🔧Valves & clearance

How it works

Valves are the "doors" of the combustion chamber on a four-stroke engine: intake valves let the air-fuel mixture in (or air alone on direct injection), exhaust valves let burned gases out. They're driven by the camshaft, which is turned by the timing drive: the cam lobes push the valves open (directly, or via lifters/rocker arms/tappets) at the right moment, and springs snap them firmly shut against their seats to seal the combustion chamber during compression and combustion. Each valve slides in a guide and seats against a machined seat in the cylinder head. A crucial detail is valve clearance: a small, precise mechanical gap between the cam and the valve compensates for thermal expansion when hot and ensures the valve closes fully. This clearance is adjustable (screw and locknut, or shims) and needs checking at regular intervals, with the engine cold. Failure modes: incorrect clearance (too much = ticking and reduced breathing; too little = a valve that no longer seats fully and burns), burnt valves or seats (loss of compression, popping), worn guides (oil consumption), tired springs (valve float at high revs), worn valve stem seals (blue smoke at startup). Signs of a valve problem: ticking from the top end, power loss, hard starting, misfires, smoke. Sticking to the periodic valve clearance check is the key preventive maintenance job for preserving top-end health.

Maintenance tips

• ✓Stick to the valve clearance check interval recommended by the manufacturer (varies a lot between models): it's the key preventive maintenance job for the top end, and a neglected clearance can lead to a burnt valve.
• ✓Always have the clearance checked and adjusted with the engine COLD (room temperature): a warm engine has expanded parts and will completely throw off the measurements.
• ✓Watch for signs of incorrect clearance: a steady ticking from the top of the engine, louder when cold, often signals too much clearance and a check that's overdue.
• ✓Too little clearance is more dangerous than too much: the valve no longer closes fully, can't cool on its seat, and may burn. Never "fix" a noise by tightening things up on a guess.
• ✓Use a quality oil and stick to oil changes: oil lubricates the camshaft, lifters, and valve guides; degraded oil speeds up their wear.
• ✓While checking clearance, take the opportunity to replace the valve cover gasket if it's leaking, and have the guides and valve stem seals checked if oil consumption is suspected.
• ✓On a shim-type engine, plan ahead: adjustment may require ordering shims of precise thicknesses, which lengthens the job.