[s-cars] sheared crankshaft-gear key

[qshipq at aol.com]

 Dave:

I sure wish this phenomenon was rare, it's just not.? For understanding
on how Audi designed that key to fail, take a gander at pages 23-27 of
this article:












http://www.mfge.atilim.edu.tr/Courses/mfge306/Material/Course%20files/MFGE306_Lecture%205-Locking%20Devices.pdf





 





 I claim that key was
never designed to take the reverse shear load in removal of the sealant
coated (audi glued) crank bolt.? The key is designed to fail, or it
would be a long square key to spread that type of load and shear
stress.? If you do some of those calcs vs 332lb/ft of bolt torque
(times a multiplier on removal with hardened sealant), we should expect
to see more failures of that key not less!? My theory is that key
failure happens more often on removal than on install.? Never seen
one?? I can't be that unlucky, but I also remove that gear and inspect
it under a mag-glass, because a hairline fracture is all that's
needed.? Again, I claim Audi was aware of the issue, and made the TB
R&R on the v6 to be without removing the crank bolt or gear to
reduce this chance of key failure.? I also claim it's not an 'old'
concept unique to the I5 either, this later Audi crank gear key is
wider than the I5:



http://www.blauparts.com/audi_technical_tips/audi_timing_belt/audi_crankshaft_pulley.shtml



300 belts without 1?? Can't fathom that frankly, maybe it's a
salty-chicago thing, but I really doubt it.? I witnessed my first key
failure well over 10 years ago, and have seen many since, and just
won't take the risk.? The threads and contact surface of the crank bolt
should be coated with Audi sealant AMV 188 001 02 (watch yer wallet!),
which over time turns to a really hard cement (part of the key-shear on
removal problem IMO - see page 23 above).? Torque of the crank bolt on
the 20vt is 350NM (with 2079) or 450NM (without 2079).



If you are doing all this without replacing the bolt and sealing (by
definition: 'lube' on install) it, the torque spec you use is no longer
significant, nor does it correlate to how Audi put these parts together
at the factory.? That bolt has an elastic phase to it, and on reuse a
more dangerous plastic phase to it.? 



Minimal Bottom Line:? Buy the new bolt, pull that gear off the pulley
and inspect it carefully.? Sealant on the crank bolt threads and bolt
head face, and torque to spec.? 



You guys scare me sometimes.



SJ





-----Original Message-----

From: djdawson2 at aol.com

To: qshipq at aol.com; forgied at shaw.ca; s-car-list at audifans.com

Sent: Thu, 22 Jan 2009 2:00 pm

Subject: Re: sheared crankshaft-gear key













 Agreed... if there is a
bolt that reluctantly threads in, it has yielded, effectively changing
the pitch. That bolt should be replaced... crank, cam, or anywhere else
a bolt is used.




It should also be noted that more responses to this post are coming to
me off-list than on-list... due to "sheer" (pun intended) desire to
avoid entering the debate. Most recently, one shop, that I am vaguely
familiar with, touched base claiming in excess of 300 timing belt
procedures without a single keyway failure... and elaborating that they
haven't even seen one. To me, this is just another set of data points
to consider.




Again, for me it's a numbers game... and the numbers don't justify
modification to the method/materials. It makes no difference to me if
everyone on this list makes it part of their standard practice, it just
isn't part of mine. I've shared my opinion, you've shared yours... and
anyone reading this stuff can try and make an informed decision.




I still believe that these failures are due to the use of impact
tools.? I've seen a couple of keys damaged that I replaced, but I was
not the last person that serviced the car.? I have NEVER pulled one
apart that I had assembled and found a failure.





Peace.





Now, let's talk politics...





Dave