Timing Chain

A timing chain performs exactly the same essential task as a timing belt, mechanically linking the crankshaft to the camshafts so that valve movement stays perfectly synchronised with the motion of the pistons. The difference lies entirely in its construction and consequent durability: where a belt is made of reinforced rubber, a timing chain is a metal chain, broadly similar in form to a bicycle or motorcycle chain, running on toothed sprockets at each end. This metal construction is the source of its principal advantage.

The chain meshes with sprockets on the crankshaft and camshaft and is kept under correct tension by a hydraulic tensioner, which is fed with pressurised engine oil and takes up slack as the chain wears slightly over time. Guide rails, usually faced with low-friction plastic, hold the chain on its path and damp its motion. Because the chain runs inside the engine, it is continuously bathed in and lubricated by the engine's oil, which is fundamental to how it achieves its long service life.

The great practical benefit of the chain is longevity. A well-designed and properly maintained timing chain is generally expected to last the entire working life of the engine with no scheduled replacement, sparing the owner the recurring expense and inconvenience of the belt-change service that rubber-belted engines require. This durability, along with the security of a component far less likely to snap suddenly, is why many manufacturers favour chains, particularly on engines intended for high mileage or heavy duty.

The chain is not entirely without drawbacks, however. It is heavier, noisier and more expensive to produce than a belt, and its reliance on the engine's oil for both lubrication and tensioner operation is also its chief vulnerability. Infrequent oil changes, the wrong grade of oil, or a sludged engine can starve the tensioner and accelerate wear on the chain and its plastic guides. A worn or slack chain commonly announces itself with a distinctive rattle, often most audible on a cold start, and if ignored it can stretch enough to jump a tooth and throw the valve timing out, with the same potentially destructive consequences as a failed belt in an interference engine.

In essence, the choice between a timing chain and a timing belt is a long-standing engineering trade-off. The belt is quieter, lighter and cheaper but must be replaced periodically; the chain is more robust and effectively maintenance-free in normal use but demands diligent oil care and accepts a little more noise and weight. Both ultimately serve the identical, critical purpose of keeping valves and pistons in step.