Camshaft

How it works

The camshaft opens and closes the valves at the right moment. It is a shaft with oval lobes, the cams, that press on the valves directly or through finger followers, rockers or buckets. It is driven by the crankshaft through the timing drive (chain, belt or gears) and turns at half engine speed on a 4-stroke. Many bikes have two overhead camshafts: one for intake, one for exhaust. The lobes and journals are constantly lubricated by engine oil: lack of oil, degraded oil or low pressure quickly wear the surfaces, which changes valve lift and costs power, sometimes with a metallic ticking from the top end. The camshaft's timing relative to the crankshaft must be exact: if the cam chain jumps a tooth, valves and piston can touch. Valve-clearance checks and adjustment are done with the camshafts in place, at the interval set by the manufacturer.

Maintenance tips

• ✓Change the oil on time: oil quality directly governs cam wear.
• ✓A regular ticking from the top end can mean out-of-spec valve clearance or wear: have it checked.
• ✓Follow the valve-clearance check interval in your manual.
• ✓If the cam chain is noisy, don't wait: a jumped tooth changes camshaft timing.
• ✓Setting valve timing needs precise marks and tools: it is workshop work.