How to Gap a Motorcycle Spark Plug
Reviewed by CrankDex editorial
Gapping a spark plug just means checking and, if needed, adjusting the small space between the two electrodes so it matches your bike's specification. It's a quick check worth doing before any plug goes in.
The gap is the small space between a spark plug's center electrode and its ground electrode, the little hook-shaped arm bent over the tip. That gap has to fall within a specific range for the plug to spark reliably and for the engine's ignition system to work the way it was tuned to. Checking it takes a minute and a couple of inexpensive tools.
Why the gap matters
Too narrow a gap and the spark is small and weak, which can lead to a rougher idle or harder starting. Too wide a gap and the ignition system may struggle to reliably jump the spark across it, especially under load. Manufacturers specify a gap range for each engine and plug combination, so matching that number, not just installing whatever gap the plug arrived with, is what keeps ignition consistent.
Checking the gap
A wire gauge or feeler gauge is the standard tool. Slide the gauge for your specified thickness between the two electrodes; it should pass through with a light, even drag, not sliding through freely and not forcing its way in. If it's too loose or too tight, the gap needs adjusting before the plug goes into the engine.
Adjusting the gap
Adjustments are made by bending the ground electrode, the bent arm, never the center electrode. A dedicated gapping tool usually has a small notch or lever built for this. Work in small increments and recheck with the gauge after each adjustment rather than trying to nail it in one bend; overcorrecting and having to bend it back the other way stresses the electrode more than necessary.
Iridium plugs need a gentler touch
Because iridium electrodes are ground down thin to hold a fine point, they're more fragile than a standard copper plug's thicker electrode. Bend gently, use a tool designed for gapping rather than improvising with a flat screwdriver, and never press or pry against the center electrode itself, only the ground electrode. A cracked or bent center electrode ruins the plug.
Before you install it
Give the gap one final check right before threading the plug in, since gauges and pockets and handling can nudge it slightly after you've set it. Once it's confirmed, install the plug per your manual's torque spec and you're done.
Browse spark plug gapping tools on Amazon (we may earn a commission, at no extra cost to you), or look up your bike to confirm the exact plug and gap before you buy.
Frequently asked
Do all new spark plugs come pre-gapped correctly?
Many do, but not all, and shipping or handling can nudge a gap out of spec even on plugs that left the factory correct. It's good practice to check the gap against your bike's specification before installing any plug rather than assuming the box got it right.
Where do I find the correct gap for my bike?
Your owner's manual lists the specified gap, usually as a small range in millimeters or thousandths of an inch. It's sometimes also printed on a sticker under the seat or inside a panel. Don't guess a gap from a different bike or engine, since it varies by application.
Why do iridium plugs need extra care when gapping?
Iridium's whole advantage is a very fine, thin center electrode, and that thinness makes it more fragile than the thicker electrode on a standard copper plug. Aggressive bending, or using a tool that presses directly on the fine tip, can crack or deform it. Adjust iridium plugs gently and only at the ground electrode.
What tool do I need to gap a spark plug?
A wire-style gapping gauge or a feeler gauge, both inexpensive and widely available, along with a small gapping tool to bend the ground electrode. Many gapping tools combine the measuring gauge and the bending tool in one piece.
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