A timing belt is a toothed belt that connects the engine crankshaft to the camshaft or camshafts as you can see in the picture. 

A timing belt synchronizes the camshaft to the crankshaft position, so the valves will open and close at the proper timing in relation to the position of the pistons. 

The camshafts rotate at exactly 1/2 speed of the crankshaft; meaning two revolutions of the crankshaft are equal to one revolution of the camshaft.

In some engines a timing belt can also drive additional components such as a water pump, balance shaft, intermediate shaft, injection pump and an oil pump. 

A balance shaft, an intermediate shaft and an injection pump must also be synchronised with a crankshaft.

To work properly a timing belt needs to be under certain tension that is controlled by a timing belt tensioner. Some older cars have an adjustable timing belt tensioner that must be re-adjusted if the timing belt gets loose.

Newer cars have an automatic timing belt tensioner that doesn't need any adjustment. If the timing belt gets loose, it may skip a tooth and the proper timing will be lost.

Timing belt

Timing belt or timing chain?

Not all cars have a timing belt - many newer cars use a timing chain instead of a belt. A timing belt wears out over time and needs to be replaced at a certain mileage. A timing chain can last as long as the engine itself and doesn't need to be replaced unless there is a problem with it.

If you don't know if your vehicle has a timing belt or a chain, you can check your owner's manual. If you don't have an owner's manual, many car manufacturers offer to download it from their websites.  You also can call your local dealer service department or ask your mechanic - they have this information.

When a timing belt must be replaced?

A timing belt must be replaced at recommended by the manufacturer intervals, usually from 60,000 miles to 105,000 miles (from 96,000 km to 168,000 km). Honda also specifies the time interval (every 84 months).

In addition, the timing belt must be replaced if it has any damage like cracks (see the picture), cuts, or if it is soaked in oil leaking from the engine, or if it shows any signs of excessive wear.

If not replaced in time, the timing belt can break. If you are approaching the mileage for a timing belt change, or if you bought a used car and you don't know if the timing belt has been replaced, it's a good idea to have your mechanic to inspect the timing belt condition.

This timing belt is worn out and has many cracks

What happens if the timing belt breaks?

If a timing belt breaks, the engine will no longer work. If a timing belt breaks while driving in an interference engine, the camshaft stops turning leaving some of the engine valves in the open position. 

The heavier crankshaft will continue rotating by inertia, moving pistons up and down. This will cause the pistons to strike the valves that left open. This may result in a heavy damage to the engine with broken or bent valves, damaged pistons and, possibly, destroyed cylinder head and block.

The damage will be less extensive in a non-interference engine but in either case, the engine will stall, leaving you stranded.

Q: How can I set the timing belt