Harley Davidson Spark Plugs
This shelf covers ignition parts for every Harley engine we stock, and most of it isn't plugs at all. Fourteen of the fifty-two listings are spark plugs; the rest are Screamin' Eagle wire kits, individual OEM leads and a couple of decorative covers. Which plug and which lead you need depends on the engine generation under the tank, not just the model name, and several of the older leads here are sold one cylinder at a time, front or rear, not as a pair.
Of the fifty-two ignition parts on this shelf, only fourteen are actually spark plugs. Twenty-five more are Screamin' Eagle 10mm Phat wire kits, sold by model and year, not as one generic set, and eleven are older-style plug cables sold singly, one per cylinder. That last point catches people out. A Harley twin needs a lead for the front cylinder and a lead for the rear. Several of these are titled "front cylinder" or "rear cylinder" on their own, and buying one when you needed the pair means a second trip.
Plugs and leads are matched to the engine, not just the model
Which plug or lead your bike takes changes with the engine generation under the tank, not only with the model name on it. One Phat wire kit here, 31964-89C, fits a '91-to-'98 Dyna, an '85-to-'99 Softail, or an '80-to-'84 Touring bike. Another, 31937-99C, fits only a '99-to-'17 Dyna. Same family, same "Dyna" word in both titles, and the two Dyna windows don't overlap at all. Plugs follow the same pattern, though each one needs checking on its own: Harley-Davidson's own fitment page puts 32337-04 on '48-to-'74 FLH and FX models, a Big Twin plug, not a Sportster one, despite the year range on our own listing. 32336-04 spans '54-to-'56 KH, '57-to-'70 XL-900, '71-to-'85 Iron Sportster and '48-to-'74 FX and FLH models, and 32338-04 fits '71-to-'85 Sportster models — both running later than our own titles show. Riders report the split continuing into the current Milwaukee-Eight engine, commonly said to take a different plug again from the older Twin Cam. We won't guess which one fits yours off a forum post. Match the listing to your exact year, or ask us to check it.
Heat range and gap are the most argued-over numbers on any spark plug thread, and long-time owners mostly agree on one thing: leave them alone unless you're running a genuinely modified or racing engine. We won't quote a specific gap or torque figure here: every one we found online was someone's own note of what their manual said, not the manual itself. Your handbook has the right number, and we can talk you through fitting it.
Buying leads individually? Check whether it's one cylinder or two
Most of what's listed here as a cable or lead is one cylinder's worth, not a pair. 31981-99C is stamped front cylinder, 31960-99A is stamped rear, and the same split runs through 31600008A (front) and 31600009 (rear), 31985-91A (front) and 31983-71C (rear). A V-twin needs both. If you're replacing leads individually instead of buying a Screamin' Eagle kit, check which cylinder the listing is for before you order, and be ready to order the other one too.
The job gets harder as the engine gets newer
Owners describe an older Evolution engine as a two-plug job, done in a few minutes with everything in easy reach. A Milwaukee-Eight is a longer afternoon: Harley-Davidson's own guidance says reaching the plugs may mean removing the seat, the fuel tank or other components first. Plugs binding fast or stripping the thread in the aluminium cylinder head comes up on the forums too, repeatedly, across more than a decade of owners' own reports. Good reason to have it done properly instead of in a hurry. Our workshop in Leeds can fit whichever plugs and leads your bike needs, if you'd rather book it in.
How to tell a lead has failed, and how often to change a plug
Harley-Davidson doesn't set a fixed replacement interval for the leads themselves. There isn't one. So the real question is how you'd know they'd failed, and their own guidance is specific: if the engine misses or runs roughly in wet weather, the insulation on the lead could be cracked or damaged, letting damp through to the spark. Most Harley riders in this country will recognise that symptom from a wet commute. Worth having it checked. Not ignored. Fitting a replacement is also the moment to add a little dielectric grease inside the boot, which is Harley-Davidson's own advice, to help keep moisture out and make it easier to remove again at the next service. Spark plugs do have a stated interval — Harley-Davidson recommends replacing them in Milwaukee-Eight engines every two years or 30,000 miles, whichever comes first. Older engines are on their own schedule. Check your handbook, or bring it in and we'll tell you where you stand.
The Phat wire kits here are a factory Harley-Davidson option, sold through dealers alongside the standard wires, not an aftermarket performance part. For Stage kits and engine tuning, that's a job for our Screamin' Eagle range. This page is service parts, not tuning. Every plug, lead and cover on this shelf is a genuine Harley-Davidson part, for sale with 10% off RRP and free UK delivery available, and the rest of our Harley-Davidson parts range is next door.