Spark Plug Gap Adjustment and Exchanging Spark Plugs on Harley-Davidson Motorcycles
What’s the Job of a Spark Plug in a Harley-Davidson Motorcycle?
Harley-Davidson V2 and single cylinder engines burn a fuel-air-mixture. In order for the combustion to start each time at the right point, a spark is generated between the spark plug electrodes — @ 3,000 rpm of the crankshaft 25 times per second, e.g.
Due to this hard job, Harley-Davidson spark plugs will inevitably wear, and will have to be exchanged every now and then. Worn or defective spark plugs can be the cause of bad starting, rough and uneven engine run, and missed emission standards on emission controlled vehicles.
Spark Plug Basics
Spark plugs are sold in various versions. Anyway, the basic components are the same for all of them:
- a single center electrode that is connected to a terminal
- a ceramic insulator that is gas-tight placed in a
- threaded steel case that carries a hex profile
- one or more ground electrode(s) are welded to this case.
As for differences between spark plugs, here are the main ones:
- the center electrode and the top terminal may be connected by a resistor for radio suppression purposes
- the central electrode varies in terms of protusion into the combustion chamber
- the steel casing will vary in terms of hex size, thread diameter and length, and the seat area that provides seal on the cylinder head
Length of the center electrode and its ceramic insulator and the space between it and the steel case will influence heat absorption of the spark plug, the contact area to the cylinder head (thread, seat) will determine heat dissipation. This is why some people may speak of „hot“ and „cold“ spark plugs.
Harley-Davidson spark plug heat ranges go from #2 (hot) - #12 (cold).
Roughly speaking the more power an engine produces the colder the spark plugs should be. This is why a 1926 model J uses a pair of #2 spark plugs while high compression Buells and V-Rod models use #10-12 spark plugs.
How to Choose Spark Plugs
The spark plug you choose must first and before all meet the threads of your cylinder head. Look for:
- thread diameter (14, 12, or 10 mm on newer Harleys, 18 mm on mid-20es to mid-50es bikes, 7/8” on stock pre-1926 Harleys)
- thread length (varies from 3/8” to 3/4”)
- seat (flat with a crush washer or conical)
Too long a thread: the threads that project into the combustion chamber will anneal and be difficult to screw out again. The protruding section may be hit by a high domed piston.
Too short a thread: The spark will be created in a niche, combustion will be affected negatively, as a result engine may overheat. The threads of aluminum cylinder heads that are not covered by spark plug threads will anneal, carbon will deposit and by a pain in… to remove.
Next thing to look for is how far the center electrode protrudes. Farther protruding electrodes ignite the fuel-air-mixture better. If protruding too far, the piston may hit the spark plug.
Check and compare the spark plug heat range.
If you want to keep it simple: have a look into your rider’s handbook or the Harley-Davidson Service manual that covers your model. Look up the spark plug Harley-Davidson recommends.
I Want the Optimum Choice: Which Spark Plugs Are Best for My Harley-Davidson?
There aren’t perhaps as much debated subjects as the „right“ engine oil and the „right“ spark plug. Every now and then a new brand pops up that claims to have the ultimate product. But stay calm, lean back, and ask these people for authoritative experimental proof series: and you won’t hear from them again. Whatever your spark plugs, have they two, have they three or even four ground electrodes: there's only one spark every time the plug ignites.
There are lots of rumour about „the right brand“ of plugs and the „one and only true“ spark plug. Sorry to disenthrall all those myths. In the end of the day there’s one spark plug brand that can be recommended: the one that the engine manufacturer recommends and uses. In the case of US-designed Harley-Davidson motorcycles: the US-made Harley-Davidson spark plug.
For all these older bikes which the factory no longer provides genuine spark plugs for, Champion spark plugs have proven a reliable choice. So, Champion brand spark plugs is our choice where H-D plugs aren’t available.
When Should I Change Sparks Plugs on My Harley-Davidson Motorcycle?
As components that underlie wear spark plugs should be checked and exchanged regularly.
Minimum check intervalls according to Harley-Davidson:
- 2,000 – 2,500 mls Pre-Evolution models
- 5,000 mls Evolution Sportster and Big Twin models, Twin Cam models
Replace spark plugs if in doubt. The price of a spark plug is not worth to run risks. Replace spark plugs regardless of function every second check. Defective spark plugs may cause not only power reduction and bad starting, but damage to the cylinders and heads.
