Timing Belt or Timing Chain: When to Replace and What If It Fails

Belt or chain: what is the difference?

A timing belt is a toothed rubber belt, usually kept dry behind a cover. It is quiet and light, but rubber ages, so it has a replacement schedule. A timing chain is a metal chain running inside the engine, lubricated by engine oil. Chains are often expected to last longer, though they are not necessarily for life.

Chains can stretch, and the tensioners and guides that keep them tight can wear. Rattling at start-up or a timing fault code can be a sign that a chain system needs attention.

• Belt: rubber, replaced at set intervals, normally outside the engine
• Chain: metal, runs in oil, often longer-lived but can still wear
• Tensioners, guides and idler pulleys wear in both systems
• Neither should be ignored if the engine makes new noises

When should a timing belt be replaced?

There is no single answer, because manufacturers set different intervals by mileage and by age, and some engines are more demanding than others. A car that has covered few miles still needs its belt changed on age, since rubber deteriorates with time as well as use.

The reliable guide is your handbook or the service schedule for your exact model. If you have bought a car and have no record of the belt ever being changed, treat it as unknown and have it looked at. Guessing is not worth the risk.

Why the water pump is often changed at the same time

On many engines, the water pump is driven by the timing belt or sits behind the same cover. Getting to the belt means taking apart most of what is needed to reach the pump, so replacing both at the same time adds relatively little labour and avoids doing the job twice.

A pump that fails later can leak coolant onto the belt or seize and take the belt with it. Tensioners and idlers are usually changed with the belt, along with the coolant if it is due. At Banks of the Tyne in Hebburn we fit timing belts, timing chains and water pumps, and we will explain what is included in a kit.

What happens if it fails?

On a non-interference engine, a snapped belt stops the engine, and that is the end of it. On an interference engine, which many modern engines are, the pistons and valves share the same space at different times, and when the timing is lost they can collide. The result can be bent valves, damaged pistons or a damaged cylinder head.

Which kind of engine you have depends on the model, so we will not guess here. If you are not sure, ask. The consequence of failure is why intervals matter.

• Engine stops suddenly, often without warning
• Bent valves and damaged pistons are possible on interference engines
• Repair can mean cylinder-head repairs or an engine rebuild
• Recovery is usually needed because the engine will not restart

Warning signs and what to do

A belt rarely gives much notice. Possible clues include ticking or rattling from the front of the engine, a squealing noise, a belt cover that is cracked or oily, difficulty starting or poor running. Oil leaks near the belt cover can also soften the rubber.

If you hear something unusual, stop driving the car and get it looked at. Catching a worn belt, chain or tensioner early is far cheaper than dealing with an engine that has failed on the road.

Frequently asked questions