Belt tension

Unread post by **alan s** » 21 Dec 2006, 21:35

Malcolm,

What they seem to do (no pun intended) is just stop either at TDC or at the timing marks, then lock the cams and crank using some pretty heavy duty locking devices, slacken the jockey wheel or tensioner, slip the old belt off, slide the new one on, use the car in gear to relieve the tension, if it's a bit tight, spanner it all up and test. If it flaps they tighten it and if it whines they slacken it. I have even heard of a Stanley knife being used to slice a belt in half laterally and then slip a new one one on. When almost on, slit the old belt and remove; all done in minutes without locking anything up.
In theory should work well, but they be braver souls than I.

Alan S

Mandrake · Unread post by **Mandrake** » 21 Dec 2006, 21:48

Good point about whining when too tight Alan... I was just about to add that to this thread - there is such a thing as too tight...

When I did my Xantia (2.0i 8 valve petrol) I initially tightened the belt too much and the awful warbling whining noise it made around 1500rpm had to be heard to be believed:

http://www.frenchcarforum.co.uk/forum/v ... 9&start=30

Slackened it off again a bit, and noise gone. Of course too loose is bad as well so it's definately hard to know where to set it...

Oh and bxbodger:

I've never actually measured a belt tension-I just crack the tensioner on until it's hard to push the belt by thumb on its longest run...after all, it's not going to get any tighter by itself, only slightly slacker which the tensioner will take up anyway.

Thats what I thought too - until I realised that the belt gets much tighter when the engine heats up due to expansion of the block and head!

In fact I found the increase in tension from a cold engine to a hot engine was MUCH more than the gradual loss of tension over 3 years as a belt ages...

So IMHO a belt being checked by the "twist 90 degrees" or "press with thumb" rule of thumb methods should really be rechecked when the engine is hot - you might be stunned to find out just how much tighter it is when the engine is hot. (I was)

I would say that the "factory figures" for belt tension using a tension gauge are for a cold engine, but take into account the expansion of the engine (and increase in belt tension) as it heats up.

Regards,
Simon