The majority of cars utilise cam belts, often referred to as timing belts. These belts form an integral part of the engine and are vital for its function. Manufacturers recommend changing these are certain mileage/kilometres and this varies from make to make, or if it has reached a certain age without reaching the required mileage/kilometres. It is common to replace the timing belt tensioner at the same time as the belt is replaced.

In modern engines cam belts and matching components are subjected to a great deal of stress. High rotational speeds, vibrations and extreme temperatures can easily have a negative effect on the functioning of parts. Cracks below the metal surface, overheating and worn-out sealing rings of ball bearings lead to increased metal wear and even the jamming of pulleys. So they can easily cause belt damage. If a timing belt snaps when the engine is running, the engine can sustain very costly or irreparable damage as some valves will be held open when they should not be and thus will be struck by the pistons.

 

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 unravelling of the fibre cores. 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.

In the internal combustion engine application, the timing belt connects the crankshaft to the camshaft(s), which in turn controls the opening and closing of the engine’s valves. A four-stroke engine requires that the valves open and close once every other turn of the crankshaft. The timing belt does this. It has custom teeth to turn the camshaft(s) synchronized with the crankshaft and is specifically designed for a particular engine. In some engine designs, the timing belt may also be used to drive other engine components such as the water pump and oil pump.

A timing belt is typically rubber with high-tensile fibres running the length of the belt as tension members.

Rubber degrades with higher temperatures and with contact with motor oil. Thus the life expectancy of a timing belt is lowered in hot or leaky engines. Newer or more expensive belts are made of temperature resistant materials such as “highly-saturated nitrile” .

Older belts have trapezoid shaped teeth leading to high rates of tooth wear. Newer manufacturing techniques allow for curved teeth that are quieter and last longer.

Timing belts are typically inaccessible and difficult to inspect. The manufacturer or your mechanic may also recommend the replacement of other parts, such as the water pump, when the timing belt is replaced because the additional cost to replace the water pump is negligible compared to the cost of accessing the timing belt.