Spark Plug Gapping
STEP 1. Use the correct spark plug for your application. Pay particular attention to material type, heat range, and thread length.
STEP 2. Use the proper tools. The video shows which tool for adjusting the ground strap and a feeler gauge to measure the air gap. Do not use the wire loops. It is not recommended to tap the spark plug on a flat surface to lower the air gap. This could potentially damage the plug if the center electrode and ground strap make contact.
NOTE: Refer to your installation manual for the proper spark plug gap to be used. Although the spark plugs are pre-gapped from the factory to a certain standardized specification, your application may require a smaller gap for optimal performance and to prevent misfires.
STEP 3. Carefully bend the ground strap to open or close the air gap of your spark plug. Avoid contacting the center pin or galling or scratching the ground strap as this could lead to pre-ignition of the air/fuel mixture due to hot spots and premature wear.
STEP 4. Using the side of the feeler gauge, carefully measure the gap. Do not force the gauge between the ground strap and center pin. The gap is correct when you feel a slide drag and the next size up feeler gauge will not fit in the gap, generally 0.001" or .025mm larger than the desired gap.
STEP 5. Install your spark plug. Unless directed to do so via the factory repair manual, refrain from using pastes or sealants on the threads of the spark plug as this can weaken the ground connection between the plug and cylinder head. If at any time during the procedure, a spark is dropped, DO NOT USE IT, dispose of it immediately. The insulator could have a hairline fracture that is not visiable and could break off in the combustion chamber potentially causing internal engine damage.
Finally, when tightening the plug, use a torque wrench and tighten to the factory specification. Ideally, do this when the engine is at room temperature. Over-tightened plugs can damage the threads in the cylinder head and be a costly repair or require a new cylinder head. Under-tightened plugs will cause a poor electrical ground connection and can cause premature ignition coil failures and/or allow hot cylinder pressure gases to impede performance or melt the ignition coil.