What is a timing belt or chain?

The timing belt or, depending on your car, timing chain is critical component in the engine, ensuring that the internal components operate in perfect synchronisation.

Without the timing belt or timing chain, the engine wouldn't work properly, and the failure of a timing belt or chain can lead to significant damage. In this article, we'll go into more detail about what they do and what the difference is between them. 

What is timing?

In an internal combustion engine, be it petrol or diesel, precise timing is essential. The pistons must rise and fall, valves must open and close, and spark plugs must fire at exact moments. This coordination ensures optimal engine performance and efficiency, and if the timing is off then the engine could run rough or even mulch itself by slamming components into each other.

The timing belt or chain is key to this essential synchronisation. It links two of the engine's key rotating parts – the crankshaft, which converts the up-down motion of the pistons into rotary motion, and the camshaft, which opens and closes the engine's valves in time with the piston movements.

Timing belt vs timing chain – what are their advantages?

Timing belts, also known as cambelts, are typically made from high-strength rubber with reinforcing fibres. They work quietly and are generally less expensive, both to make and replace. However, they have a finite lifespan, which means you'll need to replace them from time to time. The timing belt is usually located inside a plastic or rubber cover on the outside of the engine and don't need any lubrication.

Timing chains are made from metal, which makes them more durable and usually last longer than timing belts. They use engine oil for lubrication, which extends their life, and are usually located deeper inside the engine than a belt, which makes them less accessible but more robust over time.

How do I know if my timing belt needs replacing?

Manufacturers give specific intervals for timing belt replacement, typically between 60,000 and 100,000 miles, or every five to six years, whichever comes first. As an example, Ford recommends replacing the timing belt every six years or 100,000 miles.

Signs that your timing belt may need replacing include engine misfires or rough idling, or ticking noises from the engine. You may also be able to see visible wear or damage on the belt, and it's worth checking it annually – just in case.

Sticking to the manufacturer's recommended schedule is vital, as a failed timing belt can cause severe engine damage.

How much will a timing belt replacement cost?

The cost of a timing belt (and fitting it) varies depending on the car, but you can expect to pay between £300 and £650, including parts and labour. It's often a good idea to replace the water pump at the same time, as it's usually driven by the timing belt and located in the same area of the engien bay.

How do I know if my timing chain needs replacing?

Once again, the car manufacturer will give instructions on when to replace your car's timing chain. They're designed to last longer than belts and will often do more than 100,000 miles, but they can still wear out or become damaged. Signs that your timing chain needs looking at include a rattling noise from the engine, especially on startup, engine misfires or poor performance, or metal shavings in the engine oil. Some cars will show an illuminated check engine light.

Because the chain is lubricated by your car's oil, regular oil changes can help extend its life.

It's worth noting that a timing chain is only as good as the strongest component it works with. Timing chain tensioners are a known weak spot on some cars – Jaguar used to make them out of plastic – and if they fail, the effect can be just as disastrous as if the chain snaps. The same goes for chain guides. 

How much will a timing chain replacement cost?

While a timing chain is more durable than a belt, it also costs more to replace. The chain itself costs double the price of a belt and it's generally more labour intensive to fit (along with the replacement water pump). The cost can vary between £600 to more than £1200, depending on the car.

What's a non-interference engine?

On most cars, a failed belt or chain will destroy your engine. But thehre are some engines where the pistons and valves don't occupy the same space in the cylinder. This means that even if the timing belt or chain fails, the design prevents the pistons from hitting the valves, thereby reducing the risk of severe engine damage.

Conversely, inference engines have pistons and valves that share the same space at different times. If the timing mechanism fails, the pistons can smash into the valves, which means significant engine destruction.