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How to Gap Your Car's Spark Plugs

Mia Bevacqua
December 18, 2018

To ignite the air-fuel mixture that makes an engine run, spark plugs have a gap that voltage jumps across. For the engine to run perfectly, the gap — the distance between the plug’s two electrodes — must be set just right.

When spark plugs are replaced, the gap is checked and altered if necessary. If the gap is too large, the plug may not fire at all. If it’s too small, there may not be enough of a spark to ignite the air-fuel mixture. Either scenario can cause a misfire that can damage your car’s inner workings.

Do all spark plugs need to be gapped?

A couple decades ago, nearly all plugs were the copper core variety and had to be gapped. But things are different now; many vehicles come from the factory with platinum or iridium plugs, or those with a dual-electrode design. These plugs are often pregapped, and no adjustment may be needed.

Check your owner’s manual to find the correct type of plug for your vehicle. Then check out the product description to see if gapping is recommended.

Regardless of whether a plug comes pregapped, the gap should be checked before the spark plug in installed. This is necessary to ensure the gap didn’t change during handling.  

Not ready to do it yourself?

How to gap a spark plug

If you need to check that a spark plug gap is set right, here’s what to do:

1. Select a gapping tool: There are two different designs, “coin-style” and wire. The coin type has a ramp of increasing thickness with corresponding measurement marks. The other style has various gauge wires sticking out of the side, which are sized to measure gap.

2. Determine the correct gap for your vehicle: Check the owner’s manual. Alternatively, your local auto parts store can look it up for you.

3. Measure the gap: Select the proper gap size on your measurement tool. Then, pass the tool between the plug’s electrodes. It should go through with a slight drag. If it passes through without touching, the gap is too wide. If it can’t go through at all, the gap is too tight.

4. Adjust the gap: Wire-type gapping tools have a built-in adjustment prong, which fits around the plug’s top electrode. Once in place, the tool is moved up or down to adjust the gap.

If you’re using a coin-style gapping tool, the gap can be decreased by gently pressing the top electrode against a forgiving surface, such as a wooden tabletop. To increase the measurement, slide the electrode through the hole in the center of the tool, then gently pull up on it to increase the gap.

5. Recheck the gap: After you’ve made the necessary adjustments, recheck the gap. Make any additional changes as needed.

Mia Bevacqua

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.

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