What does a "timing belt" do?
A timing belt, timing chain or cam belt is the part of your engine that synchronizes the rotation of the crankshaft and the camshaft(s) ensuring that the engine’s valves open and close at precisely timed intervals.
Your engine’s valves operate in such a way as to allow fuel and air into and out of the engine’s combustion chamber as part of the combustion process. Where combustion (the normal ‘explosion’ of fuel and air being ignited in your engine) occurs – the valves push the pistons in your engine up and down, providing the power to drive your car forwards.
If the valves and pistons do not open and close at the right times due to a timing belt issue, they can collide with each other. As the pistons are larger, where this happens they will damage the valves, potentially locking up your engine.
How do I know when to replace a timing belt
Unfortunately there isn’t an easy way to know if a belt needs replacing just by looking at it. To the untrained eye there may be no obvious signs that there is a problem or that your belt may be about to fail – it can just happen.
For this reason most manufacturers will have guidelines for replacement of timing belts / chains – but a reasonable rule of thumb is to replace every 60,000 – 100,000 miles.
I've heard replacing a timing belt can be expensive
Replacing a timing belt will typically cost 300 to 500 euros depending on your vehicle. Where a timing belt is replaced it’s common practice to also replace the water pump.
However, if your timing belt breaks and damages your engines’ valves, pistons or water pump the cost can grow to twice or three times that amount. If you’re really unlucky the engine (and usually also the car) can be a write-off.