All current automobile piston engines have a camshaft and a crankshaft. The crankshaft transmits the power from the pistons as they move up and down and changes it to a rotating motion. (This is like the pedals on a bicycle changing the up and down motion of your legs to a rotating motion.) The crankshaft drives the CAMSHAFT, which opens and closes the valves in the engine. A camshaft has eccentric LOBES or CAMS all in a row: one lobe for each valve. This lobe presses on a cam follower or valve lifter which opens the valve.

For each cylinder, the camshaft first opens the intake valve to let in fuel and air to be burned. After this, the camshaft opens the exhaust valve to release the burned fuel gasses, which pass out the exhaust pipe. This all must happen at exactly the right time. This coordination of the crankshaft with the opening and closing of the valves via the camshaft is called Valve Timing or Camshaft Timing.

The camshaft is turned by the crankshaft. 3 different methods are used to connect them together.

1) THE TWO GEAR METHOD
This is the most reliable method. A gear on the crankshaft meshes directly to a gear on the camshaft. These gears last the life of the engine and almost never fail.

2) THE TIMING CHAIN METHOD
Although not as reliable as the gear to gear system, a timing chain still will last 80,000 to 100,000 miles or more before wearing out. As the chain stretches, however, performance will be affected. Also, on many vehicles the camshaft sprocket is made of plastic. When these vehicles are overheated even slightly this plastic can melt and the chain will skip teeth. The engine will stop immediately and will not run again until the chain is repaired. If a valve opens all the way at the same time as the piston comes up, it can bend the valve and possibly destroy the engine.

3) THE TIMING BELT METHOD
This method requires belt replacement every 40,000 to 60,000 miles. The camshaft sprocket is connected to the crankshaft sprocket by a toothed drive belt. This is becoming the dominant method. Over half of the cars on the road use timing belts. There are several reasons for this:
a) It's easier to connect the camshaft and crankshaft together with a belt when the camshaft is far away from the crankshaft (like with overhead cam engines).
b) They claim the belt is quieter. Belts are lighter than chains, and every ounce counts to the manufacturers.
c) It's much cheaper and simpler to make a belt drive engine than any other way.
d) When the belt breaks it can destroy the motor, so it increases new car sales or repairs.