lawn mower spark plug size

Lawn Mower Spark Plug Size: What Size Do You Need For Your Mower?

Spark plugs are a small component of your lawn but they are one of the most crucial. These parts are there to ignite a spark so that the air and fuel mixture can burn causing the lawn mower to start running.

From time to time, your spark plugs will need to be changed and it’s important to make sure that you use the right size.

If you want to make sure that your lawn mower runs smoothly and without any issues, you will need to ensure the correct lawn mower spark plug size. But how do you know which to use and what happens if you use the wrong size? In this guide, we’ll tell you everything you need to know.

When it comes to changing your lawn mower spark plug, you have to remember that there’s no such thing as one size fits all. However, there are a limited number of spark plug sizes so your lawn mower is bound to use one of these.

In order to understand the different sizes of lawn mower spark plug, it can help to know how these sizes are measured. When measuring a spark plug, it is the thread that is taken into account. This includes the length of the thread and the thread size.

But this isn’t the only difference between spark plugs as the heat rating will also vary between spark plugs. This basically means that each spark plug is designed to be able to work at different temperatures.

Once you have a greater understanding about lawn mower spark plug sizes, you will be able to choose one much more easily.

Thread Size

When it comes to the thread size on a lawn mower spark plug, there are two things that are taken into account. The first is the thread pitch and the second is the thread diameter.

The thread pitch tells you the distance between the threads and since these measurements are so small, they’re typically given in millimetres.

The thread diameter tells you the width of the threaded part of the spark plug.

spak plug thread

Thread Length/Reach

The next thing that you need to understand when it comes to spark plug size is the length or reach of the thread. This tells you how long the spark plug is as well as how far it’ll sit within the engine.

The choice of thread lengths is a lot more varied than the choice of thread pitch or diameters and it is critical that you pay close attention to what will be compatible with your lawn mower.

The reason for this is that the spark plug may gather gas and oil at the tip if it isn’t far enough into the engine. On the other hand, if it sits too far into the engine then you may have problems with things like oil or fuel accumulating on the thread.

Spark Plug Gap

Spark plugs have two electrodes; one on the side which is a ground electrode and a second on in the middle which is the live electrode.

It is at the live electrode that the spark is created and this then travels to the ground electrode via a gap that separates the two.

This gap can be altered and many times, you may need to do this if you are having problems getting your lawn mower engine to start. You will find information on the correct spark plug gap in your user manual or online so always make sure to check this if you’re having starting issues.

The problem is that, while the manufacturer will have set this gap correctly, it can shift during shipping so may not be right. When you install a spark plug, take the time to check the gap to avoid problems down the line.

You might be wondering how the gap can make all that much difference. But it’s important to note that it must be set to the specified width. If it is too wide then the spark created at the live electrode is going to have trouble moving to the ground one and the engine won’t ignite. On the flip side, it is possible for the gap to be too small which would mean that there isn’t enough space to create a spark, again giving you starting issues.

The best way to measure the spark plug gap is by using a feeler gauge. This has a selection of stripes, all of varying thicknesses and you can use these as a guide once you know the correct width for your lawn mower spark plug gap.

Heat Rating

When it comes to lawn mower engines, temperature really matters as each engine will run at a different temperature. What this means is that you need to use a spark plug that’s compatible with this as different ones are designed to work at different temperatures.

When you buy a spark plug, it will have a heat rating between 2 and 12. Lower numbers indicate that the spark plug heats up a lot more easily so has the tendency to overheat more quickly.

Lawn Mower Spark Plug Sizes Chart

lawn mower spark plugs

In order to choose the right spark plug for your lawn mower, we have put together this simple table with all of the relevant measurements.

Thread Measurement Pitch Diameter Reach
Riding mower 1mm - 1.5mm 10mm - 12mm 9mm - 12mm
Push mower 1mm - 1.5mm 12mm - 14mm 15mm - 19mm

How To Find Your Lawn Mower Spark Plug Size

Now that you understand the importance of using the right sized lawn mower spark plug and what the different measurements mean, you’re probably wondering how you find the correct size for your machine.

Even with all the information on the standard sizes, it can feel confusing but there’s no need for it to be. The most important thing you can do is to take a look through your user manual as this will provide you with all the information you need and will tell you exactly what size spark plug to choose.

If you no longer have your lawn mower user manual then there are a lot of digital user guides online which are really handy to download. You’ll refer to this time and again so it’s always good to take the time to download a copy.

Failing that, you can take a look at the part number of the existing spark plug and find another one that matches this. Or if you cannot see that, you can simply take the old one to the store with you and have someone help you find what you are looking for.

What Happens If You Use The Wrong Spark Plug In A Lawn Mower?

It’s really important to make sure that you use the right spark plug in your lawn mower. Not doing so is going to affect how the lawn mower performs and you may end up damaging your machine or causing problems that are much trickier to fix than just replacing a spark plug.

You see each lawn mower is designed to work with a spark plug of a specific size and heat rating. If you get the wrong one, the first and most obvious thing that might happen is that you simply cannot screw it in because it won’t fit.

You might also have problems with the engine misfiring or things like rough starts. In some cases, the engine just won’t fire up at all. If the spark plug does not have a compatible heat rating, this could result in things overheating which again, can cause more serious problems. And don’t forget that using the wrong spark plug could result in oil and gas deposits on the thread.

It’s always a good idea to follow the instructions in your user manual and choose spark plugs that are compatible with your lawn mower.

FAQs About Lawn Mower Spark Plugs

The socket you use will depend on the size of the spark plug you are installing. However, generally speaking, the socket for changing a spark plug will be either 13/16 or ¾ of an inch.

Depending on the thread size and other measurements, it may be possible to use a car spark plug in your lawn mower. However, they aren’t all going to be suitable so you’ll need to check before using anything that it is compatible.

You should check the spark plugs for dirt, carbon build up or damaged components. This usually signals that the spark plug needs changing. However, you may also need to replace the spark plug if the recoil start is hard to pull

Final Thoughts

Lawn mower spark plugs are a crucial part of any riding or push petrol lawn mower. Without them, the spark required for combustion wouldn’t be produced so you’d end up with a lawn mower that doesn’t fire up.

Spark plugs are parts that will wear or become dirty over time and this means that you will have to replace them from time to time.

One thing that confuses a lot of lawn mower owners are the different sizes of lawn mower spark plugs. But don’t let this deter you from shopping for parts. You simply need to make sure you have the right thread size and heat rating for your mower. Details of this can be found in your user manual.

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Andrew Fisher

Andrew Fisher

Andrew is a dedicated father of three who really takes pride in his lawn and garden. You'll find Andrew behind the scenes of almost everything Edge Your Lawn produces. When he's not helping readers find all the information they need, he's in his backyard working on his lawn and garden landscaping. This year he hopes to build an outdoor deck and sort out his veg patches.