When it comes to small gas engines, maintaining the spark plug is a really important process. If there isn’t a consistent spark in the combustion chamber, you’re going to run into all kinds of problems. When people talk about how to tell if a lawn mower spark plug is bad, they usually describe sooty or oil fouled plugs that have been used for quite a while. One thing that gets missed a lot of the time is making sure that your spark plug has the right gap between its center electrode and grounded electrode. Luckily it’s super easy to check and adjust your spark plug’s gap if you have the right tool on hand. I’m going to describe how to gap a lawn mower spark plug and try to answer some related questions that I’ve noticed people ask pretty often.
Are Lawn Mower Spark Plugs Pre-Gapped?
Tons of people wonder, are lawn mower spark plugs pre-gapped? And, do you have to gap lawn mower spark plugs even if they’re new? Well, yes and no. Most spark plugs are sold as “pre-gapped” but you shouldn’t take the manufacturer’s word for it.
If you take a look at the box your spark plug comes in, I can almost guarantee that it will say it has been set to a certain gap. However, you should always measure a new spark plug’s gap yourself before you install it.
In my experience, you can’t trust manufacturers to accurately pre-gap all of the spark plugs they sell. More often than not, brand new spark plugs have a gap that is not what the box says it should be. Since adjusting the gap on a spark plug is so easy, do yourself a favor and double-check the gap on any spark plugs you buy.
How to Tell If You Need to Gap Your Spark Plug
My advice is to check the gap on every spark plug, new or old, before putting one in your engine. Over time, spark plugs tend to wear out and their gap can be altered. And as I mentioned above, you can’t rely on manufacturers to perfectly gap all of their spark plugs. If you take a few minutes to measure your spark plug gap and adjust it accordingly, you’ll save a bunch of time by not having to remove it and gap it later.
You might also run into a situation where your current spark plug isn’t performing how it should be. This could be due to an incorrect gap. Some common signs that you’re having spark plug issues are:
- Engine misfiring
- Trouble starting
- Rough idle
- Running out of gas quickly
Knowing how to test a spark plug on a lawn mower, and how to gap a lawn mower spark plug definitely come in handy here. So, let’s get into the actual process of measuring and adjusting the gap on a spark plug.
What You’ll Need to Gap Your Lawn Mower Spark Plug (Equipment List)
Before figuring out how to remove your lawn mower’s spark plug, or going to buy a new one, make sure you have the necessary equipment to test and set the gap on the spark plug for your mower. Here’s what you’ll need:
- A feeler gauge or a spark plug gap tool (coin or wire style)
- Owner’s manual or mower’s model number
If you haven’t done this process before, you’ll need to invest in either a special spark plug gap tool, or a feeler gauge. Unfortunately, there is no other reliable method that I would recommend to measure a spark plug’s gap without one of these tools. But don’t worry, both of these tools are inexpensive and can be used for any spark plugs that you might encounter.
The last thing I should mention is that you’ll want to have a spark plug wrench and a pair of pliers in order to remove and/or install the spark plug itself.
How to Gap a Lawn Mower Spark Plug (Step By Step)
Measuring and setting the gap on a spark plug is pretty much a universal process for all spark plugs. Before I go into too much detail about how to gap a lawn mower’s spark plug, here’s an overview of the process:
- Find the recommended gap using your owner’s manual or mower’s model number.
- Measure the current gap on your spark plug.
- Adjust the spark plug’s gap to the recommended width.
Now that you’ve got a general idea of what setting the gap on a spark plug looks like, let’s talk about the details. Be sure to handle your spark plug with care as the tip of the plug where you’ll measure the gap is fragile.
1) Find the Recommended Spark Plug Gap
The best place to find out what the gap of your lawn mower’s spark plug should be is in your owner’s manual. You can also find out what type of spark plug to buy in the same section if you need a replacement.
The standard gap size for most lawn mower spark plugs is between 0.02 and 0.035 inches. Though my guess is that your manual will have you set your spark plug at about 0.03 inches, you should be sure to check. The gap on your spark plug needs to be set at a very specific width to function properly.
If you don’t have an owner’s manual for whatever reason, find your mower’s model number and use that instead. You can use this as a reference either online, or at your local small engine mechanic’s shop to pull up specific engine and spark plug specifications.
2) Measure Your Current Gap
Taking a measurement of your spark plug’s gap is done a little bit differently depending on what tool you have. I’ll cover how to measure a spark plug’s gap with the three most common types of gapping tools.
A) Feeler Gauge
Using a feeler gauge is my favorite method of gapping a spark plug. This is because they are the most gentle on the electrodes of a spark plug, and they can also be used to take other mechanical measurements. With that being said, a feeler gauge is probably the most expensive of the three tools.
A feeler gauge is made up of a bunch of pieces of wire or metal that are different thicknesses. Usually, they progress in thousandths of an inch. So, all you need to do is use the gauge that has the thickness you’re looking for and insert it into your spark plug’s gap to see if they’re the same width.
B) Wire Gap Tool
A wire gap tool is used basically the same way a feeler gauge is. The difference is that rather than having long “feelers”, this tool is circular (about the size of a silver dollar) with a variety of small, rectangular, metal wires around its edge. These wires also differ in thickness by a thousandth of an inch respectively.
A wire gap tool is usually meant just for spark plugs and is pretty easy to find. Again, choose your desired thickness, and compare it to the current gap on your spark plug.
C) Coin Gap Tool
Using a coin-style gap tool works just fine, but runs the risk of damaging the spark plug’s center electrode. Coin gap tools have a tapered edge that you insert into the spark plug’s gap at the thinnest part, and then turn until it wedges itself in between the two electrodes. If you aren’t careful when doing this, you may damage the center electrode or accidentally widen the spark plug’s gap.
Still, this tool will give you an accurate reading and is also inexpensive. When using this style gap tool, just be careful when measuring the gap and turn it slowly.
3) Adjust the Spark Plug’s Gap Accordingly
If you measure your spark plug’s gap and it isn’t exactly what the specifications in your owner’s manual say it should be, you need to adjust it. The three tools listed above will all have a slot or hole that you can use to gently bend the grounded electrode.
When setting your tool onto the curved electrode, make sure that you aren’t touching or applying pressure to the “puck” (center electrode) underneath it. Then, bend the grounded electrode either slightly wider, or slightly narrower. Only move it tiny amounts at a time as you could snap it off completely with too much force.
After bending the electrode a bit, take another measurement to see where you’re at. With some luck, it might only take one try to set the perfect gap. If the gap still isn’t what it should be, repeat the process by bending it a little further. Eventually, you should end up with a gap that is the exact width of your feeler gauge or spark plug gap tool.
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