A timing belt, timing chain or cambelt is a part of an internal combustion engine that synchronizes the rotation of the crankshaft and the camshaft(s). The engine’s valves open and close at the proper times during each cylinder’s intake and exhaust strokes.
Timing belts must be replaced at the manufacturer’s recommended distance and/or time periods. Failure to replace the belt can result in complete breakdown or catastrophic engine failure, especially in interference engines. The owner’s manual maintenance schedule is the source of timing belt replacement intervals, typically every (50,000 to 80,000 km) It is common to replace the timing belt tensioner at the same time as the belt is replaced.
The usual failure modes of timing belts are either stripped teeth (which leaves a smooth section of belt where the drive cog will slip) or delamination and unraveling of the fiber cores. Breakage of the belt, because of the nature of the high tensile fibers, is uncommon. Often overlooked, debris and dirt that mix with oil and grease can slowly wear at the belt and materials advancing the wear process, causing premature belt failure.
Correct belt tension is critical – too loose and the belt will whip, too tight and it will whine and put excess strain on the bearings of the cogs. In either case belt life will be drastically shortened. Aside from the belt itself, also common is a failure of the tensioner, and/or the various gear and idler bearings, causing the belt to derail.