Squealing Cam

[James Bell]

Dr .I. M:

You need to do the hot engine/cold engine,
observation test. In order to calibrate your
perceptions. Hands on learning.

The belts do not run under tension when hot!

The 20 valve boys know this one by heart, they
have to. The learning curve is rather steep. Ask
them about (cold engine) belt "Flutter" noise. The
16 Valvers in VW'S realm know the sound too.

Sounds like you need to find a mechanic, whom
knows his trade, for Audi's. This is Audi 101 for
most techs. Very basic stuff.

J.B. in ID.

----- Original Message -----
From: "Dr. Ian McArthur" <sutul at telusplanet.net>
To: <quattro at audifans.com>
Sent: Monday, July 21, 2003 11:33 PM
Subject: Squealing Cam (more info)


> This is a multi-part message in MIME format.
> --
> [ Picked text/plain from multipart/alternative ]
> 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.
> ************************************************
> Dr. Ian McArthur, Consulting by Acronym, Cochrane, Alberta
> sutul at telusplanet.net
> --
>