Yes-most manufacturers quote milage AND age.

I would like to agree with the previous comment, but in my experience the optimum is somewhere in between. Having been involved with diesel engine development, I would make the following comments:

The belts are indeed very durable and if correctly rated for their duty will practically last the life of an engine - PROVIDED nothing else in the drive train fails or degrades. This is the real problem.

We can neglect pressure lubricated cam and crank bearings - but not their oil seals. A leak here will kill a belt. Idlers and tensioners are usually sealed ball race bearings, grease lubricated. The grease will harden with age (engines are hot places) as the volatiles evaporate off. Seals will become less effective as their polymer running edges harden with age. Water pumps (some are cambelt driven) can develop leaks at the seal which usually means some water ends up in the spindle bearing. Finally, some designs are just unfavourable. The tiny anti-flutter roller which VAG fit to some of their diesel drives is too small and so, although very lightly loaded, it runs at an insane speed. Over time, the grease is centrifuged out of the bearing and an old one will rattle when you take it off.

Generalisations are difficult, but in view of my experience of the above, I would not leave any camshaft drive unchanged for the normal life of an engine. The VAG "4 year" rule is just a money making scam, but I would seriously consider changing all of the drive components after 6 years. Mileage depends on the design - which varies a lot but one point to bear in mind is that you can arrange an integer number of belt changes for your period of ownership if you know roughly at what mileage you intend to sell the vehicle.

659.

Edited by 659FBE on 24/02/2011 at 15:08

There's not a lot of measurement evidence to suggest that the belt is more highly stressed under cranking or starting conditions. If the tensioner design is sub-optimal the belt may be loose with a cold engine, leading to the possibility of tooth shear.

The real problem other than the failure modes of the components described above, is drive resonance. It's very difficult to arrange not to have a resonance at some useful engine speed and high forces can be generated in the drive under these conditions. It is an important part of engine development to seek out and limit such resonances. Damped sprung tensioners such as the Litens are an excellent way of both regulating belt tension over a wide range of conditions and limiting potentially destructive resonances.

659.

Edited by 659FBE on 24/02/2011 at 16:11