Lawn Mower Spark Plug Size: Which is Right for Your Mower?

Keeping rhythm with the precise timing of gas engines, spark plugs create a small spark and ignite the fuel and air mixture that gets drawn into the cylinder. This is the case for any gas-powered engine. But even though the basic process is the same, there are a bunch of different-sized spark plugs. Size differences between the engines inside a car, snowmobile, or airplane, for example, are reflected in the size of their spark plugs. But when it comes to lawn mower spark plug size, a bunch of different concerns come into play. Are all lawn mower spark plugs the same size? How are spark plugs measured? What happens if you try to use the wrong type of spark plug? Let’s find out.

Are Lawn Mower Spark Plugs Universal?

In short, no, not all lawn mowers use a spark plug that is one universal size. With that being said, most lawn mower engines use spark plugs that fall into a common range of sizes. To explain this, I should describe how spark plugs are measured first.

Spark plugs are actually measured by their threads. The two thread dimensions that vary on a spark plug are thread size and thread length. Aside from this, the final difference between spark plugs is the heat rating. Different spark plugs are rated to function at different temperatures. Knowing a bit about the way spark plugs are sized will definitely help you navigate the parts aisle a little bit more effectively. Here’s some more info about these three characteristics:

Thread Size

The size of the thread on a spark plug actually refers to two numbers, thread pitch and thread diameter. Thread pitch measures the distance between each individual thread. This is a very small distance and is measured in millimeters. Thread diameter, on the other hand, is a measure of the width of the threaded section of the spark plug.

Thread Length/Reach

The reach or length of a spark plug has to do with how long the spark plug is and how deeply it will be seated in the engine. There is a wider range of thread lengths and it is very important to use the correct one. If your spark plug isn’t seated deep enough, it could accumulate oil or fuel around its tip. If the spark plug is screwed in too far, overheating can occur and gas/oil buildup could occur on the threads.

Heat Rating

Since different engines run at different temperatures, spark plugs are designed to be able to tolerate a certain level of heat. This is described by a standardized rating on a scale from 2 to 12. The lower the number, the more easily the spark plug will heat up. For example, a spark plug with a heat rating of 6 will overheat at a lower temperature than a spark plug with a heat rating of 8.

Standard Lawn Mower Spark Plug Sizes (with Chart)

As is the case with cars and motorcycles, lawn mower spark plug sizes have a few standard measures. Whether looking at Toro, Craftsman, Honda, or Troy Bilt lawn mower spark plug sizes, you’ll find that they all fall into a similar range of thread pitch, diameter, and reach. Here’s a lawn mower spark plug size chart with the typical thread measurements for both riding and push mower spark plugs:

Tips on Finding Your Lawn Mower Spark Plug Size

If you’ve taken a look at the standard lawn mower spark plug sizes but are still wondering how exactly to go about finding the plug you need, look no further. The best advice I can give to find the right size spark plug is to take a look at your owner’s manual. Getting information from the manufacturer is always a good idea.

But, if you’ve misplaced your manual or bought a used mower without one, there are a couple of other options: read the part number written on the spark plug itself, or bring your spark plug into the hardware store and ask for some advice. If you’re looking to replace your spark plug, I would definitely recommend bringing your old plug with you so that you’re sure to get the exact same size.

What Happens if You Use the Wrong Spark Plug in a Lawn Mower?

Lawn mower engines are designed to use spark plugs with certain dimensions. If you try to use the wrong spark plug in your lawn mower, you could end up with some issues. Some of the biggest problems that could arise are:

• Your new spark plug won’t screw in
• Engine misfiring
• Trouble starting the engine or rough running
• Oil fouling the tip or threads of the spark plug
• Spark plug overheating and failure

As mentioned in the section about spark plug measurements, using the wrong size spark plug can cause all of these issues. Ultimately, the wrong size spark plug will cause your engine to run poorly or not at all.