The Complete Guide to Small Engine Spark Plugs

The spark plug is a small part with a big job — it ignites the fuel-air mixture in your engine's cylinder thousands of times per minute. When it's working properly, you never think about it. When it's not, nothing works.

How often should you replace it? Most small engine manufacturers recommend replacing the spark plug every 100 hours or once per season. However, if your mower is hard to start, runs rough, or lacks power, the spark plug is one of the first things to check.

Reading your spark plug can tell you a lot about your engine's health. A light tan or gray color on the electrode and insulator means the engine is running correctly. Black, sooty deposits mean the engine is running rich — check the air filter and carburetor. White or blistered deposits mean the engine is running too lean or overheating. Oily, wet deposits suggest worn piston rings or valve seals.

Choosing the right spark plug matters. You need to match the reach (thread length), thread size, and heat range specified by your engine manufacturer. Using the wrong spark plug can cause poor performance or engine damage. Common spark plugs for small engines include the Champion RJ19LM, NGK B2LM, and Briggs & Stratton 796112S.

Gapping is important too. The spark plug gap — the distance between the center and ground electrode — must be set correctly. Most small engines call for a gap between 0.028 and 0.032 inches. Use a gap tool to check and adjust.

Installation tips: don't overtighten. Spark plugs should be snug — about a quarter turn past finger tight for new plugs with a crush washer. Over-tightening can strip the threads in the aluminum cylinder head, which is an expensive repair.

We stock the most common spark plugs for Briggs & Stratton, Honda, Kohler, and Kawasaki engines. Stop by and we'll get you the right one.