Quick answer
Most four-stroke lawn mower engines use a spark plug gap of 0.030 inch (0.76 mm). That covers most Briggs & Stratton, Kohler, Kawasaki and Tecumseh engines. Honda mower engines use 0.028–0.031 inch (0.7–0.8 mm). Two-stroke trimmers and blowers are often smaller, around 0.020–0.026 inch. Your owner’s manual or the engine’s spec sheet always has the final number.
The gap is the small space between the spark plug’s center electrode and the bent ground electrode. Too wide, and the coil may not jump it reliably, causing hard starting and misfires. Too narrow, and the spark is weak, causing rough idle and fouled plugs. Setting it takes two minutes and a cheap gauge.
Lawn mower spark plug gap chart
| Engine brand | Typical gap | Notes |
|---|---|---|
| Briggs & Stratton (most walk-behind and riding) | 0.030″ (0.76 mm) | Common plugs: Champion RJ19LM / B&S 796112 |
| Kohler (Courage, Command, 7000 Series) | 0.030″ (0.76 mm) | Check spec for your engine model |
| Honda (GCV, GX, GSV) | 0.028–0.031″ (0.7–0.8 mm) | Common plug: NGK BPR5ES or BPR6ES |
| Kawasaki (FR, FS, FX) | 0.030″ (0.76 mm) | Riding mower V-twins |
| Tecumseh (older mowers) | 0.030″ (0.76 mm) | Discontinued, but many still running |
| 2-stroke trimmers and blowers | about 0.020–0.026″ | Varies by brand, so check the manual |
Some engines differ from these typical numbers, especially older engines or EFI models. If your manual gives a different figure, use the manual.
How to check and set the gap
Step 1: Remove the plug safely
With the engine cool, pull off the plug wire and unscrew the plug with a spark plug socket. Most mowers use a 5/8″ or 13/16″ socket.
Step 2: Inspect it
- Light tan or gray deposits: normal.
- Black and sooty: running rich (dirty air filter, choke) or a weak spark.
- Wet and oily: oil getting into the cylinder (overfilled crankcase or worn rings).
- Worn, rounded center electrode or cracked porcelain: replace it.
Step 3: Measure with a wire gauge
Slide the correct wire of a wire-type gap gauge between the electrodes. It should drag slightly. Wire gauges are more accurate than flat feeler blades on used plugs, because the electrodes wear unevenly.
Step 4: Adjust
To close the gap, tap the ground electrode gently on a hard surface. To open it, pry it gently with the gauge’s bending tool. Never lever against the center electrode or the porcelain.
Step 5: Reinstall
Thread the plug in by hand to avoid cross-threading. Then tighten a new plug with a gasket about ½–⅔ turn past finger-tight (or to the torque in your manual), and refit the wire.
Common replacement plugs
- Champion RJ19LM: fits many Briggs & Stratton walk-behind engines. Equivalent to Briggs part 796112, also sold as a genuine Briggs 2-pack.
- NGK BPR5ES: used in many Honda GCV mower engines and other small engines.
- Always match the plug part number listed in your manual or on your old plug.
Frequently asked questions
Do new spark plugs come pre-gapped?
Often close, but not always to your engine’s spec, and the gap can shift in shipping. Always check a new plug before you install it.
How often should I replace a mower spark plug?
Once a season, or about every 100 hours, is common guidance from engine makers. Replace it sooner if it’s fouled, cracked or the electrodes are worn.
Can a wrong gap stop a mower from starting?
Yes. A gap that’s far too wide or closed up can prevent a reliable spark, especially with a weak ignition coil. Check the gap whenever a mower is hard to start.
What gap does a riding mower use?
The same as other four-stroke mower engines, usually 0.030″ for Briggs, Kohler and Kawasaki V-twins. Check both plugs on twin-cylinder engines.
Method: Gap values are typical figures from engine manufacturers’ published specifications and owner’s manuals. Specific models can differ, so always confirm with your engine’s manual or spec number.
Update log: November 2026: rewrote the guide. We corrected the gap chart by brand, added a step-by-step gapping method, plug inspection signs, common replacement plugs and an FAQ.