How to Gap a Spark Plug

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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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Gustavo

16 years of experience
75 reviews
Gustavo
16 years of experience
Ford Edge - Spark Plugs - San Antonio, Texas
So happy I found, Mr. Gustavo!! I have already recommended him to friends/family. He was prompt, nice and knowledgeable. Your Mechanic is the way to go!!
Toyota Corolla - Spark Plugs - New Braunfels, Texas
He was very friendly and did the work in a timely manner. He was informative about my car. I will definitely request him again.

Matthew

32 years of experience
734 reviews
Matthew
32 years of experience
Nissan Sentra - Spark Plug Replacement - Newport News, Virginia
This was my first time using the MyMechanic services. Mr. Lerman was early for my appointment and finished in a timely fashion. Very knowledgeable and professional. Glad there are still great mechanics out there that take time and explain things to you!!
Jeep Cherokee - Spark Plugs - Newport News, Virginia
Matt was outstanding . Highly recommendable and i was recommended to this site and specifically already told I’ll be lucky if I get Matt cause he’s great .. and I was and he is .

Horus

10 years of experience
122 reviews
Horus
10 years of experience
Jeep Commander - Spark Plugs - Universal City, Texas
Very experienced and friendly. Let me know exactly what he was doing. Showed me the parts that was replaced
Infiniti I30 - Spark Plugs - San Antonio, Texas
Horus was really helpful and friendly he provided me valuable information on my vehicle. Unfortunately with age of my vehicle I'm not sure what I may do next. But great insight was given.

Joshua

7 years of experience
6 reviews
Joshua
7 years of experience
Dodge Dart - Spark Plugs - Little Elm, Texas
Very friendly, courteous, and overall helpful. Finished the services in a timely manner under the estimated time given. Also he let me know if I had any other issues with the vehicle. Would definitely book again!

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