Prepare the Change
… and get prepare the necessary tools:
- Fitting socket and handle
- Torque wrench of adequate range
- Compressed air (canned or air gun)
- Brake cleaner
- Rag
- Feeler gauges
Make sure engine has cooled down sufficiently. Ignition switch must be OFF. Changing spark plugs requires the area aroung the plug casing to be clean. Use brake cleaner and compressed air and/or a rag.
How to Change Spark Plugs on Your Harley-Davidson Motorcycle
As said above, cylinder heads should have cooled down. There’s no thing as easily stripped as a hot spark plug thread.
Gently pull the spark plug cap and put the socket on the spark plug. Press firmly, as the insert in the socket should be snug on the spark plug insulator. This insert serves for two purposes: it protects the insulator while you spin the socket. And it holds the spark plug while you pull it from its seat or start to re-engage the threads for screwing the plug in. Do not tilt the socket as the insulator may brake.
Untighten the spark plug and unscrew it.
Inspect it keenly. If in doubt replace it.
Replace the spark plug. Take care to correctly place the spark plug threads on the receiving threads in the cylinder head. Most thread stripping damage starts at this point, so take your time.
When the spark plug is fully seated apply torque as per Harley-Davidson spark plug torque schedule:
- 14 mm thread in cast iron cylinder heads: 18 - 28 ft-lbs = 24 - 38 Nm.
- 14 mm thread in aluminum cylinder heads: 18 - 22 ft-lbs = 24 - 30 Nm
- 12 mm thread: 11 - 18 ft-lbs = 15 - 24 Nm
- 10 mm thread: 89 - 133 in-lbs = 10 - 15 Nm
On roadside emergency checks and repairs you may want to use a 13/16” box wrench instead of a socket. There’s nothing to be said against, but act with caution as you may easily break a spark plug insulator just with a touch of a box wrench. If it says ‚ding‘ it’s too late.
If no torque wrench is at hand tighten a spark plug the following way:
- a new spark plug with an unused sealing ring is tightened 90° = 1/4 turn,
- a used spark plug with a squeezed sealing ring is tightened only 45° = 1/8 turn.
Motorcycle Spark Plug Care and Maintenance
Clean the area around the spark plug hex regularly. You don’t want that goo top build up.
Lift of and inspect the spark plug cap at a regular base. Coat the rubber lip with silicone grease or similar to keep the boot splash-tight.
Inspect the unscrewed spark plug for soot and signs of overheating.
- Soot build up on the thread face and electrodes tell you that the air-fuel-ratio is badly on the rich side.
- A small blueish ring around the ground electrode bend is normal. A thoroughly blued ground electrode is a sign of overheating. Check ignition timing and/or air-fuel-mixture.
- Actual unleaded fuels make it difficult to read a spark plug. The insulator should show a even greyish coloring with a tiny brownish ring around the center electrode. Don’t believe those that want you to believe that medium brown is the only color an insulator should show.
Check the spark plug gap. To adjust the gap only bend the ground electrode slightly. Do not touch the center electrode. Harley-Davidson recommends gaps as follows:
- .020” for magneto ignitions
- .025 … .030” for battery coil ignitions and short thread spark plugs (Flatheads, Knuckleheads, Panheads, Iron Head Sportsters and Shovelheads →1978)
- .038-.043” all Big Twins 1979- (incl. Shovel, Evolution, Twin Cam) and Sportsters 1980→
- .060” for 1979 Sportsters only with N° 4 plugs
Take care not to drop a spark plug. Once dropped to ground it should be replaced. The insulator is made of ceramics, any heavy shock can make it crack unnotedly.
Brushing soot away with a metal brush is an option only for roadside repair. Use brass brushes if any. But be aware that even soft brass brushes may leave metal traces on the insulator that can make a spark rather follow them instead of jumping the electrode air gap.
Where to Get Spark Plugs for Harley-Davidson Motorbikes?
Spark plugs for air-cooled classic V-twin Harley-Davidson engines and vintage singles are listed under Ignition/Spark Plugs. OEM reference numbers, heat range numbers, and model applications, supplemented by thread size and hex socket informations.
Use this section to buy the right spark plugs for your model F and J, your Flathead or Knucklehead, your Panhead and Shovelhead, Evolution ode Twin Cam engine.
To find all the tools mentioned above for a smooth motorcycle spark plug change, take a look at our Workshop, Garage & Tools category. They might even inspire you to go the extra mile and perform additional maintenance work. Our Tech Tips could be just the friendly help you need.