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Timing belts are reinforced bands of rubber used to coordinate the turnings of crankshafts and camshafts in internal combustion engines. The inner surface of most timing belts have teeth which correspond precisely to the gears of a specific engine design. In earlier automobiles, timing belts were only responsible for synchronizing camshafts and crankshafts, but modern engines often use them to drive water pumps as well.
Timing belts play a vital role in keeping the valves and pistons working in a precise order. Each valve and piston combination must push down on the individual cams on a camshaft at just the right moment. Think of a bicyclist waiting until one pedal has reached the top before he can bear down on it again. If he tried to push down on the other pedal too soon, his foot might fall through or the pedal could break off. Timing belts work to prevent the pistons and valves from pushing on the camshaft before it has reached a full cycle.
Timing belts are engineered to last at least 60,000 miles, with some newer designs lasting the life of the engine. Vehicle owners are urged to replace timing belts according to the car manufacturer's recommendations. If a timing belt fails completely, the entire engine will grind to a halt and the car will become inoperable. In a worst case scenario, the valves and pistons may drive themselves into the camshaft and become hopelessly bent or broken. There is usually very little warning before timing belts fail, so a timely replacement may be the best prevention.
Replacing timing belts is generally not a job for amateur mechanics. The belts are generally inaccessible without removing quite a few other peripheral parts, like the distributor and possibly the water pump. The replacement timing belts must match the old ones precisely, which may require a time-consuming custom order. Once the timing belt itself has been replaced, the mechanic must synchronize the camshaft and the crankshaft with a special strobe light. Repairing broken timing belts can take several hours, and the vehicle will have to be towed into the shop.
It is not unusual for a mechanic to suggest replacing the water pump at the same time as the timing belt, even if there is no obvious damage. It is not a requirement to take this advice, but it may pay off in the long run. Almost all of the labor required to replace a water pump has already been accomplished while replacing the timing belt. If the water pump should fail later on, the mechanic would have to go through all of those preliminary steps again. Timing belts may only have to be replaced once in a car's lifetime, so it may pay to take advantage of the offer if the added expense of a water pump is not a concern.