What is a timing belt?

A timing belt is a toothed belt that connects the engine crankshaft to or camshafts, as you can see in the pictures on this page.

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 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.

How does the timing belt work?

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

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 camshaft rotates 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, an intermediate shaft and an injection pump must also be synchronised with a crankshaft. 

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

When a timing belt is replaced, it’s very important to set the timing properly. Before a new timing belt is installed, the crankshaft, the camshaft and other components synchronised with a crankshaft must be aligned in a certain way. Improperly set timing will cause a variety of problems such as lack of power, vibration, misfiring, etc.

When a timing belt must be replaced?

A timing belt must be replaced at intervals recommended by the manufacturer, usually from 60,000 miles to 105,000 miles. You can find the recommended replacement interval in your owner’s manual. In addition, the timing belt must be replaced if it has any damage like cracks, 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.

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 to rotate 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, possible, 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.