Are Lawn Mower Spark Plugs The Same As Car Spark Plugs?

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Written By Janine Clarke

I am Janine Clarke AKA Equipment Girl, a nerdy girl with an unhealthy knowledge about power tools and gardening! You can contact me here.

Can a lawn mower spark plug really be the same as an automobile spark plug? Are lawn mower engines really that similar to car engines? And, if so, does that mean you don’t have to spend hours finding the ‘right spark plug’, because the two are interchangeable?

These are all questions I’ve wondered in the past, so today I’m going to answer them, and hopefully you guys will find the information useful if you ever need to change your lawn mower spark plugs in the future.

To start with, let’s look at lawn mower spark plugs first, to find out if all lawn mower spark plugs are exactly the same.

Is Every Lawn Mower Spark Plug The Same?

To be clear, every spark plug does the same job in any automobile gasoline engine (including lawn mower engines and car engines) but just because they do the same job, doesn’t mean they’re the same.


As a quick recap, every spark plug in every gasoline engine has a center electrode (which receives a high pulse voltage when you start the engine) and a ground electrode (which doesn’t receive any voltage). The high potential difference this creates then goes on to make an electric spark.

The spark ignites an air fuel mixture in the internal combustion engine, which then goes on to keep the engine running.

Basically, all spark plugs work on the same energy principle, by having a spark jump over the spark plug gap from one electrode to another to burn the fuel in the combustion chamber, which then allows the engine to start and run.

But Are All Mower Spark Plugs The Same?

Despite doing the same job, no, not all lawn mower spark plugs are the same. Many lawn mower engines are created differently. Yes, they will have all the same basic parts to make the engine start and run (including a spark plug, of course), but each lawn mower engine model will have physical differences in the way those parts are put together.

These physical differences then lead to certain specification differences, too. Things like differences in:

  • thread size
  • thread length
  • heat range
  • specific engine model

So, lawnmower spark plugs aren’t even universal or interchangeable – but perhaps car spark plugs might be suitable for certain lawn mower models?

Is Every Car Spark Plug The Same?

Much like each lawnmower engine model is different, so too are cars’. And if each car engine is different, then so too are the automobile plugs they use.

Because all automobile engines and spark plugs are different, that also means you’ll notice differences in thread size, thread length, heat range, and engine model, just like with lawnmower engines and spark plugs.

So the real question isn’t whether or not all lawn mower spark plugs and car spark plugs are the same (because it’s clear to see that they’re not, since they don’t even have the same spark plugs in every lawnmower engine or every automobile engine), but rather whether or not there are any lawn mower spark plugs and car spark plugs that are similar enough to be used in each other’s engine.

Can You Use A Car Spark Plug In A Lawn Mower Engine And Vice Versa?

In general, no, you can’t use a car spark plug in a lawn mower or a lawn mower spark plug in a car – because the chances are you won’t find the same spark plug that can be used in both engine types.

It’s hard enough sourcing the right spark plug for your mower model or car model specifically, so to find a spark plug that can be used in both just doesn’t seem likely.

Even if you could find the specific spark plug that works in both, it still wouldn’t be recommended by most because it can cause all sorts of problems. I’ll cover what would happen if you mixed up the different spark plugs in each other’s engine more below, but for now, let’s dive further into the key differences between lawn mower spark plugs and auto plugs.

Key Differences Between The Different Spark Plugs

Thread Size

Many automobile plugs have a thread of 14mm as standard. Lawn mowers tend to have a thread of 10 or 12mm, so right off the bat you’re dealing with two different spark plug types.

The reason for the differences in threads is the size of the engine. Think about it, walk behind lawn mowers are much smaller than cars. Even ride-on mowers are significantly smaller than cars too. And with a smaller vehicle comes small engines which need different sized plugs to fit them.

Thread Length

Using the wrong thread length can spell disaster for the different engines you’ve used them in. And the thread length between car and mower spark plugs is different yet again.

Car spark plugs tend to be either 1/2 inch or 3/4 inch in length, but yet again, a lawn mower’s spark plug is much smaller in length at 3/8 of an inch. The shorter thread length is, again, due to the smaller engine, and the tapered hole that the plug is designed to fit is shorter as a result.

Heat Range

Even if you’ve managed to find a spark plug for a car that is the right spark plug in terms of the thread length and size for your lawnmower, there’s still one crucial factor to consider: the heat that the spark plug is capable of dealing with.

Most walk behind lawn mowers have an air-cooled engine because it’s so much smaller and less demand is being placed on it. But a car’s spark plug is designed to deal with much more heat.

If you have a cold plug (because the engine just doesn’t get hot enough for the spark plug to work), then the chances are the ignition system won’t work, and you’ll end up gas fouling the spark plug which will mean you’ll need to replace the old spark plug with a new one because once fouled, the spark plug becomes too worn to function as it should.

Not using the correct spark plug can mean that it’s too hot or too cold to actually start the car or lawn mower properly, so it’s highly unlikely you’d be able to find the same plug for your auto engines and lawn mower due to the different demands placed on them in terms of the heat they can deal with.

Engine Model Number

Then, after all that, there’s the engine model number too. Each lawnmower engine is unique, and it’ll need different boxes to be ticked in order for a spark plug to work in it.

Checking your lawn mower owner’s manual, talking with an expert at a hardware store, or doing some further investigation yourself with a spark plug wrench ought to show you what your current spark plug looks like, and that’ll give you an idea about what any new plug would need to look like too.

Things like the spark plug gap, the way the plug seals the cylinder head in the engine, or even the exact place the spark plug wire connects can all be different depending on the lawn mower.

And that all applies to cars too. Because everything is so different and there are so many tweaks to each element of a spark plug that make it perfect for the specific engine it was designed for, you’ll almost definitely not be able to find a spark plug for a car that’s also suitable for a lawn mower.

What Happens If I Try To Put The Wrong Spark Plug In?

Whether you’ve tried to put a spark plug for a car in a lawn mower or simply bought the wrong spark plug to begin with, you’ll notice some problems.

Problems With The Thread

The first problem you’ll notice if you don’t use the right spark plug with the right thread length is that it just won’t fit properly. A bad spark plug here will fit poorly, and you might not be able to attach it to your engine at all.

Fuel Efficiency

If you somehow manage to get a spark plug for a car in a lawn mower, then you’ll also notice problems with your fuel. It’ll burn through more fuel more quickly when you’ve not used the right spark plug, and that’s true even if you’ve just used the wrong lawn mower spark plug.

Engine Damage

Internal engine damage and poor engine performance is a very real possibility when you replace your old spark plug with a wrong new one. If you don’t use the spark plugs that are recommended in the owner’s manual, then you can put an unnecessary strain on your engine and they’ll either perform poorly or be damaged completely.

Spark Plug Gap

Without the proper gap, the electric spark can’t jump from the central electrode to the outer electrode effectively. The old plug will be designed to fit so that the gap is as it should be, but if you replace your old spark plug with a new one that isn’t the perfect match, then the gap can be compromised and your engine just won’t start.

Final Thoughts

I think we can say pretty definitively after today’s deep-dive that lawn mower spark plugs and car sparks plugs just aren’t the same.

With differences in thread length and size, heat, and engine design, finding the perfect spark plug to fit both types of engine just isn’t possible.

But at least now you won’t make the novice mistake of believing they’re the same, or worse, trying to fit the wrong spark plug in the wrong engine!