How to change motorcycle spark plugs: step-by-step guide

To change motorcycle spark plugs, let the engine cool, clear the area around each plug, pull the plug cap straight off, unscrew the old plug with the right plug socket, check the gap on the new plug against your manual, thread it in by hand, and finish with a torque wrench. On most bikes it takes an hour or two, and the hard part is usually reaching the plugs, not the plug itself.

This post is the hands-on how-to. For help deciding when plugs are due and what a worn plug tells you about the engine, see our guide to motorcycle spark plugs.

When to change motorcycle spark plugs

Your owner's or service manual lists an interval for your engine. Follow it. Plug life depends on the plug type, the engine and how you ride, so do not rely on a number you read for a different bike.

Replace them sooner if you notice:

• Hard starting that is not explained by the battery
• A rough idle or misfire
• Poor fuel economy or noticeably weaker acceleration
• Black, wet or oily deposits, or a cracked insulator, when you pull a plug

Our motorcycle maintenance schedule shows where plugs fit among other routine jobs, and the valve adjustment guide is worth reading because both jobs often happen at the same service visit.

Tools and parts

Parts

• New spark plugs of the exact type your manual lists. The heat range matters. As Wikipedia's spark plug article explains, the heat range balances keeping the tip hot enough at idle to avoid fouling against keeping it cool enough under heavy load to avoid pre-ignition, and brands number their heat ranges differently, so numbers should not be treated as interchangeable. Use the part number in your manual or the plug maker's cross reference.
• Optional: a small amount of dielectric grease for the inside of the plug boot, only if your manual allows it
• Optional: new crush washers, if your plugs are the washer type and the washer is separate

Tools

• Spark plug socket of the right size, often with a rubber insert or magnet. Many bikes need a thin-wall or long-reach socket for deep, narrow plug wells.
• Ratchet and extension
• Torque wrench (required)
• Wire-style or feeler gauge for checking the gap
• Compressed air or a small brush, and a clean rag
• Sockets or hex keys to remove the tank, fairing or airbox if they are in the way
• Gloves and eye protection
• A flashlight

What needs the service manual

Item	Where to find it
Correct spark plug part number and heat range	Owner's manual or service manual
Plug gap	Service manual
Plug tightening torque, and whether it is different for new vs reused plugs	Service manual
Firing order and which cap goes on which cylinder	Service manual
Steps to remove the tank, fairing or coils	Service manual

Do not guess the torque (our guide on using a torque wrench on a motorcycle covers the basics). Spark plugs thread into aluminum cylinder heads on most bikes, so too much force can strip the threads, and too little can let the plug loosen or overheat.

Wikipedia's spark plug article also notes that tapered-seat plugs need less installation torque than washer-sealed plugs, and that the two designs should never be swapped between heads. Know which type you have before you tighten anything.

How to change motorcycle spark plugs: step by step

1. Let the engine cool

Work on a cold engine. Hot aluminum expands, and a plug removed or installed hot is easier to damage. If you want to read the old plugs, run the bike briefly first, but let it cool before you remove them.

2. Get access

Many bikes make you remove the seat, tank, side panels or airbox to reach the plugs. Disconnect the battery's negative terminal if you are removing the tank or working near wiring. Support or tie off the fuel tank safely, and never let fuel lines kink or pull off. If this step looks like a large project for your bike, you can still have a shop do the whole job.

3. Mark and label the plug caps

On a multi-cylinder bike, label each cap with tape so it goes back on the same cylinder. If the bike has coil-on-plug, take photos of the connectors.

4. Clean around each plug

Use compressed air or a brush to clear dirt from the plug well. Debris that drops into the cylinder when the plug comes out can damage the engine.

5. Remove the plug cap

Pull the cap straight up, twisting gently if it sticks. Never yank on the wire. A coil-on-plug cap may have a retaining bolt.

6. Loosen and remove the old plug

Fit the socket squarely over the plug and turn counterclockwise. Break it loose with a steady pull, not a jerk. If it will not move, stop. Do not force it, because a seized plug can pull threads out of the head.

Unscrew the last few turns by hand if the socket allows, or keep the socket straight on the plug the whole way out.

7. Look at the old plug

The tip and insulator tell you something about how the engine has been running. Wikipedia notes that a light brownish tip indicates proper operation, while a sandblasted look can suggest detonation, and heavy deposits or a porous, glazed look can suggest the heat range is wrong. For a useful read, plugs are best pulled after a run at steady load. Photograph them for your records. If anything looks off, see the overview of reading plugs and consider having a shop check the engine.

8. Check the new plug and its gap

Compare the new plug to the old one: same thread diameter, same reach, same seat type. Check the gap with a gauge against the number in your manual. If it needs adjusting on a conventional plug, bend only the ground electrode gently, never the center electrode. Fine-wire iridium and platinum plugs are generally not meant to be re-gapped, so if the gap is wrong, get the right plug rather than bending it.

9. Start the new plug by hand

Thread the plug in by hand until it seats. If it does not turn smoothly with fingers, back it out and try again. Forcing a crooked plug is the most common way to wreck a cylinder head thread. A short piece of rubber hose slipped over the plug's terminal can help you start it in a deep well, and you can feel it slip if it is crossing.

10. Torque the plug (torque wrench step)

Once the plug is finger-tight and the washer or taper has just touched the head, tighten with the torque wrench to the value in your service manual. If your manual gives a "turn past finger-tight" method instead, follow that. Do not use a long extension that wobbles, because it can throw off the reading.

11. Refit the cap and reassemble

Push each cap on until it clicks or seats fully, and make sure it goes on the right cylinder. Reinstall the tank, airbox and panels. Reconnect the battery.

12. Start the engine

Start the bike and let it idle. It may crank a little longer if you disturbed fuel lines. Listen for a smooth idle, check for fuel leaks and look at the plug area for any smoke. Take a short test ride and recheck that nothing has loosened.

Common mistakes

• Cross-threading the plug and damaging the head
• Skipping the torque wrench and overtightening
• Using the wrong heat range or the wrong reach
• Bending the center electrode or re-gapping a plug that should not be adjusted
• Mixing up plug caps on a multi-cylinder engine
• Letting dirt fall into the cylinder
• Pulling on the wire instead of the cap
• Forgetting to reconnect a sensor or connector after moving the tank

Cost: DIY or shop?

A DIY change costs the plugs and a good socket, which varies a lot by bike and by plug type (iridium plugs cost more than standard ones). A shop adds labor, and bikes with hard access can take longer. Our tune-up cost guide shows typical shop ranges. Varies by bike, region and shop.

When to see a mechanic

Book a shop visit if:

• The plug is seized, or a thread strips or pulls out with the plug
• A plug breaks and part of it falls into the cylinder
• The engine misfires, idles badly or smokes after you change the plugs
• You see signs the plugs are wearing abnormally, such as oil fouling or melted electrodes
• You do not have the manual's plug specs or torque values
• The tank, fairing or wiring is too complicated for you to reassemble with confidence

A shop can find the underlying cause, such as a fuel mixture issue, instead of just replacing parts. Browse our maintenance services page, or compare shops in the motorcycle repair directory.

Quick checklist

• Use the exact plug and gap from your manual.
• Work on a cold engine and clean the plug wells first.
• Label the plug caps.
• Start every plug by hand.
• Torque to spec.
• Test the bike and recheck for leaks and loose parts.