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A timing belt (cam belt) is made of rubber usually reinforced with glass fibre strands or similar flexible cords. The belt has equally spaced rubber teeth on one side which mesh with teeth on the crankshaft pulley, camshaft pulley and on a diesel engine the fuel pump pulley. The reverse side of the belt is smooth rubber which provides a surface for the belt tensioner pulley and guide pulleys to act on. The water pump may also be driven by the timing belt using either the toothed or smooth side of the belt.
The timing belt does not require lubrication so therefore can be mounted on the outside of the engine.
A timing chain is a chain made of metal. They can be either a single row or double row design. The chain fits around sprockets on the crankshaft, camshaft and other driven items such as the fuel pump and any guide sprockets. The timing chain must also be tensioned, and this is achieved by either a spring-loaded tensioner or a hydraulic tensioner using oil pressure from the engine.
The timing chain sits inside the engine and requires constant lubrication to ensure smooth and quiet running.
The timing belt should be replaced according to the vehicle manufacturers recommended time / distance. If you have no record of the timing belt being replaced at the correct intervals it is always worth having the timing belt replaced (belt failure can cause engine damage).
Due to its design timing belts can degrade and stretch with age. They may also degrade if they become contaminated with oils and other fluids. For that reason, it is important to ensure that the engine remains free of oil leaks.
The timing chain does not usually have a recommended interval for changing. When the chain begins to rattle that is an indication of the time to replace the chain (chain failure can result in engine damage).
It is important with a timing chain to always ensure that you have regular engine oil and filter changes, in order to prolong the life of the timing chain (contaminated and degraded oil can result in timing chain tensioner failure thus resulting in chain rattle or breakage).
It is not possible to check a timing belt for wear. The belt fails due to the reinforcing strands inside the belt becoming weakened. There is nothing to see on the outside of the belt until it snaps!
The timing chain cannot be physically checked either although you would be advised if there is any sign that the timing chain is rattling.