How To Check Dirt Bike Valve Clearance

What Valve Clearance Is

When a valve is shut, there is a small gap between the cam lobe's base circle and the follower that pushes the valve. That gap lets the valve close fully on its seat even as parts grow with heat. Honda's manual puts the two failure modes plainly. Too much clearance makes noise and can eventually damage the engine. Too little, or none, stops the valve from closing, which causes valve damage and power loss.

On a dirt bike the gap almost always shrinks. Every time a valve slams shut the face and seat wear a little, the valve sits deeper in the head, its stem tip rises toward the cam, and the clearance closes. Drag the slider in the figure to watch that happen.

Check The Valves Step By Step

1. Work on a cold engine at TDC on compression. Let the engine cool to room temperature, remove the seat, tank and valve cover, and turn the crank to the TDC mark. The cam sprocket marks should sit level with the head surface. If they do not, turn the crank one more full turn.
2. Measure the intake valves. Slide feeler blades between each intake lobe's base circle and its follower. The right blade slides with a light drag. Write down every reading.
3. Measure the exhaust valves. Do the same on the exhaust side and record each valve.
4. Compare with the spec. Honda's 2025 CRF250R spec is 0.10 ± 0.03 mm intake and 0.20 ± 0.03 mm exhaust, cold. KTM's 2024 250 SX-F is 0.08 to 0.15 mm intake and 0.12 to 0.19 mm exhaust at 20 °C.
5. Work out the new shim. Honda's formula is new shim = (measured clearance minus specified clearance) plus old shim thickness, with the old shim measured by micrometer. Round to an available size.
6. Fit the shim and recheck. Follow your service manual to remove the cams, swap the shim, retime the cams and torque the caps, then measure again.

The figure is a generic bucket and shim head, the layout Yamaha's YZ250F uses. Honda's CRF250R puts its shims under rocker arms and KTM's 250 SX-F uses finger followers, but each measures the same gap between the cam's base circle and the follower. The installed shim sizes in the figure are examples, and the cut-away draws the gap about 40 times larger so you can see it.

Specs By Model

Always use the manual for your model and year; Honda's own numbers changed between the 2018 and 2025 CRF250R.

• Honda CRF250R 2025. Intake 0.10 ± 0.03 mm, exhaust 0.20 ± 0.03 mm, cold.
• KTM 250 SX-F 2024. Intake 0.08 to 0.15 mm, exhaust 0.12 to 0.19 mm, at 20 °C.
• Yamaha YZ250F 2026. Intake 0.12 to 0.19 mm, exhaust 0.17 to 0.24 mm, cold.

Doing The Shim Math

Honda's formula is A = (B − C) + D, where A is the new shim, B the measured clearance, C the specified clearance and D the old shim. Measure D with a micrometer instead of trusting the printed number. If an intake measures 0.05 mm against a 0.10 mm spec and the old shim is 1.850 mm, the new shim is (0.05 − 0.10) + 1.850 = 1.800 mm. Honda sells shims from 1.200 to 3.000 mm in 0.025 mm steps; Yamaha's pads run 1.20 to 2.40 mm in 0.05 mm steps with its own rounding rule.

Keep a log of every reading. If the same valve needs a thinner shim again soon, stop shimming and look at the valve and seat. Honda warns not to lap its titanium valves, and Motocross Hideout notes that a valve that keeps tightening usually needs replacing and the seat recut.

How Often To Check

The factory intervals are far apart, and the shortest ones are written for racing. KTM checks its 250 SX-F motocross race bike after the first hour, then every 45 operating hours, and its 2025 450 XCF-W enduro bike on the same 45 hour schedule (PDF p. 54). Honda inspects the CRF250R race bike every six races, about 15 hours. Yamaha's YZ250F race schedule checks after break-in and about every 7.5 hours. Honda's CRF250F trail bike goes every 600 miles (1,000 km) or 6 months (PDF p. 52), and so does the CRF450X on its non-competition schedule (PDF p. 45). Follow the schedule printed for your own bike and the way you ride it. Motocross Hideout says a 250F that keeps hitting the rev limiter needs checks far more often than one trail ridden at lower rpm, and that only a feeler gauge reading against spec tells you whether to adjust. If the clearance is in spec, leave it alone. A leak-down test can point the same way, since a hiss at the intake or exhaust means air is getting past those valves (DIY Moto Fix). Hard starting, especially when a cold engine suddenly takes many kicks or only bump starts, and a loss of power are reasons to check sooner. A dusty engine wears seats faster, so the air filter guide is part of valve care. Our valve clearance guide covers symptoms and costs in more depth.

FAQ

What happens if dirt bike valves are too tight?

A valve without enough clearance cannot close fully. Honda says that causes valve damage and power loss, and KTM lists too little valve clearance as a cause of low engine power. Tight intake valves also make cold starts harder.

Do you check valve clearance hot or cold?

Cold. Honda specifies cold clearances, KTM gives its values at 20 °C (68 °F) and Yamaha says room temperature, because the gap changes as the engine heats up.

How do I find TDC on the compression stroke?

Turn the crank to the TDC mark and check that the cam sprocket timing marks line up with the head surface. If the marks are not in place, you are on the exhaust stroke, so turn the crank one more full revolution.

How do you calculate a new valve shim?

Honda's formula is new shim = (measured clearance minus specified clearance) plus old shim thickness. Measure the old shim with a micrometer and pick the nearest available size.

How often should I check valve clearance on a 250F?

Follow the manual for your own bike. KTM checks the 250 SX-F motocross race bike every 45 operating hours after the first hour, Honda checks the CRF250R race bike about every 15 hours, and Yamaha checks the YZ250F race bike about every 7.5 hours. Honda's CRF250F trail bike schedule checks every 600 miles or 6 months.

Why do valves keep getting tighter?

The valve face and seat wear, the valve sinks deeper into the head and the gap closes. If a valve needs smaller and smaller shims in a short time, inspect or replace the valve and seat instead of shimming again.