Electrical

Most Milwaukee-Eight Softails specify 0.038 to 0.043 inches (0.97 to 1.09 mm) for stock plugs — always verify against your service manual, as the Evo and Twin Cam specs differ. On fuel-injected M8s the tolerance matters: a gap even slightly wide will cause misfires under load because the injection system expects precise ignition timing.

A fouled plug produces a rough idle that usually clears when the engine warms up, and it pins the fault to one cylinder. A failing coil is intermittent and temperature-sensitive — often worse when hot, sometimes cutting out under load and then recovering. Change the plug first (cheap, quick); if the misfire shifts cylinder when you swap it or persists with a new plug, the coil is next.

Yes. SE plugs are typically spec'd tighter from the factory — around 0.030 to 0.032 inches in higher-compression applications. Running them at the wider stock NGK gap will cause premature gap erosion and reduce the performance gain. Always check the SE documentation for your specific plug number; for big-bore kits in particular, confirm the gap with the kit supplier.

The rear cylinder runs hotter because it sits in restricted airflow — the front cylinder gets first crack at incoming air. That extra heat produces slightly richer combustion residue on the rear plug even when fuelling is dialled in. A moderate colour difference between cylinders is normal; a notably dark or fuel-wet rear plug is worth investigating, but don't call it a fault just because they don't match.