Timing chain vs timing belt: what a buyer should know

A timing belt is a rubber part with a replacement date. A timing chain is a metal part designed to last much longer, but it is not maintenance-free: it can stretch, and it depends on clean oil changed on time. For a buyer, that means two different questions. With a belt, ask for the invoice for the last change. With a chain, ask for proof of regular oil changes and listen to the engine from cold.

What both parts do

Every petrol or diesel engine has to keep its crankshaft and camshafts turning in step, so the valves open and close at exactly the right moment. A timing belt or a timing chain links them. If that link slips or breaks, the timing is lost. On many engines the pistons can then hit the valves, which can do serious damage.

So the job is the same. The difference is how each one wears and what that means for the car you are looking at.

Timing belts: a scheduled replacement

A timing belt, often called a cambelt, is a toothed belt made of rubber with reinforcing cords. It usually runs dry behind plastic covers, though some newer engines run the belt in oil.

Belts age with time and miles. That is why each manufacturer sets a replacement interval in years and miles, whichever comes first. A belt that goes past its interval is a risk even if it looks fine, because it can fail without much warning.

The good news is that a belt job is predictable. You know it is coming, you can get a quote and you can plan for it. Our guide to cambelt intervals covers what to check and what a good invoice shows.

Timing chains: designed to last, but not forever

A timing chain is a metal chain, a little like a bicycle chain, that runs inside the engine in oil. Chains are usually designed to last a long time, and many manufacturers do not list a fixed replacement interval for them.

That does not mean a chain never needs attention. Two things wear:

• The chain itself can stretch. As the links and pins wear, the chain grows slightly longer. The timing starts to drift.
• The tensioner and guides wear. A tensioner keeps the chain tight, and on many engines it is pushed by oil pressure. Plastic guides keep the chain on its path. Worn guides or a weak tensioner let the chain slap about.

Oil is the main thing that keeps a chain healthy. Old, dirty or wrong-grade oil wears the chain and its parts faster, and low oil pressure can stop the tensioner doing its job. That is why a chain engine's service history is about regular oil changes, on time and with the right oil.

Signs of a stretched timing chain

The classic sign is a rattle when the engine starts from cold. It is often loudest for the first few seconds, before oil pressure builds and the tensioner takes up the slack. As the wear gets worse, the rattle may last longer or be heard at idle once warm.

Other signs include:

• The engine management light. When a chain stretches, the engine's computer can see that the camshaft and crankshaft are no longer perfectly in step. It may store a fault and light the engine management warning.
• Rough running or poor starting, as the timing drifts.
• A whine or rattle that changes with engine speed.

None of these proves the chain is the cause. A rattle can come from other parts, and the engine management light has many possible triggers. But any of them is a reason to have a mechanic look before you buy.

Why a cold start matters

A warm engine can hide a chain rattle. If you only hear the car after the seller has driven it to meet you, you miss the moment the noise is most likely. Ask the seller not to start the car before you arrive. Put your hand on the bonnet when you get there. If it is warm, ask why.

Then start it yourself and listen from outside, with the bonnet open if you can, for the first ten or so seconds.

How to find out which one a car has

You do not need to name the engine to find out. Use one of these:

1. The handbook or service schedule. If the schedule lists a timing belt replacement at a set interval, the engine has a belt. If it lists none, it may have a chain, but check.
2. A franchised dealer or independent garage. Give them the registration or VIN and ask. They can look up the engine and its schedule.
3. The service invoices. A previous cambelt job on an invoice tells you it is a belt engine.

Be careful with general claims online that a certain model "has a chain". The same model can be sold with several engines, and some use a belt while others use a chain. Check the engine in the car you are looking at.

What the MOT shows

The MOT does not look at the timing belt or chain. The test is done without taking the engine apart.

It does check the engine malfunction indicator lamp, often called the engine management light, on most petrol cars first used from 1 July 2003 and diesel cars first used from 1 July 2008. The DVSA inspection manual makes a lamp that does not work, or that shows a malfunction, a major defect. So a chain fault that has lit the warning can show up as a fail. Our guide to dashboard warning lights covers this, and the MOT history guide shows how to read past results.

What to ask the seller

For a belt engine:

• When was the belt last changed, at what mileage, and can I see the invoice?
• Was the water pump changed at the same time?

For a chain engine:

• How often was the oil changed, and can I see the invoices?
• Has the chain, tensioner or any guide been replaced? Is there an invoice?
• Has the engine management light ever come on?

For either, ask whether the car has been serviced to the manufacturer's schedule. Our guide to full service history explains how to check stamps and invoices.

If the answers are vague, or you hear a rattle from cold, see the car and ask a mechanic before you agree a price.

Frequently asked questions

Is a timing chain better than a timing belt?

Neither is simply better. A belt needs replacing on a schedule, which is a known cost. A chain is designed to last longer but can stretch, especially if oil changes were missed. What matters is how the car was looked after.

Do timing chains need replacing?

There is often no fixed interval, but chains, tensioners and guides can wear out. A rattle from cold or a stored timing fault are signs it may be due. A mechanic can confirm.

How do I know if a car has a timing belt or a chain?

Check the handbook or service schedule. If it lists a belt change at a set interval, it has a belt. A franchised dealer or garage can also look it up from the registration or VIN.

What does a stretched timing chain sound like?

Usually a rattle for a few seconds after a cold start, before oil pressure builds. As wear gets worse, the rattle can last longer or be heard once the engine is warm.