How to 'gap' a spark plug so your engine fires like a cannon

Wednesday, January 25, 2023
Maintaining the spark plug of a lawnmower is important because it plays a crucial role in the engine's combustion process. Ensuring the plug has the correct ‘gap’ is really important to ensure the engine can burn fuel efficiently.

"Gapping" a spark plug refers to adjusting the distance between the center electrode and the ground electrode, this distance is called the "gap".

The gap must be set to the manufacturer's recommended setting, which can be found in the owner's manual for say the lawnmower or chainsaw.

The gap setting is important because it affects how the spark plug fires.

If the gap is too wide, the spark may not be strong enough to ignite the fuel, resulting in poor engine performance or even engine failure. When the engine is running, the ignition coil sends a high voltage current to the spark plug through the spark plug wire. This current flows across the gap between the center electrode and the ground electrode of the spark plug, creating a spark. This spark ignites the fuel in the combustion chamber, which powers the engine.

If the gap is too narrow, the spark plug may become damaged from the high heat and pressure.

So, you need it to be just right, like Goldilocks’ porridge.

A spark plug gapping tool is used to adjust the gap of the spark plug.

This tool typically has a small ruler or gauge on one end and a small adjustment tool on the other.

The adjustment tool is used to bend the ground electrode to change the gap.

How to correctly gap a spark plug

To gap the spark plug, first, measure the gap using the gauge on the gapping tool by inserting it into the gap using the thin part of the tool. 

Turn it around until it stops - you can then check this point to measure the gap by using the mm gauge on the tool:

Then, use the adjustment tool to bend the ground electrode as necessary to achieve the correct gap as specified by the manufacturer:

Be gentle and do not overbend the ground electrode.

Finally, check the gap again to confirm that it is set to the correct level. You are now good to install the spark plug into your machinery.

Here’s a really excellent video guide to help you visualise what to do:


It's important to note that many spark plugs come pre-gapped and do not need to be adjusted.

If you've got a stuck plug, here's how to loosen it.
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