The timing belt kit on a car runs on gears and has the critical function of synchronizing the movements between valves and pistons. The engine valves are opened and closed by arms actuated off the camshaft that is rotated by the timing belt. The timing belt is used intensively and is a key component of the engine. The rupture of this component in operation will cause immediate and extensive damage which will lead to the engine breaking down, and even a serious accident.

The water pump pumps the coolant around the engine, keeping the engine cool. On some vehicles the timing belt also drives the water pump. Both the timing belt and the water pump are a part of an integrated system. Many garages therefore also choose to change both the timing belt and the water pump at the same time, since both wear, and it is inconvenient and more expensive having to change the one part and perhaps the other a half a year later.