KV6 timing belts: is the tensioner the problem?
 

I now have a Gates timing belt kit K015527XS and so I am able to compare a new primary belt with that which has covered 90,000 miles.

Here is a general view of both. The old belt, shiny in appearance is at the top.

https://www.the75andztclub.co.uk/for...09abff3d21.jpg

Then, using vernier calipers I measured the thickness of the backing and here are the results. First the original factory fitted belt:

https://www.the75andztclub.co.uk/for...09ac0353dc.jpg

And the brand new Gates replacement:

https://www.the75andztclub.co.uk/for...09ac05462e.jpg

As can be seen, the readings are identical. So although, in theory, some wear must have taken place over 90,000 miles and 19 years, it is so small as to be insignificant.

I then measured the depth of the teeth from peak to trough. Again, in both cases the reading was 3.6 mm.

I am surprised I must admit. I expected a few tenths of a millimetre difference.

By far the most important finding was that the bearing of my tensioner pulley was running extremely roughly. It felt and sounded as if it had been kicked around on the beach; the friction was easily detectable. The Gates replacement is beautifully smooth. This has led me to consider a possible explanation for the very few KV6s which experience unexplained timing belt failure within the 90,000 mile/6 year period.

It seems to me that it's not the belt that is the weak point but the tensioner pulley. If the operating friction reaches a critical point I would suggest that the risk of the belt being thrown off it becomes very real. Should this happen, the driver would be under the impression that the belt had suddenly broken without warning. Of course it had, but the guilty party is not the belt but the tensioner pulley.

In any case, judging by the condition of my tensioner pulley, MG Rover should have specified the precautionary replacement of this part (it does not appear in RAVE or anywhere on the service schedule, not even at 90,000 miles) because the evidence shows that this is the weak link in the chain, if you'll excuse the pun. :} Since RAVE states that a timing belt removed for any reason at or after 45,000 miles service, should not be refitted, that would have been a reasonable mileage to specify renewal of both the tensioner pulley and the timing belt.

But the marketing department would not have liked it. ;)

Even tempered comments are welcome but please don't resurrect arguments from the closed thread because that is contrary to club rules.

Simon

Could you get a vernier with better resolution and re measure please Simon ;)

Also even with the naked eye, and there is no parallax error, it appears the upper belt tooth pitch is elongated.

Anyway, now that you have the parts, are you now going to put this back together? as this has to be the longest KV6 belt change in history :getmecoat:

Brian :D

Yes, that vernier is pathetic!
But even if you have a better vernier, there will not be much difference.

My brother has always assured me that amongst folk who maintain industrial machinery this is an accepted fact , Simon . Pulleys and idlers which wobble , vibrate, come loose etc are the usual cause of belt displacement / failure if left unchecked.

I've said for a very long time that the tensioner, idlers/guide rollers and/or water pumps (when driven by the timing belt of course) are the main cause of belt failure rather than the belt itself.
Hence the importance of fitting a full timing belt kit rather than just the belt (or belts).
Personally, I won't take on a timing belt job unless I'm authorised to fit a complete kit & water pump where required.

It is my view that an accuracy of 0.1mm is satisfactory for this purpose.
Thanks Chris and Mike. The inspection and measurements which I have carried out have convinced me of this.

Wouldn't it be interesting to take the study a stage further; to replace the components with worn bearings but running with the same belt.

Simon

I always change the tensioner etc when renewing timing belts.
I always check any auxiliary belt tensioners & guides for wear & noise before renewing them.

Put the original belt back on with new tensioner and jockey pulleys then Simon ;)

Your poundshop plastic vernier measured with the jaws at an angle to suit your purposes doesn't impress me at all, in fact I suspect you may have measured the new belt, locked the slide, then transferred the vernier to the old belt, thus "demonstrating" that no wear had taken place :getmecoat:

There is a visual to the naked eye difference in the pitch of the belt teeth in your picture, which is evidence the belt has deteriorated to the extent that there is at least a 1/4 mm variation over 16 teeth of a 176 tooth belt, but in your usual way you have selectively chosen to ignore a valid point and simply quote out of context once again to suit your point of view.

The top and bottom of the story is simple, you have been fortunate not to suffer a failure of the belt, and whether that is attributable to deterioration of the tensioner, jockey wheel, water pump etc etc is immaterial, you have not demonstrated anything, or proven anything bar your attempt to justify your skimping on maintenance.

Can I suggest you box it back up and pat yourself on the back, and congratulate yourself as to your saving of the cost of doing the job at the correct service intervals.

Brian :D

I'm not quite sure that the vernier gauge used is much an issue. For the purposes of here its no doubt fine, yes I suppose one capable of measuring to 0.01mm would be better or a depth gauge and a micrometer to back the results up. However I suspect they wouldn't tell us a great deal more.