Harley Davidson spark plugs service has changed with the M8. Harley M8 models have four spark plugs. Two per cylinder. This not only aids in cleaner emissions but in better performance. Two of the Harley Davidson spark plugs are located on the left side of the motorcycle where they have historically been located. These are easy to access.
The other two Harley Davidson spark plugs are located on the top of the motorcycle’s front and rear heads. They are located down in between the front and rear rocker boxes. To access these two spark plugs, it is necessary to remove the fuel tank.
To do so, begin by removing the motorcycle’s main fuse. Remove the dash trim and dash assembly. Use a soft towel to safely set the dash on the handlebars without unplugging.
Next, remove the front and rear fuel tank retaining hardware. Use a small block or shim under the rear of the fuel tank to aid in accessing the fuel line quick connect, a vent line, and electrical plug. Unplug the electrical plug and the vent line. Before unplugging the fuel line, it is helpful to spray the quick connect with lube. Lift up on the quick connect while pulling straight down on the fuel line to release the line. The tank will now be free to lift and remove from the motorcycle. Be sure to set the fuel tank in a safe, clean location and cover with a soft blanket or towel. Remove the spark plug wires from the Harley Davidson spark plugs. Be careful not to damage the spark plug wires. It is important to use a spark plug boot pliers especially for the center spark plugs.
Replace the spark plugs with genuine Harley Davidson spark plugs. Verify that the plugs are gapped correctly. If they are not, the gap must be corrected. Use a small amount of never seize on the threads of the new spark plugs. Thread in by hand, then torque to the factory spec of .031″ to .035″. Apply a small amount of dielectric grease into the spark plug boots and snap the plug wire onto the new spark plugs. Reassemble the fuel tank and dash assemblies in reverse and spark plug replacement is completed.