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Timing Belt
The job of the timing belt is to turn the camshaft(s) at exactly 1/2 the speed of the crankshaft while maintaining a precise alignment.  This means that the crankshaft will make two revolutions for every revolution of the camshaft.  Engines will have at least one camshaft, or as many as four camshafts in some of the V-type engines.  The camshaft causes the intake and exhaust valves to open and close in time with the pistons which move up and down in the cylinders.  The valves must open and close at exactly the right time in relationship to the piston movement in order for the engine to run properly.

Timing belts are typically inaccessible and difficult to inspect. Replacement at specific intervals is recommended by the manufacturer. The manufacturer may also recommend the replacement of other parts, such as the water pump, when the timing belt is replaced because the additional cost to replace the water pump is negligible compared to the cost of accessing the timing belt. Failure of the timing belt will leave the engine non-functioning. Depending on the design of the engine, the piston and valve paths may “interfere” with one another and incorrect timing in their movements may result in the piston and valves colliding. (Such designs are also called “interference head” or “interference engines”. Conversely, non-interfering engines are called “free-wheeling” or “non-interference” engines.)

The automobile engine uses a metal timing chain, or a flexible toothed timing belt to rotate the camshaft. The timing chain/belt is driven by the crankshaft. The timing chain, or timing belt is used to “time” the opening and closing of the valves. The camshaft rotates once for every two rotations of the crankshaft.