Timing belt or timing chain: what matters when you buy used

The crankshaft turns because the pistons turn it. The camshaft opens the valves. The two have to run in a fixed ratio to one another — in a four-stroke engine the camshaft turns at half crankshaft speed, and not approximately but to within a few degrees. That link is made either by a toothed belt or by a chain.

The belt is a reinforced polymer strap with teeth that runs dry over sprockets and is held in tension by an idler. The chain is a steel link chain that runs in engine oil, guided by plastic rails and tightened by a hydraulic tensioner that takes its pressure from the oil circuit.

Everything else follows from that. The belt is a wear item with an appointment: it gets replaced before it breaks. The chain is not a service item, but it is not everlasting either — it stretches, and its tensioner and guides wear. The difference is not durable versus fragile, it is planned versus unannounced.

For the belt the manufacturer names two limits: a distance and a time. Whichever comes first applies. The time limit is the one that gets overlooked — a car that has covered little ground in eight years has an eight-year-old belt, and polymer ages whether or not it is turning. Which figures apply to a given engine is in the maintenance schedule; two engines from the same family can differ.

A belt change is never just the belt. Tensioner and idler pulleys go with it, because their bearings have the same years behind them and one seizing bearing will destroy the new belt in short order. Where the water pump is driven by the belt — true of many engines, not all — it goes too, because its failure takes the same belt with it and the labour is identical anyway.

An invoice that says only “timing belt replaced” is therefore half an answer. The complete one names the date, the mileage and each part fitted.

A timing chain does not stretch because the steel elongates but because pins and bushes wear at every link. A hundred links with a fraction of a millimetre of play each add up to enough to retard the camshaft against the crankshaft. The tensioner takes it up until it runs out of travel.

Two things are audible before that point. The first is a brief metallic rattle in the first seconds after a cold start: the hydraulic tensioner has bled down overnight, and the slack chain knocks against its guide until oil pressure builds. The second is a rattle that follows engine speed and no longer goes away.

The third sign is in the fault memory. The control unit compares the crankshaft and camshaft sensor signals and logs a fault when their positions no longer agree — usually described as a correlation fault. It can also come from a phaser or a sensor; the chain is the most expensive of the possible causes and therefore the one you want ruled out first.

If the belt breaks or the chain jumps a tooth, the camshaft stops and the crankshaft keeps turning. Whether that becomes engine damage rests on a single design decision: do valves and pistons share the same space at different moments, or not. In the first case — normal for modern high-compression engines — the piston meets the open valves.

The result is not a belt job but a cylinder head: bent valves, often damaged guides, in the bad case a marked piston. The difference between the scheduled change and the failure is therefore not a surcharge of a few per cent but a different order of magnitude.

Whether a particular engine is of the non-interference kind cannot be inferred from displacement or model year. It is in the workshop data for that specific engine code, and the answer is worth a phone call before you write off an overdue belt as a mere bargaining chip.

A third design is now common enough to be worth knowing: the wet timing belt. It sits inside the oil space and, depending on the engine, drives the camshaft, the oil pump or both. The idea behind it is lower friction than a chain in a similar package.

The price is a dependence on the oil that a dry belt does not have. A belt in oil is designed for one oil specification; the wrong oil, or one left in too long, attacks the material. What comes off does not stay in the belt drive but travels with the oil — and the oil pump pickup screen is where it collects. A belt problem then becomes an oil pressure problem.

In practice, for a purchase: on an engine with a wet belt, the oil grade and change interval are not a side issue but part of the belt question itself. The previous owner’s invoices should name the specification, not just the words “oil change”.

A stamp in the service book is a hint, not an invoice. Anyone who has paid for a belt change also has a document about it, and that document says more than the workshop’s name.

A belt change that is due is not a defect but an appointment. It still belongs in the price, because it is coming to the buyer and because its size can be established: one call to a workshop with the engine code and model year is enough for a quote that works as evidence. Evidence moves a price; an assertion does not.

Conversely, a freshly changed belt with a complete invoice is a seller’s argument, and a fair one. If you are selling, put the document in the listing instead of hunting for it when asked.

And for the chain: a rattle that lasts longer than an instant is not a bargaining item but a reason to end the viewing and look at another car — unless a quote is already on the table and the price has been set around it.