How to Gap a Spark Plug

The spark plug is used to ignite the air/fuel mixture inside the engine. The spark plug must be in good condition and must be properly gapped for proper engine performance.

spark plug gaps

The size of the gap affects the spark plug's firing temperature which in turn is related to the combustion of air and fuel in the engine.

A gap that is too small will cause a low firing voltage resulting in incomplete combustion. On the other hand, if the gap is too large, the supplied voltage may not be sufficient to bridge the gap in the required time.

measuring spark plug gap

For this reason, it is essential to check the size of the spark plug gap during spark plug replacement and when trying to diagnose an ignition misfire.

You can learn to properly gap spark plugs by measuring and adjusting accordingly.

Method 1 of 1: Gap the spark plug

Materials Needed

spark plug gap specifications

Step 1: Find the gap specifications for your vehicle. You can find the spark plug gap specifications at several locations on your vehicle.

You can usually find the information on the emissions decal located on the underside of the vehicle hood.

If the information is not there, you can consult the factory repair manual for your vehicle.

  • Tip: Your local auto parts store should be able to provide you with the information. All you have to do is call them with the year, make, model and engine size of your vehicle.

cleaning spark plug with a wire brush

Step 2: Clean the spark plug. If you are installing new spark plugs, you do not need to do any cleaning.

However, you’ll want to clean plugs that have been in use for some time. This will ensure you get a an accurate reading when setting the gap.

You can clean the plug contact points with either 90% alcohol or a fine metal brush. Of course, if the plugs are fouled or exceptionally dirty, they should be replaced instead of cleaned.

hand measuring spark plug gap

Step 3: Measure the spark plug gap. The next step is to measure the spark plug gap. Do this by inserting the gauge between the center wire and side terminal or ground electrode.

spark plug diagram

If the tool does not touch the electrodes at the appropriate measurement, the gap is too large. Likewise, if the tool doesn’t fit between the electrodes the gap is too small.

hand adjusting plug with dedicated tool

Step 4: Adjust the spark plug gap. You can adjust the spark plug gap if it was not set correctly.

To increase the gap, gently bend the ground electrode towards the center electrode. To do this, use either a pair of needle nose pliers or a dedicated spark plug gapping tool.

  • Tip: To decrease the spark plug gap, you can use the tools mentioned above, or you can push the ground electrode against a soft surface, such as a wooden table.

  • Note: Be careful when bending the ground electrode and never adjust it more than .02 in. at a time.

Step 5: Re-measure the gap. Measure the gap again and readjust the distance until the desired gap is achieved.

Spark plug replacement and gap measurement can be tedious. If you’d prefer to have a professional inspect and/or replace your spark plugs, the team at YourMechanic is here to help.


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William

34 years of experience
303 reviews
William
34 years of experience
Cadillac STS - Spark Plugs - Phoenix, Arizona
He came to the first appointment explained what was happening he was able fix and get me a quote for the issue I had and scheduled me quick for the other issue I had was able to explain with my certain items were not needed even though online portal stated was needed. Only paid for the services required to fix the issue. He kept in contact with me to advise on the repair status and the final result. Nice to have the mechanic come to you. Would recommend William very courteous.
Chevrolet Cobalt - Spark Plugs - Phoenix, Arizona
Very nice. Experienced with cars that are from other parts of the country, so he knew how to deal with our car from back East and issues that are common to that area. Was very willing to explain things! Worked quick and hard to get the job done!

Stephen

32 years of experience
99 reviews
Stephen
32 years of experience
Chevrolet Traverse - Spark Plugs - Denver, Colorado
He’s awesome! Went over everything, made sure I understood, & let me ask as many questions as I needed. Definitely a great mechanic from YourMechanic
Toyota RAV4 - Spark Plugs - Denver, Colorado
This was the second time I used Stephen's services and again I am totally satisfied. He is prompt and does excellent work. I definitely recommend him to anyone and will continue to use his service.

Paul

19 years of experience
247 reviews
Paul
19 years of experience
Ford Focus - Spark Plug Replacement - Tampa, Florida
Paul was timely, professional, and honest. Got the job done quickly and went above and beyond. I would highly recommend him to anyone.

Greg

22 years of experience
47 reviews
Greg
22 years of experience
Mazda CX-9 - Spark Plugs - Apache Junction, Arizona
Greg was very professional and was not in a rush. He was very knowledgeable and informed me about what was being done. When I was asking questions he gladly answered them. I will be booking him for future appointments.

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