Are Lawn Mower Spark Plugs Pre-Gapped?

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The spark plug gap determines the heat range that the spark will generate to ignite the fuel. The gap has to be a specific length, otherwise the mower might not work. If you are going to install a new spark plug, this post will show you how it is done. As you will see, the process is really simple.

Majority of lawn mower spark plugs are pre-gapped, but it is a good idea to check it yourself. The spark plug gap for mowers should be 0.02 to 0.30 inches. You can use a feeler gauge to measure this.

Is Your Lawn Mower Spark Plug Gapped?

Spark plugs play an important role in lawn mowers. Just like solenoids, you have to make certain it is installed properly. Most lawn mower manufacturers’ pre-gap the spark plugs and you can check the owner’s manual to confirm this.

Even if the brand is reputable, you should inspect it. And if you need to install a new plug you will have to set the gap properly. If you bought an old mower there is the chance its spark plugs are not gapped. If the engine is not starting or consumes fuel rapidly, there could a spark plug problem.

Even if the manufacturer states the spark plug is gapped, situations might cause it to misalign. Assuming that the plugs are properly installed, it is still a good idea to know how to measure the gap for future spark plug installations. The following steps show you how to do this. It works with the Craftsman M105 140cc and other mowers.

How to Gap Lawn Mower Spark Plugs Step by Step

Whether you want to double check the gap on your new mower or installing a spark plug, the following instructions will work.


Step 1

Look in your lawn mower manual for the suggested gap. As stated earlier this is usually 0.02 to 0.30 inches. Do not make assumptions though. It is best to check the manual to be sure. Depending on the lawn mower, the gap may be different.

Aside from the manual, you can find the spark plug gap online. Go to the manufacturer website and look for your lawn mower make and model. You will find either an online manual or technical specifications including the spark plug gap.

Step 2

Once you have the exact gap, it is time to measure it. There are three ways to measure it, with a feeler gauge, wire gap or coin gap.

Feeler gauge. A feeler gauge is comprised of wires that vary in thickness. Pick the one that matches the recommended plug gap. If the gauge fits right in, the space is correct. The feeler gauge is the best tool for checking the gap as it will not damage the electrodes.

Coin gap. To use a coin gap, you place its edge in the gap. Keep running it until it is between the electrodes. This is the cheapest method but also the riskiest. The danger is you cold damage the electrodes or make the gap bigger. These days, coin gap tools are only used on older lawnmowers.

Wire gap. A wire gap tool is a coin-shaped utility with several metal wires on its edges. These wires are in varying thickness and used just like a feeler gauge. If your tool cannot go between the electrodes, the gap is too small. If the tool gets in the gap but does not touch the electrode, the gap is too big. If it fits well, the gap is right.

If the measurement is correct, you are done. If the gap is too small, proceed to step 3.

Step 3

If the gap is too small or too large, use any of the tools above to adjust it. All of them have openings which can be utilized to bend the electrode.

Use your preferred tool to hold the spark plug. The electrodes should face down towards the ground.

  • If you want to increase the gap, bend the electrode outwards
  • If you have to narrow the gap, bend it inwards

Do not bend more than 0.02 inches at a time. Spark plugs are delicate and bending too much can damage it.

Once you have made an incremental adjustment, check the measurement again. If the gap is still too narrow or large, adjust a little more. Keep doing this until the gap is correct. The key is to be patient. If you break the electrodes you have to buy a new one and start over.

Why Do Spark Plugs Have Gaps?

The spark plug is the device that generates a charge in a lawn mower engine. This charge ignites the fuel and air mix and activates the engine. The gap refers to the space between the ground and center electrodes. This gap has to be at a precise rang for the spark plug to work.

While most lawn mowers today are shipped with pre-gapped spark plugs, problems can crop up. During shipping, the lawn mower might be accidentally dropped and cause the gap to widen or narrow. Mishandling or misusing a mower can also lead to faulty gaps.

The environment can also be a factor. If it is too hot the electrodes might deteriorate. Abrupt movements or jolts might cause a gap misalignment too. You can tell the gap has increased as the anode gets narrow. You can find the anode within the cylinder. While you can use a utility to reset the gap, it will wear out at some point and must be replaced.

The gap is one of the reasons why an engine will stop running. However not everyone is aware of the role the gap plays, or that there is even a gap. By being knowledgeable about this you can prevent problems with your lawn mower in the future.

Why Spark Plug Gaps Must be Exact

If you are wondering why lawn mower spark plug gaps range from 0.02 to 0.35, it is because the voltage and amps vary. If the gap is not correct, the cylinder is going to malfunction and cause startup failure.

If the gap is too small, the sparkplug will not have the room needed to ignite the fuel and air mixture. The most obvious sign is the voluminous emissions and poor performance.

If the gap is too wide, the spark generated in the center electrode will not reach the ground electrode. You might get the engine to run, but it is going to exert more effort to ignite the fuel. The harder the engine works, the faster it will wear out.

The type of spark plug used and the environment also affects performance. The heavier the workload of the mower, the more pressure there is for the spark plug and other components. Using the wrong fuel type also drags down performance. The greater the load, the higher the voltage is needed.

If the engine is misfiring, chances are the gap is incorrect. There are many reasons why a lawn mower engine will not start, but checking the spark plug gap is a good place to start.

As to which is worse, a wide or narrow gap, both are bad. But in some ways, a wider gap is going to cause more damage. If it is too narrow, the motor probably won’t start at all. But if the gap is too big, the engine will still run.

The problem is it is going to tax the components. Unless you know the gap is incorrect, you could end up using the lawnmower or long periods with a large gap. Over the long term this will damage the engine, which is going to cost more than a simple sparkplug gap fix.

One more thing needs to be pointed out. If the spark plug is worn out or damage, the engine will misfire. It does not matter if the gap is correct or not. If the plug is worn out or broken, it must be replaced with a new one.