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Spark Plug Secondary Terminal Designs

Bulletin 1013

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All internal combustion engines that require spark ignition.


Spark plugs have different configurations of terminal nut designs. The terminal nut is the critical element of the spark plug that makes the connection to the ignition system secondary system. This bulletin will describe each of the most common designs.


The design of the terminal nut is directly related to the spark plug resistor cap, coil on plug, or spark plug wire on the engine application. The spark plug must share the same characteristic for it to make the critical secondary ignition system connection, improper mating between the two will result in a weak spark leading to an ignition system misfire condition. In the application guide spark plugs are available with different terminal nut configurations having the same part number but with different stock numbers, the only difference in the spark plug is the terminal nut configuration.


The most common design is the loose terminal nut design. This spark plug has a terminal nut that is removable. Notice the internal threads through the top of the nut. This design will let the spark plug act as a stud or solid terminal design.


The stud type is mostly used on powersports or motorcycle application. This spark plug does not come with the common secondary terminal nut. The previous loose terminal nut design will work in place of the stud type terminal. To interchange these spark plugs the installer will simply need to remove the nut from the loose terminal nut design spark plug.


The solid design terminal is a spark plug with a permanent secondary terminal. Again, the loose terminal nut design spark plug can take the place of this solid design by tightening a terminal nut onto the threaded portion of the spark plug allowing it to function as a solid type.


The last design is the newest of terminal nut configurations and is used primarily with European applications and is not interchangeable with other designs. Check the application guide for the correct part numbers.


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