Cam chain (timing chain)

The cam chain, or timing chain, links the crankshaft to the camshaft(s). In a four-stroke engine the camshaft turns at half crankshaft speed, and the chain keeps valve timing exactly in step with the piston.

The chain is kept taut by a cam chain tensioner (manual, automatic or oil-pressure operated) and guide rails. As the chain stretches or the tensioner weakens, the chain starts to rattle – a metallic clatter from the top of the engine, especially on a cold start or at idle. A badly stretched chain can jump a tooth and let the valves hit the piston.

The chain is inspected and replaced together with the guides and, if needed, the tensioner. When setting the timing, align the crank and cam marks carefully according to the service manual.