When does a cambelt need changing, and what happens if it goes?

Why a cambelt is replaced on a schedule rather than when it looks worn, why the time limit matters as much as the mileage, and how to find out whether yours has ever been changed.

Difficulty: beginner
Time: 12 min
Read: 4 min
Safety: caution

Short answer

Cambelt intervals are set by the manufacturer as a mileage or an age, whichever comes first, and the age half is what catches out low-mileage cars — rubber degrades sitting still. There is no reliable way to inspect one and judge it, so if there is no invoice proving it was done within the interval, assume it has not been. Have the water pump and tensioner done at the same time; the labour is most of the cost.

The cambelt — timing belt — keeps the top of the engine turning in exact step with the bottom. On the great majority of modern engines, which are what the trade calls interference designs, the valves and the pistons occupy the same space at different moments. If the belt snaps, that timing is lost instantly and the pistons hit the open valves. The result is bent valves at best and a wrecked cylinder head or engine at worst, from a part that costs a fraction of that to replace on schedule. It is the clearest example in car ownership of a job where the whole value is in doing it before anything is wrong.

Safety

A cambelt is a scheduled replacement item, not a wear item you can inspect and judge. On most engines a snapped belt causes immediate, severe internal damage that usually costs more than the car is worth. If you do not know whether yours has ever been done, treat that as overdue rather than as probably fine.

Step by step

Find out whether your engine has a belt or a chain.A timing chain runs in oil inside the engine and is generally designed to last the engine's life, though not always. A belt is rubber, runs behind a plastic cover, and has a replacement interval. Your handbook, the manufacturer's site or any garage can tell you which you have from the registration.
Look up the interval for your exact engine, not your model.The same car can be sold with several engines on quite different intervals. Get it against your registration or engine code.
Check both halves of the interval.It will be expressed as a mileage or a number of years, whichever comes first. A ten-year-old car with 40,000 miles is usually overdue on age even though the mileage looks low.
Search the service history for the invoice.A cambelt change is a big enough job that it is always invoiced and usually stickered under the bonnet. If you cannot find evidence, it counts as not done — a verbal "I think it was done" from a previous owner is not evidence.
Have the water pump and tensioner replaced with it.On many engines the water pump is driven by the same belt and sits behind the same covers, so replacing it costs little extra now and a second full labour bill later. A failed tensioner or pump takes the new belt with it.
Keep the paperwork somewhere you will find it.It is one of the first things a buyer asks about, and being able to produce the invoice is worth real money at sale.

Common mistakes

Assuming a belt can be inspected and passed — Belts fail from internal fatigue and heat ageing, not from visible cracking. A belt that looks perfect can be one journey from failing, which is exactly why the interval exists.
Doing the belt and leaving the old water pump — If the pump fails a year later, you pay the same large labour bill twice, and a pump that seizes can destroy the new belt on the way.

If it doesn't work

Buying a used car and the belt history is unknown

Cause: No invoice, or a seller who is vague about it — Fix: Price the job and negotiate it off, or have it done immediately after purchase. It is the single most common expensive surprise on a used car.

Rattling or whining from the timing cover area

Cause: Failing tensioner or idler pulley — Fix: Stop driving it and get it looked at. A bearing that seizes will shred the belt, and the noise is the warning you get.

Engine turns over but will not start, suddenly

Cause: Possible snapped belt — Fix: Do not keep cranking it. Repeated cranking with a broken belt causes more contact damage. Have it recovered and diagnosed.

Car has a chain, and there is a rattle for a second or two on cold start

Cause: Chain tensioner losing oil pressure when the engine stands — Fix: Get it investigated. Chains are not all lifetime parts in practice, and a brief cold rattle is the classic early symptom of a stretched chain or failing tensioner.

Questions people ask

How do I know if my engine would be damaged by a snapped belt?

Most modern engines are interference designs and would be. Some older, lower-compression engines are non-interference and simply stop. A garage can tell you which yours is, but the practical advice is identical either way, because even on a non-interference engine you are stranded.

The car has done very few miles. Do I still need to do it on time?

Yes. Rubber ages by heat cycling and oxidation whether the car moves or not, which is why the interval has a years figure at all. Low mileage is the most common reason a belt is left far too long.

Is it worth doing early if the car is due to be sold?

Usually yes if it is close to due, because an unproven belt is the first thing a buyer discounts for, often by more than the job costs.