What is a timing belt or chain?

The timing belt or – depending on model – timing chain is one of the most important components in your car. It's tasked with ensuring your engine and its hundreds of individual components run in perfect unison.

In terms of importance then, not many single parts can have as big an effect – if your belt or chain goes pop, in most cases, your engine (or a significant part of it) is toast. So what's the difference between a timing belt and a timing chain, what exactly do they do, why do some cars have belts while others use chains? For the answers to theses questions and more, join us on a voyage of discovery as we delve deep into the great timing belt/chain mystery. 

What is timing?

Before we plunge into a deep dive on timing belts and chains we must first answer the question: what is timing? 

Well, it kind of does what it says on the tin, in order for your engine to run efficiently – or indeed, at all – everything needs to be timed perfectly. The pistons need to rise and fall, the valves need to open and close and the spark plugs need to fire at exactly the right time. 

This is your engine's mechanical timing. If its out slightly, it could mean your engine runs rough, worse than that and it could lunch itself entirely as its pistons and vales meet in an unholy matrimony. The result? Bent valves and damaging slivers of metal pumped around your engine. It's worth avoiding.

Which is why belts and chains exist, they're the part that keeps the whole shebang in sync.

Timing belt and a timing chain – what are their advantages?

If your car hasn't got a timing belt  then it most likely has a timing chain (we'll leave pushrods for another time, but nowadays they tend to be fitted to old-school V8s). The most obvious distinguishing feature between a timing belt and a timing chain is the construction.

A belt is made from fibre-reinforced rubber that's extremely durable, but also operates quietly and is relatively cheap to replace. A chain? Well, it's made of metal so it is even more durable, but also more expensive to replace and – because it usually runs internally – trickier to inspect, although being plumbed into the engine's lubrication system means they last even longer.  

How do I know my timing belt needs replaced?

Your car's manufacturer will schedule a timing belt change into your car's recommended service schedule, but usually you'll need to replace it every 60,000 miles or three years, whichever comes first. That being said, given how important a timing belt is to the running of your car, it makes sense to inspect it annually – checking for frays, as well as perished and dried rubber. 

Some manufacturers – Alfa Romeo in the '90s for example – were notoriously over confident with the gaps they recommended between cam belt changes, with predictable explosive results.

How much will a timing belt replacement cost?

A timing belt is relatively cheap to replace but it's good practice to change your water pump as well – it's an affordable part that makes sense to do when you're already paying for the labour of removing the belt. A timing belt and water pump kit from a reputable parts supplier such as Bosch will set you back £100 all in, but you can expect to pay double that again on the labour to have them fitted.

How do I know my timing chain needs replaced?

Like with a belt, your car's manufacturer will recommend when your car's timing chain should be inspected and replaced, but you can expect a well maintained chain to have twice the life of a belt. Only on a well maintained car, though. 

As a timing chain is lubricated by the same oil that works around the engine, poor oil quality will greatly reduce the life expectancy of your timing chain. A chain that's on the way out will likely have stretched pushing the timing out of sync and making your engine feel sluggish – known as engine retardation – and flashing up the 'engine warning' light, it'll also likely sound like a box of spanners, buzzing and rattling you round the twist.

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 its out of plastic – and if they go, the effect can be just as disastrous as if the chain snaps. Same goes for chain guides. 

How much will a timing chain replacement cost?

A timing chain may well be more durable than a belt, but it also costs more to replace – the chain itself costs double the price of a belt and it can also be more labour intensive to fit, factor in the replacement water pump and you could be looking at a bill of more than £1000. Although that's vastly preferable to replacing your engine. 

What's a non-interference engine?

On most cars a failed belt or chain will destroy your engine but this isn't always the case, in non-interference engines (the type fitted to early Mazda MX-5s) valves and pistons keep a safe distance from each other so self destruction is impossible. 

Think of it like dancing. You get your non-interference engines, their valves and pistons are like you and your other half at your first school dance – you get within the vicinity of each other, but never too close. Well, not until the smuggled booze takes hold. 

Interference engines are like dancing pros. Their valves and pistons invade each others space with frightening regularity but – like a band's syncronised dance routine – they get close but never touch, moving in and out in unison. That all goes to pot if your belt or chain goes – pistons and vales hit and it's all over – your engine's busted.