When it comes to keeping your lawn mower running smoothly, the spark plug plays a much bigger role than many people realize. At first glance, you might think that as long as the new spark plug screws in and fits the hole, it will work just fine. But that's not the case. Using the wrong spark plug in your mower can lead to all kinds of problems—from poor performance to serious engine damage.

Let's look at why choosing the correct spark plug matters.

1. Heat Range Mismatch

Every spark plug is designed to run at a specific temperature. This is called its heat range.

If you install a plug that runs too hot, it won't get rid of heat quickly enough. The tip can overheat, causing pre-ignition (the fuel ignites before the spark), which can damage pistons or valves.

If you install a plug that runs too cold, it won't stay hot enough to burn off carbon deposits. That leads to fouling, hard starts, and misfires.

The ignition timing itself doesn't change, but the wrong heat range can make the combustion process act like the timing is off.

2. Wrong Thread Reach

Spark plugs also come in different lengths, known as reach.

If the plug is too short, the spark sits too far back in the combustion chamber, and the fuel mixture may not ignite efficiently.

If the plug is too long, the tip can extend into the chamber and get struck by the piston, or overheat because it's exposed to too much combustion heat. Either way, this can destroy both the plug and the engine.

3. Electrode Design and Gap

Different small engines are designed around specific spark plug shapes and electrode styles. If the wrong type is installed, the spark may not fire in the ideal location. Combine that with an incorrect gap, and you'll notice:

Rough running

Loss of power

Increased fuel use

4. What You Might Notice with the Wrong Plug

If you've accidentally put in the wrong spark plug, your lawn mower may:

Run rough or stall under load

Lose power compared to normal

Show a black, oily, or carbon-covered spark plug tip

In severe cases, make knocking sounds or suffer engine damage

Just because a spark plug screws into your mower doesn't mean it's the right one. Using the wrong plug can cause fouling, pre-ignition, poor performance, and even permanent engine damage.

Always check your mower's owner's manual or parts guide for the recommended spark plug model. Sticking with the correct plug ensures your engine runs efficiently, lasts longer, and starts up without trouble.