HarleyMotorcyclePartsSpark Plug Replacement and Reading
Changing them is easy. Reading them tells you what the engine has been doing.
Plugs are a cheap service item, but two things about this job matter more than the plugs themselves: not dropping debris into the cylinder, and not over-tightening into aluminium heads. Both mistakes are far more expensive than the parts.
Tools
- Correct spark plug socket with a rubber insert
- Torque wrench
- Extension and swivel
- Feeler/gap gauge
- Compressed air or a brush
- Service manual
Before you undo anything
Work on a COLD engine. Aluminium heads expand, and removing a plug from a hot head is how threads get pulled. This is not a precaution you can skip because you are in a hurry.
Clean around the base of each plug before removal — compressed air or a brush. Anything sitting in that recess falls straight into the cylinder when the plug comes out.
Remove plug leads by the boot, not the wire. Pulling on the wire separates it from the terminal inside the boot and you will be buying leads as well.
If you are removing more than one lead, mark which goes where before you start.
Removal and reading
Loosen each plug a little, then blow out the recess again before removing fully.
Keep the plugs in cylinder order as they come out. Which cylinder a plug came from is the entire diagnostic value.
READ THEM. A light tan or grey firing end is a healthy engine. Dry black soot suggests a rich mixture or a lot of short trips. Oily and wet suggests oil reaching the chamber — rings or valve guides. White and blistered, or an eroded electrode, suggests running too hot or too lean. A single plug that differs from the other is the useful finding: that is a cylinder-specific problem, not a fuelling problem.
Fitting
Check the gap on the new plugs against the figure for your model, with a proper gauge. The gap specification is in your service manual. Plugs are not reliably "pre-gapped" to your engine.
Adjust the gap by moving the ground strap only, and never lever against the centre electrode or the insulator.
Start every plug BY HAND. If it does not turn freely by hand for several turns, stop and back it out. Cross-threading an aluminium head turns a twenty-minute job into a helicoil or a head off.
Torque to the manual figure. Over-tightening pulls the threads; under-tightening loses the heat path through the seat and the plug overheats. "Good and tight" is not a spec.
Refit leads firmly — you should feel the terminal click home. A lead pushed on but not seated causes a misfire that will send you chasing everything else.
Mistakes that cost money
- Working on a hot engine.
- Not cleaning the recess and dropping grit into the cylinder.
- Pulling leads off by the wire.
- Starting plugs with a ratchet instead of by hand.
- Guessing the torque.
- Throwing the old plugs away without looking at them.
When to stop
If a plug binds going in, remove it and check the thread. A plug forced into a damaged thread will seize, and extracting it is a head-off job.
Parts for this job
Check fitment for your own model and year before ordering — add your bike and we will flag what fits.
Other Harley how-to guides
This guide covers procedure and judgement. Torque figures, fluid capacities, gaps and tension settings are specific to your model and year and must come from the factory service manual for your motorcycle — we deliberately do not print numbers we cannot stand behind for your bike. Independent aftermarket retailer, not affiliated with Harley-Davidson, Inc.