Van, Truck, Trailer, Bus and Coach Aftermarket News in Ireland

FAI Automotive offers timing belt advice

Manufacturers are continuously developing new materials to improve the performance of their products and this has led to a significant increase in the service interval for replacing timing belts, according to FAI Automotive.

Timing belts are manufactured using three basic elements: Rubber compound, Cord and Fabric. The ratios of these three components have to be optimised to achieve the required level of performance.


6. Rubber – there are many polymers available but only a few of them are suitable for timing belts. The latest generation is HNBR (Hydrogenated Nitrile Butadiene Rubber) or HSN (Highly Saturated Nitrile) – replacing the previous generation Chloroprene or Neoprene.
7. The transition from Chloroprene to HNBR is the result of exhaustive testing. The main characteristic of this polymer is exceptional heat resistance, preventing the formation of cracks over time.

8. Cord – this is the part of the timing belt that resists the pulling/elongation force. It is made by twisting glass filaments together to form a kind of string. Glass cord has a very high tensile strength but is vulnerable to crimping which can break the thin fibres. This is why all manufacturers recommend avoiding crimping the belt. High tensile strength is essential to reduce belt elongation and is a critical element in ensuring accurate pitch between the teeth of the belt.

9. Fabric – this ensures low belt abrasion and reduces friction between the belt and pulley. Nylon construction with a rubber coating is the standard specification. To cover the entire range of applications, however, there are more than ten fabrics used in a different combinations to achieve the desired characteristics of strength and flexibility to meet specific engine requirements.

Prior to the advent of automatic tensioners the most common way to tension a belt was by a movable eccentric pulley that enabled manual tensioning of the belt. Many belt manufacturers offer a special tool to ensure that this is done correctly

Despite the best efforts of mechanics, this system has two major limitations:

  • The tensioner must be set and the belt tension tested, ideally with a sonic belt tension tester and then adjusted until the system is within manufacturer specifications.
  • The fixed manual tensioner does not allow compensation for engine expansion as it reaches operating temperature, which can significantly increase belt tension.

Therefore, it is essential that, on an engine using this arrangement, the tension is adjusted to avoid problems, such as under tensioning, which can cause the belt to ride over the pulley teeth causing engine damage. Over tensioning, on the other hand, can cause noise and eventual bearing damage.

A variation of as little as 10% in optimum belt tension can reduce the life of a timing belt by upto 40%.

Automatic tensioners are a mechanism that maintains belt tension constantly, regardless of the engine temperature. The tensioner pulley moves and compensates for variations in length due to temperature fluctuations. This consistent tension increases belt life.This system is very successful, provided high quality tensioners are used and the belt is engineered correctly to work with the tensioner.

When replacing a belt, bear in mind that the metal components have completed the same mileage as the belt. Therefore, it is false economy to replace the belt in isolation which is why FAI’s extensive range of timing belt kits all contain the parts necessary to complete the task.