs extremely short insulator which lacks the needed resistance value.


For addition support contact the GoTech technical services team at 855-207-5630 M-F 7:30-6:00 CST for assistance.

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