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Spark Plug Air Gap Adjustment

Bulletin 1004

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Spark plugs with single ground electrode


Spark plug gap is the measurement between the center electrode and the ground electrode. This specification is usually measured in millimeters or thousandths of an inch. As electricity travels between the gap a spark is created causing combustion of the air/fuel mixture. This setting is adjustable and though some spark plugs may claim to be pre-set, air gap adjustments may be necessary to meet vehicle manufacturers specification. Always check the vehicle specification from the vehicle manufacture for the air gap recommendation. Spark plug gap adjustment typically should not be greater than .008” in either direction to prevent damage to the ground electrode or alter the spark path between the electrodes.  In modified engine designs, as compression or combustion chamber pressure increases it may be necessary to decrease gap to ensure ignitability. Skilled technicians will read the spark plugs following a short run to determine if the correct spark plug heat range and gap adjustment have been made.


Gap is adjusted by moving the ground electrode closer or further away from the center electrode. It is very important, especially on fine wire electrode spark plugs to use the correct gap tool. Never use a coin or slide type gapping tool on fine wire spark plugs. To safely set the correct air gap always use a feeler gauge or wire style tool along with a slotted keyway to move the electrode by hooking the side of the ground electrode, this will ensure that the fine wire electrode does not become damaged. A damaged center or ground electrode from using the wrong type tool is not warrantable. Below are a couple of examples of acceptable wire style gapping tools.


Note: Multi-ground electrode spark plugs have non-adjustable gap settings.


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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