Harley-Davidson motorcycle engines are all based on the V-twin design. As of 2010, most factory Harley engines have one spark plug per cylinder and two cylinders per engine in a V-twin configuration.
1. Find and disconnect the battery, following instructions in the Harley-Davidson service manual. Disconnect the positive side first, then the negative side.
2. Locate the spark plugs on both cylinders. Remove the spark plug wires from the spark plug ends by pulling gently and evenly on the hard cable end until the wires are removed. Move the wires aside.
3. Spark plug tool
Use the spark plug tool or a ratchet with a spark plug socket and remove the plugs in a counter-clockwise direction. Set the spark plug aside carefully when removed.
4. Inspect each electrode area of the spark plugs, or the area that was inside the engine, for damage. Replace them if necessary.
5. Check the gap between the electrodes by sliding the appropriate size wire from the spark plug gap gauge, between the two electrodes on the plug. The wire should feel like it has a slight bit of resistance when pulling it through the gap. If it is too loose, use the spark plug tool to bend the outer electrode slightly inward. If the gap is to narrow, bend it slightly outward.
6. Torque wrench
Apply a small amount of the engine oil to the threads on the spark plugs and tighten them back into the engine heads, in a clockwise direction, finger tight. Then use a torque wrench to tighten the plugs to 11 to 18 foot-pounds.
7. Reattach the spark plug wires and the battery cables–attaching the ground, or black, cable and then the power, or red, cable.