Informational Guide

How To Gap Spark Plugs

Learn how to gap spark plugs correctly with our complete guide.

It isn’t difficult to replace your spark plugs during a routine tune-up. However, you must know how to gap spark plugs to perform the job correctly.

Our guide will provide step-by-step instructions to help you learn how to set spark plug gap the right way.

spark plug parts

The spark plug gap is located between the center electrode tip and the ground electrode. Power from the spark plug travels to the center electrode tip and jumps the gap to the ground electrode.

This process helps to ignite the fuel. This gap must be precise based on what your vehicle requires. If it is too large, small, narrow or wide, you will notice performance issues or may have trouble starting the car at all.

Symptoms of Incorrect Gapped Spark Plugs

Rough Engine Idle

You will need to learn how to use a spark plug gap tool if your engine begins running rough during idling. An incorrect gap messes with the air and fuel mixture, causing trouble during combustion, which is most often noticed during idling.

Engine Hesitation

If your vehicle’s engine begins to stumble or hesitate while accelerating, it can also be a sign of an incorrect spark plug gap. When the plug is gapped too wide, the spark loses strength as it bridges the two electrodes resulting in hesitation.

Engine Missing

You might also need to learn how to adjust a spark plug gap when the engine begins to miss or otherwise run erratically. Incorrect gaping leads to uneven firing of the spark plugs, which can delay engine combustion. One or several spark plugs could be to blame.

Poor Engine Performance

In general, if you are noticing any poor performance of the vehicle, it could be the result of an incorrect spark plug gap. When the gap isn’t measured perfectly, you alter the engine combustion and weaken the spark. This process must be precise for the engine to run smoothly.

Engine Knocking

Otherwise known as engine pinging, engine knocking is a sign that the spark plugs aren’t gapped correctly. The knocking sound comes from either incomplete or delayed combustion, most noticeable during acceleration. However, knocking can also occur from using bad fuel.

Spark Plug Gap Setting

Before you learn how to properly gap spark plugs, you need to know what the gap should be. This measurement is going to be different for every vehicle, based on the engine specifications. For the best result, it’s always best to consult with your owner’s manual or a spark plug catalog.

The spark plug gap must be perfectly calibrated between the center and ground electrode to work. When you adjust this distance, you are gapping the spark plugs. With the aid of a spark plug gap gauge, you can make these adjustments with ease. If you create too large of a gap, you will need to learn how to close the spark plug gap to get the recommended measurement.

How to Gap Your Car’s Spark Plug: The Mechanic’s Advice

Follow these basic steps when figuring out how to gap your spark plugs.

  1. 1
    Use a spark plug gap gauge to get precise measurements. Some people will suggest how to gap spark plugs without a tool, but this doesn’t make sense considering how cheap the gauge is.
  2. 2
    Clean the spark plug. This isn’t a step you need to do with new plugs, only if you are removing one to check it.
  3. 3
    Measure the spark plug gap with your gauge. Once you know how to check the spark plug gap, take note of this measurement and compare it with the factory specs.
  4. 4
    Make any necessary adjustments. Once you know the correct measurement, select it on your gauge. If you can't get the tool through the gap, you must widen it. If your tool runs through without touching the spark plug electrodes, it's time to narrow the gap because it is too large. You can use the tool to widen the gap by gently bending the ground electrode in or out as needed. Just be careful, as this part wasn't meant to withstand a lot of force.
  5. 5
    Check the gap one more time. Now that you know how to measure spark plug gap, it’s time to double-check your work before installing the spark plugs.

Some new spark plugs come pre-gapped, but you should always check the measurements. Additionally, if you can easily get to the spark plugs, replacing them is a simple DIY project.

However, some vehicles make the process more difficult, making a spark plug change better suited for a professional mechanic.

brand new sparkplug

Gapping a Lawn Mower’s Spark Plug the Right Way

Learning how to gap a spark plug for lawn mower models follows the same process as working with a vehicle. In fact, many lawnmowers actually use spark plugs that are also used in passenger vehicles.

Check the owner’s manual or reference the information from the manufacturer to find out what the gap should be. You might also find the instructions for gapping a spark plug in your owner’s manual. Generally speaking, you want to change your lawnmower spark plug every spring before the season begins.

Here is a helpful video further illustrating the steps to gap a lawnmower spark plug.

People Also Ask (FAQs)

What is the proper spark plug gap?

Each vehicle requires a different gap setting, specific to the engine. Typically, the spark plug gap measurement falls between 0.028" and 0.060". You can find the information in your owner's manual or through the spark plug manufacturer.

Do spark plug gaps increase over time?

As you use your spark plugs, the electrodes are subject to constant arcing, which eventually wears away at the metal. This condition can increase the gap over time.

Can spark plug gaps backfire?

Yes. Any ignition system problem can lead to backfiring conditions, including a wide spark plug gap. Additionally, carbon buildup on the electrode can prevent proper combustion.

Does spark plug gap affect gas mileage?

As the spark plug gap becomes wider, more voltage is required to create an arch. The smoother this combustion process is, the less demand there is for engine power, which conserves fuel. When the gap isn’t correct, there is a higher need for fuel.

Does side gapping spark plugs work?

Cutting the ground electrode back exposes more of the spark to the fuel mixture. When you minimize the surface area with a shorter ground electrode, more heat can flow into the combustion chamber, resulting in less voltage needed to create a spark.


If you want the best spark plugs for horsepower, you need to make sure the gap is correct. Otherwise, you could run into engine performance issues. Our guide taught you how to gap iridium spark plugs for a well-optimized engine.