Certain engines with overhead camshafts have timing belts that have a limited lifespan. Car makers use these belts instead of more durable chains because chains are noisier andcost more to manufacture. Your vehicle owner's manual will recommend at what mileage the timing belt must be replaced. These intervals range from every 60,000 miles to every 105,000 miles. To see what is recommended for your engine, click on the link at the bottom of this article.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.