How To Check Dirt Bike Valve Clearance (And Shim It)

What Valve Clearance Is

A four-stroke opens its valves with cam lobes. Each lobe presses on a follower, which might be a bucket, a finger follower or a rocker arm, and that follower pushes the valve open against its spring. When the lobe rotates away, the spring snaps the valve shut onto its seat. Valve clearance is the small, deliberate gap between the cam and the follower while the valve is fully closed. Yamaha's 2026 YZ250F owner's service manual and Honda's CRF owner's manuals both describe the measurement at that point.

The gap exists so the valve can always seat. Honda puts the two failure directions plainly in its CRF250R manual. Too much clearance makes noise and can eventually damage the engine. Too little, or none, stops the valve from closing and causes valve damage and power loss. The figure below runs one exhaust valve train through a full cycle. Press the steps in order.

Why Valve Clearance Tightens

On a dirt bike the clearance almost always shrinks over time. Every time the valve slams onto its seat, the valve face and the seat wear a tiny amount. As they wear, the valve sits deeper in the head. Its stem tip moves closer to the cam, so the gap between the lobe and the follower closes up. Motocross Hideout describes this valve sinking as the main reason clearances go tight, and Yamaha schedules a separate valve seat and face wear check on the YZ250F, which tells you the factory expects that wear.

Several things speed it up.

• Coated titanium valves. Honda warns that its titanium valves have a thin oxide coating and must not be lapped because lapping damages it. Motocross Hideout notes that once a titanium valve's hard surface wears through, it goes out of adjustment quickly.
• Dust. A leaking or dirty air filter feeds abrasive grit straight to the valve faces and seats. Our air filter cleaning guide covers the routine that protects them.
• Carbon on the exhaust side. Honda tells owners to have the exhaust valve seat refaced if carbon deposits push the calculated shim above 3.000 mm.

Which valves go tight first varies by engine, and our sources do not single out intake or exhaust as a rule. Measure all of them every time. Clearance can also grow, but that is less common. Yamaha's troubleshooting chart lists a too-wide gap, along with worn lobes, lifters and springs, as a cause of top end noise, so a widening gap means something else has worn.

Drag the slider in the lab below and watch what wear does to valve timing. Less clearance means the lobe picks up the valve earlier and lets it go later, so the valve spends less of each cycle sitting on its seat. At zero the bucket rests on the cam and the valve never closes.

Symptoms Of Tight Valves

Tight valves give warning before they fail. These are the signs Motocross Hideout and the manuals point to.

• Hard starting. A tight intake valve does not seal fully, so cranking compression drops. Motocross Hideout describes a bike that went from 1 to 3 kicks to 10 to 20 kicks. It also notes that a tight valve may stop sealing especially as the engine reaches operating temperature, so hard hot restarts belong on the list too. Yamaha lists incorrect valve clearance among the causes when an engine cranks but will not start.
• It only bump starts. If the starter or kick will not light it but a push start will, compression is low, and tight valves are a common reason.
• Hanging or rough idle. Revs that take seconds to drop back can come from tight intakes, though air leaks and lean jetting do the same thing.
• Popping on deceleration and misfires.
• Lost power. KTM's troubleshooting table for the 250 SX-F and 450 SX-F lists valve clearance too little as a cause of too little engine power.

None of these prove the valves are tight. A worn top end, a fuel problem or a weak spark can look the same, which is why our won't-start guide walks through fuel, air, spark and compression in order. The only proof is a feeler gauge.

Valve Clearance Specs By Model

Every number below is a cold measurement from the owner's manual. Notice how much they differ, and that the exhaust gap is equal to or larger than the intake gap on every bike listed.

Specs change between model years. Honda's 2018 CRF250R manual listed 0.11 mm intake and 0.19 mm exhaust, both ± 0.03 mm, so use the manual for your exact bike.

Why The Job Looks Different On Each Brand

The valve train design decides where you put the feeler gauge and what you swap.

• Bucket and pad. Yamaha's YZ250F uses a valve lifter (bucket) over an adjusting pad. The cam has to come out to change a pad.
• Finger follower. KTM describes the 250 SX-F as DOHC with four valves controlled via cam lever. Finger followers save moving mass compared with buckets, which lets an engine rev harder.
• Rocker arm. KTM describes the 450 SX-F as OHC with four valves controlled via rocker arm.
• Honda's mix. On the 2018 CRF250R both intake and exhaust are measured between the rocker slipper and the cam lobe, with shims under the rockers. On the 2020 CRF450R the intake is measured between rocker and lobe, and the exhaust between the exhaust rocker and its shim.

How To Check Valve Clearance

Yamaha marks this as a job that needs basic service skill and says less experienced owners should not attempt it from the manual alone. KTM's owner's manuals list the spec and interval but leave the procedure to the dealer repair manual. If you have never timed cams, have a shop do the first one with you. Motocross Hideout puts a typical shop bill at about $150 to $200.

1. Start cold. Clearance changes as the engine heats up, so Honda, KTM and Yamaha all specify a cold engine. KTM's numbers are at 20 °C (68 °F).
2. Wash the bike, then open it up. Honda's procedure starts with a clean machine so dirt cannot fall into the head. Remove the seat, tank, ignition coil, spark plug and valve cover.
3. Find TDC on the compression stroke. Turn the crank (Honda and Yamaha both say counterclockwise) to the TDC mark, then check that the cam sprocket marks line up with the head surface. If they do not, the piston is at top on the exhaust stroke, so turn one more full revolution. At TDC compression both lobes point away from their valves.
4. Measure every valve. Slide feeler gauges between each lobe and its bucket or rocker, or between rocker and shim on the Honda 450 exhaust. The right blade slides with a light drag. Write down every reading, even the ones in spec.
5. Compare with last time. One tight reading is an adjustment. A trend of readings that keep closing up is a valve and seat telling you they are wearing out.

