[s-cars] sheared crankshaft-gear key

Thu Jan 22 12:00:29 PST 2009

 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

-----Original Message-----
From: qshipq at aol.com
To: djdawson2 at aol.com; forgied at shaw.ca; s-car-list at audifans.com
Sent: Thu, 22 Jan 2009 11:07 am
Subject: Re: sheared crankshaft-gear key

 The only place I would take exception to your presentation below, is with not routinely replacing the crank-bolt Dave.? It is a torque to yield.? Think about what the crank "locking" tool is locking.? It's locking the *crank pulley*? - not the crank, in a given position....? Now, you take a 'already' yielded bolt with a 1mm pitch and torque the crank pulley to the crank with 400lb/ft+ of torque, the stress of that locking tool and the shear forces of tightening is exactly and fully on the key.? You have the face of the bolt on the pulley, the threads of the bolt in the crank, and the only thing preventing shear between the crank and the pulley, is a crappy pot steel key.? The more heat cycled and prior-yielded the bolt, the more shear on the key.? The omission of the 100USD insurance I have a tough enough time grasping.? Impossible to think of a stretched+heat-cycled hardened bolt locking a crappy pot steel key in an attempt to prevent some serious shear forces as that bolt is torqued to spec.

The very physics of this assembly process dictates failure of that key.? Or (IMO) just darn good luck in your case.? I don't look at the procedure as dictating the potential failure.? I see the hardware and reuse of that same hardware as the potential failure of the procedure.? If that bolt is torque to yield, what is the torque spec you advocate when reusing a heat cycled/torque yielded bolt vs new?? Next time you do the procedure, try putting in the old bolt, take it back out and put in a new one.? You can feel the difference = yield.? BTDT.

I'm not an engineer looking at the practice of failure, I'm just well seasoned at reverse engineering failure to practices.? I don't even keep the broken crank keys anymore, but have over a dozen in 15 years.? Forgie has a picture up of what I see often when it's blatently obvious, and I have had that failed bolt pic on Audiworld for over 8 years.? I kept that bolt to remind myself what would have happened had that bolt sheared fully with the leftovers in the crank....

OR, Force Majeur sucks?

Scott J

-----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 11:36 am

Subject: Re: sheared crankshaft-gear key

 Again Scott, I think you know I respect your opinion. For those thinking this is an argument, I beg to differ... this is good natured debate.

I suppose if I were operating a shop and repairing other people's cars all the time, I would do the same thing. After all, it is cheap insurance that your customer is paying for.

Perhaps I will change my mind if I have one fail. However, as stated before, I've got 26 years of experience with the exact same setups, and haven't had a failure yet.

I guess I'll choose to think of it like this:

Insurance... the concept of paying a small amount of money on regular intervals to eventually cover for the big event *if* it ever happens. You have selected to have yourself or your customer pay $100 at each event to prevent the "large" failure.

I have chosen to not pay on my policy, and deal with the big failure "out of pocket" *if* it ever happens. If I were to count the number of times I've either done a timing belt or removed the gear to swap a cam etc... I would conservatively estimate it at 100 occurrences during 26 years. 100 x $100 = $10,000.? Even *if* this failure ever bites me, I'm still a solid $8000 ahead of the game.

Component failure is all about statistics, that's what I do for a living. I don't disagree that the failure *can* happen, I disagree with the rationale of replacing those parts at each maintenance interval, because the frequency of failure doesn't justify it... if the job is done properly, and the parts are carefully inspected prior to re-installation. I *have* replaced each of those gears on some occasion... but they looked like Dave F's picture, and the disaster never happened because the clamping force kept the gear stationary.

If I had a preventive maintenance engineering client with a fleet of Audis (that'll never happen!) I would not advise them to include replacement at each interval as a part of their PM program.? The cost is higher than the benefit.

Again, this is something that each person has to evaluate for themselves.? My engine is currently torn down... I'll be happy to tell you when I have my first failure.

Dave

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

From: qshipq at aol.com

To: forgied at shaw.ca; s-car-list at audifans.com; Djdawson2 at aol.com

Sent: Thu, 22 Jan 2009 8:22 am

Subject: Re:   sheared crankshaft-gear key

 Dave, Dave et. al.

DF, your photo is the way I see most of them....? Best case scenario.? And that crank bolt *is* definitely a "torque to yield bolt", which by definition means it's a one time use, and if you are luck will take a couple proper torques before failure.? Here's one that didn't take the 3rd torque to spec - about 50lb/ft short to be exact.? 

http://pictureposter.audiworld.com/37342/scrankbolt.jpg

BTW, I own all the proper Audi tools to do timing belt procedures on 10 and 20v I5's - and I'm absolutely most anal about Timing Belt R&R, it's the most expensive fix to the I5 - really any audi motor.? I won't take the risk on this R&R, it's not worth it. ? The Ingersol Rand air tool isn't a valid argument IMO, assuming you lock the crank, it doesn't add failure to the crank-pulley key, in fact, I'd argue it lessens shear-risk on removal.? I've found failed keys on second round timing belts that *I* did the first round!? My own experience leads me to the opinion/conclusion that it's the reuse of the stretch/hardened crank pulley bolt that causes the key to be stressed.? Read (IMO/E): The key will live a longer life if you replace the crank-pulley bolt every timing belt change.? I also encourage everyone reusing the crankgear to remove it from the pulley and inspect it carefully.? Some of those hairline cracks are hard to see.

I also support my experience and opinion by claiming this is exactly the reason Audi went to the Timing Belt procedure ala the v6 - leaving the crank gear and bolt in situ - remove serpentine pulley from the crank-gear only.

DD, I've lost count of the I5 timing belt jobs I've done over the last 15years.? I've seen key and bolt failure many many times - the key once on my watch was all I needed to believe.? I've also had to figure out how to lock the crank when the key sheared trying to remove the bolt.? You can disagree with my opinion, doesn't bother me.? You see your very first failure, you won't ever take the risk again, btdt.? Bolt and gear is 100 retail, much cheaper than pistons and valves out of time.? Mr. Librarian is welcome to put this in his reference material.? 

My .02

Scott J

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

From: David Forgie <forgied at shaw.ca>

To: s-car-list <s-car-list at audifans.com>

Sent: Thu, 22 Jan 2009 12:11 am

Subject: [s-cars]  sheared crankshaft pulley

I case you want to know what a sheared crankshaft pulley (key) looks like, go to 

this post:

http://forums.audiworld.com/s4s6/msgs/252125.phtml

I agree with Scott.  Replace with each timing belt job.

But would add the obvious, avoid impact wrenches and tighten with a proper 

torque wrench (and/or extension as per Bentley).

Dave F, 

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