What valve clearance is and why it matters on Briggs & Stratton engines
Valve clearance is the small gap between the valve stem and the rocker arm (or lifter) on your engine. On Briggs & Stratton small engines, this gap must stay within a specific range or the valves won't open and close properly. Too little clearance and the valve stays slightly open, losing compression and power. Too much clearance and the valve doesn't open far enough, starving the cylinder of fuel and air.
Briggs & Stratton publishes clearance specifications for each engine model because the gap changes as metal parts wear and expand with heat. Checking clearance is a maintenance task you can do yourself with basic tools — a feeler gauge and a wrench. Most owners only need to check it once a year or after the engine has been stored for a season.
Key Takeaways
- Valve clearance varies by engine model and whether the valve is intake or exhaust, so you must find your specific engine model number before measuring.
- Clearance is measured with the piston at top dead center on the compression stroke, using a feeler gauge between the valve stem and rocker arm.
- Briggs & Stratton publishes clearance specs in the engine manual and on their website, organized by model number and engine displacement.
- If clearance is out of spec, you adjust it by loosening the rocker arm nut and turning the adjustment screw until the feeler gauge slides through with light resistance.
- Checking clearance takes 15 to 30 minutes and requires only a feeler gauge set, wrench, and screwdriver.
Finding your engine model number and the correct clearance specification
The first step is locating your engine's model number, which Briggs & Stratton stamps on a metal plate bolted to the engine block. On most models, this plate is on the side of the engine or near the fuel tank. The model number looks like a series of digits — for example, 12H802 or 31C707. Write this number down; you will need it to find the correct clearance specs.
Once you have the model number, go to the Briggs & Stratton website and search their engine manual database, or check the printed manual that came with your equipment. The manual lists clearance in thousandths of an inch — typically 0.005" to 0.010" for intake valves and 0.010" to 0.015" for exhaust valves on horizontal-shaft engines, though this varies. Vertical-shaft engines (like those on lawn mowers) often have different specs. Write down both the intake and exhaust clearance for your model before you start work.
Preparing the engine and locating top dead center
Remove the spark plug wire from the spark plug to prevent accidental starting. If your engine has a shroud or cover over the valve area, remove it with a wrench or socket set. You now need to position the piston at top dead center (TDC) on the compression stroke — the point where the piston is at its highest point and both valves are closed.
To find TDC, slowly turn the engine's crankshaft by hand using the recoil starter handle or a wrench on the crankshaft nut (depending on your engine type). As you turn, watch the intake valve — it will open, then close. Keep turning slowly past the point where it closes. When both valves are fully closed and the piston stops moving upward, you are at TDC. Some engines have a mark on the flywheel that aligns with a pointer on the block; if yours does, use that as your reference point.
Measuring valve clearance with a feeler gauge
A feeler gauge is a set of thin metal blades, each stamped with its thickness in thousandths of an inch. You will use the blade that matches your engine's clearance spec. For example, if the spec is 0.005", you use the 0.005" blade.
With the piston at TDC, slide the correct feeler gauge blade between the valve stem and the rocker arm. The blade should slide through with light resistance — not so tight that you have to force it, and not so loose that it falls through. If the blade is too tight, clearance is too small. If it falls through, clearance is too large. If you are between two sizes (for instance, 0.005" is too tight but 0.007" is too loose), use the smaller blade and adjust until it fits snugly.
Check both the intake and exhaust valves this way. Some engines have two intake valves or two exhaust valves, so measure each one separately. Write down what you find so you know which valves need adjustment.
Adjusting clearance if it is out of specification
If clearance is out of spec, you will adjust it by turning the rocker arm adjustment screw. Locate the locknut on top of the rocker arm — it is a hex nut that holds the adjustment screw in place. Using a wrench that fits the locknut, hold it steady while you turn the adjustment screw with a screwdriver.
Turn the screw clockwise to decrease clearance (make the gap smaller) or counterclockwise to increase it (make the gap larger). Turn in small increments — a quarter turn at a time — and check with the feeler gauge after each adjustment. Once the blade slides through with light resistance, hold the adjustment screw steady with the screwdriver and tighten the locknut with the wrench. Tighten firmly but do not over-tighten, which can strip the threads.
After tightening the locknut, check clearance one more time with the feeler gauge to make sure it did not shift during tightening. Repeat this process for any other valves that were out of spec. When all valves are within spec, reinstall the shroud or cover, reconnect the spark plug wire, and you are done.
Common mistakes and how to avoid them
The most common error is measuring clearance when the piston is not at TDC. If the piston is partway through its stroke, the valve will be partially open and your measurement will be wrong. Take your time finding TDC by slowly turning the crankshaft and watching both valves close completely.
Another mistake is using the wrong feeler gauge size. Always double-check your engine manual to confirm whether you need 0.005" or 0.010" for the intake valve, because different models vary. Measuring with the wrong blade will lead to incorrect adjustment.
A third error is over-tightening the locknut after adjustment. If you crank down too hard, you can bend the rocker arm or strip the threads on the adjustment screw. Tighten until snug, then stop. The locknut only needs to hold the screw in place, not crush it.
When to check valve clearance and what to do if you are unsure
Check valve clearance once a year before the mowing or operating season, or after the engine has sat unused for more than a few months. Clearance can drift as metal wears and as carbon builds up on the valve seats. If your engine starts hard, runs rough, or loses power, out-of-spec clearance is one of the first things to rule out.
If you are uncomfortable measuring or adjusting clearance yourself, a small-engine repair shop can do it in under an hour for a modest fee. Bring your engine model number with you so they can look up the correct specs. If you have already measured clearance and found it out of spec but are not sure how to adjust it, a technician can show you the process on your specific engine, which often makes future adjustments easier to do yourself.
Frequently Asked Questions
Where do I find the valve clearance specs if I do not have the manual?
Go to the Briggs & Stratton website and use their manual search tool — you enter your model number and it displays the PDF manual with clearance specs listed in the maintenance section. You can also call a Briggs & Stratton dealer with your model number and they will read the specs to you over the phone.
Can I measure clearance while the engine is warm?
No. Always measure on a cold engine. Metal expands when hot, so clearance will be larger than it actually is when the engine cools down. Let the engine sit for at least an hour after running before you measure.
What if one valve is in spec but the other is way off?
Adjust the one that is out of spec. It is normal for intake and exhaust valves to have different clearances, and it is also normal for one valve to drift more than the other as the engine ages. Measure and adjust each valve independently.
Do I need special tools to adjust valve clearance?
No. You need a feeler gauge set (about $10 to $20), a wrench that fits your rocker arm locknut, and a screwdriver. A socket set makes it easier to turn the crankshaft, but you can do it by hand with the recoil starter if needed.
How often should I check clearance on a new engine?
Check it after the first 50 hours of operation, then annually after that. New engines can settle as parts wear in, so an early check catches any drift before it causes problems.