What's a Timing Chain in an engine?

A timing belt, timing chain or cam belt is a part of an internal combustion engine that synchronizes the rotation of the crankshaft and the camshaft(s) so that the engine's valves open and close at the proper times during each cylinder's intake and exhaust strokes. In an interference engine the timing belt or chain is also critical to preventing the piston from striking the valves. A timing belt is a belt that usually features teeth on the inside surface, while a timing chain is a roller chain.

Most modern production automobile engines utilize a timing belt or chain to synchronize crankshaft and camshaft rotation; some engines instead utilize gears to directly drive the camshafts. The use of a timing belt or chain instead of direct gear drive enables engine designers to place the camshaft(s) further from the crankshaft, and in engines with multiple camshafts a timing belt or chain also enables the camshafts to be placed further from each other. Timing chains were common on production automobiles through the1970s and1980s, when timing belts became the norm, but timing chains have seen a resurgence in recent years. Timing chains are generally more durable than timing belts – though neither is as durable as direct gear drive – however, timing belts are lighter, less expensive, and operate more quietly.

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A timing chains looks just like the chain on a bicycle, but much cleaner. The beauty of the timing chain is that it typically lasts a very long time. Some car manufacturers recommend replacing it at certain mileage or time intervals, while others claim that the chain is good for the life of the automobile. It really depends on the make and model of the car or truck. The downfall of timing chains is that they are heavier, generally known to be noisier, and if they do break, it is usually catastrophic to all of the surrounding engine parts.

The crank and camshafts work in tandem through a connection created by the timing chain. 

http://en.wikipedia.org/wiki/Timing_belt_(camshaft)

A timing belt, timing chain or cam belt is a part of an internal combustion engine that synchronizes the rotation of the crankshaft and thecamshaft(s) so that the engine's valves open and close at the proper times during each cylinder's intake and exhaust strokes. In an interference engine the timing belt or chain is also critical to preventing the piston from striking the valves. A timing belt is a belt that usually features teeth on the inside surface, while a timing chain is a roller chain.

A chain that connect and sync between the crankshaft and the camshafts.