Lawn Mower Spark Plug Size: Find the Right Plug

I won’t deny that I tend to prioritize my garden beds over my lawn when given the choice. But I still understand the importance of having a high-quality, well-maintained lawn mower in my arsenal. 

Whether mowing the lawn is a cherished pastime or just another thing on your to-do list, a lawn mower is a big investment. The more you know about taking care of and — when necessary — repairing a lawn mower, the more mileage you can get out of this purchase.

Spark plugs are one facet of lawn mower maintenance that many people struggle to understand. Rest assured, you don’t need to be an engine expert to learn how to select the right lawn mower spark plug size for your particular model. In this article, I’ll help simplify the process and also offer some tips for finding the perfect fit.

Do Lawnmowers Use Specific Spark Plugs?

Any piece of equipment fitted with a gas engine requires a spark plug to function. If your lawn mower sources its power from a battery or extension cord, a spark plug isn’t necessary. However, all gas-powered lawnmowers have spark plugs.

In terms of function and design, the spark plug used by your lawn mower is not much different than the one used by your car. They both serve the same purpose — to create a small spark and ignite the fuel as it enters the engine’s chamber. 

While lawn mower spark plugs aren’t unique in the way they work, they do differ when it comes to size. As a rule, a spark plug’s size is relative to the engine it’s designed for. So the spark plugs used in lawnmowers are, on average, significantly smaller than those used in passenger vehicles.

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Are Lawn Mower Spark Plugs A Universal Size?

Not all lawnmowers feature the same size engine. Since spark plug sizes correspond to the size of the engine, this naturally means that not all lawnmowers use the same type of spark plug.

It’s not enough to know that some lawnmowers require smaller spark plugs than others. There are several factors that go into spark plug sizing — some of which are hard to differentiate with an untrained eye — and using a spark plug with even one incorrect measurement could place undue stress on your mower’s engine.

Thread Size

A spark plug’s thread size consists of two separate measurements: pitch and diameter. Thread pitch refers to the difference between individual threads on the plug. Meanwhile, thread diameter refers to the thickness of the threaded portion of the spark plug.

These measurements correspond to how the plug fits into an engine. If you’re struggling to picture the relationship between these measurements, it may be easier to think of a spark plug as a simple screw. Just like you can’t use a too-large or small screw to put together a piece of IKEA furniture, you can’t fit an incorrectly sized spark plug into an engine.

Thread Length/Reach

Thread length (or reach) is closely related to pitch and diameter. This measurement refers to the total length of the spark plug’s threaded section — i.e., how far the plug will reach into the engine when installed.

Though it might not seem like a big deal to swap out an existing spark plug for one that is slightly longer or shorter, you absolutely shouldn’t. An incorrect plug reach can interfere with proper engine function and cause problems like overheating and oil fouling.

Spark Plug Gap

A spark plug’s gap is the space between its electrodes. Standard spark plugs have two electrodes located at the bottom of the threaded section. These include the center electrode, a piece of metal in the center of the plug, and the ground electrode, a curved piece of metal that extends over the center electrode.

In order for a spark plug to function properly, these electrodes must be a specific distance from each other. They must be close enough to create an electrical arc (or “spark”) but far enough apart to allow fuel and air to pass between them. This balance is what allows a spark plug to ignite the fuel within your lawn mower’s engine.

Unlike other factors that go into spark plug sizing, the gap is adjustable to a point. You can purchase a simple spark plug gap tool — for general home use, I recommend investing in a coin-style tool — for just a few bucks at almost any hardware store. This tool makes it easy to measure and adjust spark plug gaps as needed.

Heat Rating

Each spark plug also comes with a heat rating. This rating describes how well a given spark plug tolerates and dissipates heat. The greater the spark plug’s heat rating, the quicker it will transfer excess heat from inside the engine cylinder.

Gas-powered engines vary in how much heat they produce during operation. Because lawn mower engines are fairly small and do not generate much heat, most models call for spark plugs with relatively low heat ratings.

Unfortunately, heat ratings aren’t standardized between brands. For example, a #2 spark plug from NGK is comparable to a #10 plug from Bosch. If you’re purchasing from a different brand than previously used in your lawn mower, I recommend referencing a trusted conversion guide to determine the correct heat rating to use.

Common Lawn Mower Spark Plug Size Chart

While there is no such thing as a universal spark plug, most lawnmowers utilize spark plugs within a standard size range. These standard ranges hold true across all major lawn equipment brands.

Keep in mind that the following chart won’t tell you the exact spark plug size required by your lawn mower model. For that, you’ll need to reference your owner’s manual or inspect the engine’s existing spark plug. 

However, I still think it’s helpful to know the general size range of lawn mower spark plugs. For example, you may want to know if a local engine part retailer sells spark plugs within this range. Having these measurements at your disposal will make it much easier to find that out.

Mower TypeThread PitchThread DiameterPlug Reach
Push Mower1.0-1.5mm10-12mm9-12mm
Riding Mower1.0-1.5mm12-14mm15-19mm

Help Finding Your Lawn Mower Spark Plug Size

You don’t need to worry about details like thread pitch or diameter as long as you know the correct spark plug part number. This piece of information will make your search for a replacement spark plug much easier.

Find the Spark Plug Number

There are a few ways to find out the part number for a particular size of spark plug:

Option 1: User Manual

Your lawn mower’s user manual should provide exact spark plug specifications. This is the most reliable source of information when it comes to sourcing parts for your lawn mower.

Option 2: Online Search

In lieu of a paper manual, you may be able to search for a digital copy of your mower’s user manual online. I’ve also had great success using online forums to source specific parts, especially in older models of lawnmowers.

Option 3: Existing Spark Plug

Alternatively, you can remove the old spark plug from your lawn mower and inspect it for an identifying serial number. You can then use the plug’s brand plus its serial number to determine its size.

If the spark plug part number has rubbed off, you may be able to identify the size by bringing it into your local hardware store. Even if you’ve positively identified the part number, it doesn’t hurt to bring the old plug with you to confirm you have the right replacement.

Look up Your Lawn Mower Spark Plug Number

While there are several ways to determine what size spark plug your lawn mower requires, I think using an online catalog is the easiest. The NGK Part Finder is a particularly comprehensive search feature that lets you look up spark plug sizes by lawn mower make and model.

Using the Wrong Spark Plug in a Lawn Mower?

Even with due diligence, it’s possible to make a mistake and purchase the wrong spark plug for your lawn mower. So what are the potential consequences of trying to use the incorrect size spark plug?

The first issue you might encounter is that the spark plug simply won’t fit. If your new spark plug won’t screw in or feels unusually loose in the engine, I recommend removing it and double-checking the size.

Some spark plugs will fit despite being the wrong size for your lawn mower. This can cause problems such as the engine struggling or failing to start. You may also notice that the engine starts but runs roughly or stops unexpectedly.

A spark plug that is the incorrect length for your engine may be prone to oil or fuel buildup. This issue is commonly known as oil fouling. An oil-fouled spark plug can contribute to engine misfiring and other wear and tear.

Finally, your spark plug may overheat and fail if its heat rating is too low for the type of engine. 

FAQs Lawn Mower Spark Plugs