Squealing Cam (more info)

[Dr. Ian McArthur]

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Re: the squealing cam bearing I posted earlier.

First, thanks to all of you who provided some ideas.

It's not the seal leaking air.  The noise is definitely a metal-to-metal
squeal.  I think the problem is caused by a too-tight timing belt.

We have three 5TQs.  Both of mine had the timing belt re-done before I
bought them.  One car had the belt so tight it wore out the teeth on the
crank sprocket and trashed the idler pulley.  This is the car where the
timing belt broke after ~20K km and bent the valves.  Now the other car has
developed the same noise.

This second car has a newly-rebuilt head (installed by the previous owner
before I bought it) and about 5K km on it.  The guy moved to Borneo or
somewhere.

Back in the old days when I built Austin Mini Cooper and MGA Twin Can racing
cars all of the bearings on the cam and the crank had radial grooves which
matched the oil passages so the oil would be able to get around the
circumference of the bearing shells.  The Audi head has neither bearing
shells for the cam or radial grooves.

My supposition is that if the timing belt is done up too tight it pulls the
cam down so hard against the lower bearing surface (and oil feed hole) that
it seal off the hole and causes oil starvation on the front cam bearing.

Now here is the rub (if you will excuse the pun!!).

The "specification" for tightening the timing belt is a brilliant piece of
work.  My profession is technical training design for the railroad industry.
Carefully designed standards a crucial safety issue.

Well, here we have a "tighten until you can turn the belt 90 degrees with
your fingers."  If you make it too loose the belt jumps, you bend the valves
and pay a fortune.  If you do it up too tight it trashes the water pump, the
crank sprocket and the idler, then breaks prematurely and bends the valves.
And, just maybe, causes oil starvation, scores the cam bearing surface and
ruins the head (since there are no cam bearing shells to replace).

So I am faced with two screaming 5TQs and no really clear idea of just how
tight (or loose) to tension the timing belt.   My neighbor (a pro football
lineman) offers to do the 90 degree finger test.  His six year old son
offers also.  There might be a slight discrepancy in the tension.

Anyway, I would sure appreciate some comments on the possibility of oil
starvation from a too-tight belt, on the possibility of belt jumping (and
scrambled valves) from too-loose a belt and any advice on the "Audi standard
finger strength test for prospective belt twisters."

It occurred to me to make my own "belt twisting torque gauge."  I thought an
ordinary kitchen fork would do for the lever.  Just slide it over the belt
and twist.  Then use an ordinary postal spring scale to measure the force
required for a 90 degree twist.  Then I would have a measurable and
repeatable standard. The only problem is that a measurable and repeatable
standard is useless unless you know in advance that you are measuring an
appropriately tight belt.

Oh well ... I suppose that's nature's way of getting even with an old guy
who was so stupid he used to build Mini Coopers and MGA Twin Cams back when
everybody else was building Datsuns.

Thanks in advance for any advice.  I really don't want to destroy two more
cylinder heads.  I'm running out of spares.
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Dr. Ian McArthur, Consulting by Acronym, Cochrane, Alberta
sutul at telusplanet.net
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