Loud Knocking Diagnosed - Spun Bearing:(

[Fred Munro]

Hi Jim;

The damage to the crank looks bad, and polishing it up with Scotchbrite
won't give you a lasting repair. This is why:
The rotation of the crank inside the bearing creates a hydrodynamic pressure
wedge in the oil. This pressure wedge supports the bearing surface about
.001" or so (depends on rotational speed and load) from the surface of the
crank. The bearing metal should never touch the crank surface - this is why
bearings on a well maintained high mileage engine can show very little sign
of wear. The oil pressure provided by the oil pump ensures the cavity
between the bearing and crank is flooded with oil - the pump pressure is not
high enough to support the bearing load. If the crank and bearing surfaces
are not square and flat (i.e .001" ridges on the crank surface, or tapered
side to side), the bearing metal will wipe on the crank, leading to failure.

You should pull the engine apart, clean it out, resurface the crank, and
check the rod bores for round and true. Find out why the bearing spun, or
you may be doing this again. What do you use for an assembly lube? Unless
you pressurize the oil system before starting the engine, you need an
assembly lube which will hang in the bearings until the oil arrives. I
personally have had good success with a 50/50 mix of oil and STP (I know, I
know, but it works for me). I've heard white lithium grease works as well.
In either case, a very early oil change ( 50 miles or so) is a good idea to
purge any grit you've missed and to get the assembly lube out of the engine.

HTH

Fred Munro
'94 S4
----- Original Message -----
From: "Jim Green" <jeg1976 at yahoo.com>
To: "Quattro list" <quattro at audifans.com>
Sent: Wednesday, February 13, 2002 11:12 PM
Subject: Loud Knocking Diagnosed - Spun Bearing:(


> Well on the advise of Javad and Phil, I dropped the
> oil pan, and found a spun rod bearing in cyl #4.
>
> http://www.geocities.com/jeg1976/bearing.jpg
>
> Well that lasted a whopping 40 miles.  The crank is
> still in pretty good shape, but needs polished.  The
> picture looks worse than it is.
>
> http://www.geocities.com/jeg1976/race.jpg
>
> I dug up a post in the archives that sugested using
> 600 grit, then finishing them off with fine
> Scotchbrite.  I like this idea much better than
> pulling the engine out again.  Anyone see any reason
> why I shouldn't try it?  I'm going to check to see if
> it's out of round, but I doubt it since I only drove
> it about two blocks.  All the other bearings look
> fine.
> Thanks,
>
> =====
> Jim Green
> '89 90tq EFI
> http://www.geocities.com/jeg1976/car_home.html
>
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