What is a cambelt and when does it need changing?

Knowing what a cambelt is and when to change one is more important than many people realise; we explain all

Even if you’re not a petrol head, a car enthusiast or a talented amateur mechanic, if you drive a petrol or diesel car, it’s vital to know what a cambelt is, and when it needs to be changed.

What is a cambelt?

An engine is made up of myriad pieces of metal, many of which move about several thousand times a minute.

All internal combustion (IE petrol and diesel) engines have cylinders, in which fuel and air are mixed. Valves open and close at just the right moment to let that fuel and air into the cylinders so they can be ignited, while further valves open so that waste gases can be emitted.

These valves are controlled by camshafts – metal rods with cams, also known as ‘lobes’ (essentially metal bulges). As the camshaft turns, these lobes push down on the valve mechanisms, forcing the valves open against their springs, or releasing them so they can close.

All of this has to happen at precisely the right moment in order for an engine to run properly. If the intake valves don’t open at the right point, the correct amount of fuel and air won’t be allowed into the cylinder; similarly, the valves need to be closed when ignition occurs, while if the exhaust valves don’t open when they need to, the waste gases have nowhere to go. Some engines have twin camshafts, with one controlling the inlet valves and one the exhaust valves, but both will be rotated by a cambelt or chain.

Cambelts are made from specially treated and tough materials like polyurethane, neoprene and rubber, and they are toothed so they grip properly on the gear at the end of the camshaft, around which they are wrapped.

Cambelts are also wrapped around the engine’s crankshaft at the base of the engine, which is the component that is spun (sending power to the car’s wheels) as fuel is ignited in the cylinders. The cambelt therefore sends rotational force from the crankshaft to the camshaft, which in turn opens and closes the valves.

All of this has to be synchronised so that the valves open and close at precisely the correct moment, and this is known as ‘timing’. Engines are timed correctly by ensuring the camshaft and crankshaft are set at just the right positions when the cambelt is fitted.

Why do cambelts need to be changed?

As with any component, cambelts are affected by age and use. Age causes rubber to perish and weaken, while frictional forces cause material to wear.

If the cambelt weakens, it could snap. If this happens, at best the engine will stop running, and at worst the metal valves will collide with the metal pistons, potentially wrecking the engine.

If the teeth on the cambelt wear they can slip, changing their position on the camshaft and/or crankshaft. This is known as ‘jumping a tooth’, and at best causes incorrect timing (meaning the engine will not run correctly), or at worst, again, collision between valves and pistons.

You really don’t want this happening: if valves meet pistons, you’re looking at serious engine damage and significant repair bills – possibly for a new engine.

Part of the reason for this is that many cars have ‘interference’ engines. This means that when they are open, the valves occupy the same space as the pistons – although the valves are only meant to open when the pistons are out of the way. If the timing is wrong or the cambelt snaps, this synchronisation fails, and the valves can meet the pistons at high speed, and with catastrophic consequences.

How often do cambelts need changing?

This will be set by the manufacturer, and should be stipulated in your car’s handbook or service schedule. A typical car might require a cambelt change every five years or 60,000 miles, whichever comes first. You may well be able to find out a specific car’s cambelt change interval with a quick search online – though be sure to use a reputable source like a dealer group’s website. Contact an official dealer for your make of car if you don’t have any joy.

How much does a cambelt change cost?

Again, this will vary from car to car, and from dealer network to dealer network, while independent garages will offer their own pricing.

Some engines’ cambelts are more complex to replace than others, but a starting price of around £400 is about right, though it can be a fair bit more than that, as some cambelts are more complex than others due to how tricky they are to access, or how complicated the engine is. The belt itself is usually a relatively inexpensive component, – you’re paying more for the labour and expertise to get it fitted; be sure any quotes include associated components like cambelt tensioners, though.

It is often recommended that a car’s water pump (which circulates coolant for the engine) is changed when the cambelt is replaced, as water pumps have a finite lifespan, and tend to be easily accessible when the cambelt is being changed.

How can I tell if a car has had its cambelt changed?

A car’s service booklet should detail when its cambelt was changed, but don’t just take it as written if you see the words ‘cambelt changed’ in biro next to a service stamp.

If there isn’t a specific, printed invoice/service record detailing the cambelt service (and arguably even if there is) when you’re looking at second-hand cars, contact the garage to which the work was attributed, asking them to confirm the cambelt was changed when the service book indicates it was.

It’s also worth investigating (the internet is your friend here) what a specific model of car’s cambelt change interval is before you start car shopping.

How do I know if a cambelt needs changing?

Cambelts often give no symptoms when they need replacing, though any unusual noises from the engine should always be investigated. As such, they should be changed at the interval recommended by the car’s manufacturer, regardless of whether the engine is running perfectly or not.

Do all cars have cambelts?

No. Just to make things more interesting, not all cars have a cambelt – some have chains instead of belts – though these tend to be known as timing chains, rather than cam chains.

Timing chains perform the same task as cambelts, but while cambelts are rubber or similar to rubber, chains are metal, and tend to look a bit like a bicycle chain.

Do timing chains need changing?

Timing chains tend to last longer than timing belts, and in some instances they’re classed as ‘lifetime components’ by the car’s manufacturer, indicating that they never need to be replaced.

In other circumstances a manufacturer might recommend a replacement interval for a timing chain, but this will typically after a longer period of time/at a higher mileage than the interval for an equivalent cambelt. As always, consult an official dealer if you’re unsure.

If you’re wondering why timing chains aren’t always preferred to timing chains, it’s for a variety of reasons, including the fact that chains can be slightly noisier than cambelts, while replacing a timing chain can also be more expensive than changing a timing belt, and chains are more expensive to manufacture.

Can I check a cambelt myself?

Not really: you will need to know exactly what you’re doing and what you’re looking for. Some engines have inspection ports for the cambelt, which can be opened without removing the whole timing chain cover. Even if there is an inspection port we wouldn’t recommend this, though, because as well as looking for cracks or missing teeth, you would also be looking for any slackness in the belt, and this requires expert knowledge.

Can I change a cambelt myself?

In short, no. There is no law stopping you from changing a cambelt, but it is a job only trained professionals, or really, really, really competent home mechanics should undertake.

For one thing, even accessing the cambelt can be pretty tricky. For another, if you mess up the timing (IE the camshaft or crankshaft move position relative to each other) when removing the old cambelt or fitting the new one, Mr Valve can meet Mrs Piston when the engine is started; this will be an unhappy and expensive union.

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