Why valve clearance is critical
Valves open and close thousands of times per minute. When hot, metals expand. Valve clearance is the gap left between the cam and the valve (or bucket) to compensate for this expansion.
- Too much clearance: the valve opens less, the engine loses efficiency, you hear a ticking noise when cold
- Not enough clearance: the valve doesn't fully close when hot, combustion gases leak, the valve overheats and risks burning
Poorly adjusted clearance means an engine that loses power, uses more fuel, and wears prematurely.
Two adjustment systems
Shim system (shim-over-bucket / shim-under-bucket):
- Used on most modern Japanese motorcycles (Yamaha, Honda, Kawasaki, Suzuki)
- Calibrated shims are placed on or under the buckets
- To change the clearance, you measure the current gap, calculate the required shim, and swap it
- Advantage: stable over time, no loosening
- Disadvantage: requires camshaft removal to access shims (on shim-over-bucket systems)
Screw adjuster system:
- Used on some singles, V-twins (Ducati, some Honda), classic bikes
- A rocker arm with an adjusting screw and lock nut
- To change the clearance, loosen the lock nut, turn the screw, re-tighten
- Advantage: quick adjustment without heavy disassembly
- Disadvantage: can loosen over time
Typical clearance values
Values vary by model, but typical ranges are:
- Intake: 0.10-0.20 mm
- Exhaust: 0.20-0.30 mm
Exhaust clearance is always larger than intake, because exhaust valves run hotter (in direct contact with burnt gases).
These values are indicative. Every engine has its own specifications — an MT-07 doesn't have the same clearances as a CB500F. Always check the workshop manual.