Shim Math, With A Calculator

Honda's formula is the clearest version, and it works for any shim-under-follower design. New shim thickness equals the measured clearance minus the specified clearance, plus the old shim thickness. Measure the old shim with a micrometer rather than trusting the number printed on it.

A worked example using Honda's numbers: a CRF250R intake measures 0.05 mm against the 0.10 mm spec with a 1.850 mm shim. The new shim is (0.05 − 0.10) + 1.850 = 1.800 mm. A thinner shim makes the gap bigger, and a thicker one makes it smaller.

Shim sizes come in fixed steps, so you round to the nearest one you can buy.

• Honda offers 73 shims from 1.200 to 3.000 mm in 0.025 mm steps. If the calculated shim is outside that range, Honda sends you to the dealer.
• Yamaha offers 25 adjusting pads from 1.20 to 2.40 mm in 0.05 mm steps. Read the number on the old pad and round its last digit (0 to 2 rounds down to 0, 4 to 6 rounds to 5, 8 and 9 round up to 10, so a pad marked 148 counts as 150), then use the pad selection table in the manual.

On reassembly Yamaha oils the lifters, puts molybdenum oil on the stem ends and checks that each lifter turns smoothly by hand. Honda coats the cam lobes and journals with a mix of half engine oil and half molybdenum grease and checks that the decompressor moves freely. Then turn the engine over by hand a few times and measure again.

Common mistakes: measuring a warm engine, measuring on the exhaust stroke instead of compression, trusting the printed shim size instead of a micrometer, dropping a shim into the crankcase (Yamaha warns about exactly that), and only checking the valve that seemed tight.

When It Is A Valve Job, Not An Adjustment

A shim fixes the gap. It does not fix the wear that closed it. If a valve keeps tightening and you need smaller and smaller shims in a short time, Motocross Hideout's advice is to stop treating it as an adjustment and inspect the valve and seat. The usual fix is new valves and a recut seat, or a new seat if the valve has sunk too far.

Do not lap valves like an old car engine. Honda forbids lapping its titanium valves, and Yamaha says any valve used for lapping a seat must be thrown away and replaced. KTM's schedule for the 250 SX-F motocross bike replaces the valves, springs, spring seats and retainers as part of the major engine service at 90 operating hours.

How Often To Check Valve Clearance

The factory intervals are far apart, from about 7.5 hours on Yamaha's YZ250F race bike to 45 hours on KTM's SX-F, and trail schedules are often counted in miles and months instead. Use the schedule printed for your own bike rather than a number from the pits.

Follow the schedule printed for your own bike and the way you actually ride it. Race schedules assume racing. Motocross Hideout says the YZ250F's every-third-race check fits expert-level motocross racers, that most riders do not need to check nearly as often, and that a 250F bouncing off the rev limiter needs checks far more often than one trail ridden at lower rpm. It also says there is no set number of hours for valve life. Dirt getting past the air filter speeds up valve and seat wear, so dust, sand and mud shorten everything. Your own logged readings are what tell you whether the engine is stable or drifting. In spec means leave it alone, and a valve that keeps tightening is a valve and seat job (Motocross Hideout). A leak-down test is a good second check, since a hiss at the intake or exhaust points to that valve (DIY Moto Fix).

That log is the cheapest diagnostic you have. MotoMind lets you set valve clearance as an hour-based task, shows it as upcoming, due or overdue as you add engine hours, and keeps notes with each service, so you can write down every measurement and see the trend. For how the whole valve train works, read how a four-stroke engine works, and see the 4-stroke maintenance guide for the rest of the routine.

FAQ

How do I know if my dirt bike valves are tight?

The common signs are hard starting, cold or hot, a bike that only bump starts, a hanging idle, popping on deceleration and lost power. Those can have other causes, so the only proof is measuring the clearance with feeler gauges on a cold engine at TDC on the compression stroke.

Do dirt bike valves get tighter or looser?

Almost always tighter. The valve face and seat wear, the valve sinks deeper into the head, and its stem moves toward the cam. A gap that grows usually means a worn lobe, lifter or spring, which Yamaha lists as a cause of top end noise.

How often should I check valve clearance on a 4-stroke dirt bike?

Follow the schedule for your own bike. On the motocross race bikes, KTM SX-F models check after 1 hour and then every 45 hours, Honda CRF250R and CRF450R every 6 races or about 15 hours, and Yamaha YZ250F and YZ450F every third race, about 7.5 hours. KTM’s 450 XCF-W enduro bike is also 45 hours. Honda’s CRF450X trail schedule and its CRF250F play bike check every 600 miles or 6 months. If the clearance is in spec, leave it alone.

Do you check valve clearance hot or cold?

Cold. Honda, KTM and Yamaha all specify a cold engine because clearance changes with temperature. KTM gives its numbers at 20 °C (68 °F).

How do you calculate a new valve shim?

New shim equals old shim plus measured clearance minus specified clearance. A 1.850 mm shim with 0.05 mm measured against a 0.10 mm spec needs a 1.800 mm shim. Round to the nearest size you can buy, 0.025 mm steps for Honda.

Can I ride with tight valves?

Not for long. With no clearance the valve cannot close, compression leaks, and Honda warns this causes valve damage and power loss. Motocross Hideout adds that it can burn a valve or damage the seat.

How much does a valve adjustment cost at a shop?

Motocross Hideout puts a typical shop valve adjustment at about $150 to $200, based on 1.5 to 2 hours of labor. Shims, gaskets and any valve or seat work cost extra.