Although I'm satisfied that I understand the strengths, limitations and smoke-screens associated with VAG and their timing drives, I have a few further thoughts and observations:

HJ is quite correct in his comments concerning plastic tensioner wheels. Often made by FAG, these parts almost invariably fail before the belt lets go, with dry and loose bearings. To the best of my knowledge, plastic tensioner wheels were only ever fitted to petrol engines - all the diesels use metal wheels.

With reference to the above post, the static tension provided by the tensioner wheel is a small fraction of the running tension. The need for good quality parts is very evident and should be carefully considered before purchasing replacement parts. In my case, all parts provided in the Contitech belt kit were of exactly the same manufacture as the originals.

If the time limitation were down to degradation of the rubber in the belt, we have a few inconsistencies. There are only a few belt suppliers and most of them supply belts to most vehicle makers. Some of these vehicle makers "allow" a 10 year period for a belt to remain in operation. If the time in service were as short as 4 years, the belts and packaging would need to be date coded, with a "use by" date to guarantee satisfactory longevity.

Nobody does this of course, and it's a nonsense.

So, I'll stick to my theory of a VAG scam, but HJ's comments concerning the plastic tensioner are absolutely valid - they're rubbish. M-K obviously saw a 4 year blanket replacement scheme as a potential gold mine (probably after being hacked off by the introduction of variable servicing, which robbed the dealers of revenue) and just couldn't resist introducing it.

It's a clever scheme, because it doesn't affect the fleet market who will trade after 3 years or cover the mileage anyway. It targets the "worried" private owner and catches them just before the point at which their pride and joy would be relinquished to the independent sector for all future work. There's still just enough "dealer knows best" loyalty left at 4 years for this scheme to work.

For me, on a PD diesel 6 years or 60k miles will do fine - and I'd be happy to let it go over the mileage quite a bit if changing it didn't suit me at the time.

659.

This is how the first line of my previous post should have read

Having owned a food shop for 22 years and knowing about use by and best before dates I thought about that

Not a cambelt - but a Poly V belt on the 1.3 Multijet fitted to my Punto.

It`s on an automatic tensioner - so that rules out different tensions, but the original was badly cracked all around the internal area, at 2.5 yrs. I replaced it with an original belt (same part no) from Fiat and it`s showing not one crack a further 2 yrs on.

Same mileage at around 16,000 miles. It spent the first 6 months of it`s life sitting on a runway somewhere during the Summer 05. That accounts for the extra 6 months on the first 2.5 yrs.

The original was hard unsupple and cracked it looked like it had done 100,000 miles. It`s replacement looks supple and new on the internal ribbing - despite a very similar mileage and actual time on the road.

Edited by oilrag on 16/04/2010 at 22:06