Thrust bearing Failure
Thread Starter
Racer
Joined: Nov 2006
Posts: 259
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From: Brisbane, Australia
why is it bad to start the car with the clutch in?
A 993 the clutch, when engaged, pulls forward on the flywheel and in turn on the crankshaft, placing a load on the the #8 main bearing (thrust bearing), and in particular, on the outer shoulder of the thrust bearing, being the loaded surface. When the engine is cold, the thrust bearing becomes relatively deplete of lubrication (gravitational drainage). During the moments after the cold startup, the thrust bearing is thus exposed to load at a time when it is suboptimally lubricated exposing it to the risk of premature wear. Ultimately, that wear leads to a loss of width of the bearing's thrust surfaces, and the increased fore and aft movement of the crankshaft then exacerbate the phenomenon. This then leads to actual wear of and loss of metal on the crankshaft itself at the bearing interface. Once this happens, its new crankshaft time! Mine had 2.5mm of excessive fore and aft movement with significant wear on the inboard aspect of the crankshaft/ bearing interface. The only symptom was a 3 to 5 second squeak on startup that was put down to pulleys, belts and the belt tension sensor. Some have stated that a 0W-50 oil lacks the capacity to cling to and remain on the bearing surface when cold rendering the bearing vulnerable to a greater risk of wear. Steve Werner has suggested 15w-50 or even 20W-50 oil for this reason. For the record, I have always used 5W-50 Mobil1.
One might argue that a warm start should not create such risk as lubrication should not be as critical an issue. The manual however states that the engine should be started with the clutch out and the car in neutral (brake applied) for all startups.
Hope this verbose explanation helps. It took me a while to get it worked out.
Malcolm.
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Joined: Jun 2006
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From: Sydney Australia
I have always wanted to use that phrase but could never find just the right time!
Thread Starter
Racer
Joined: Nov 2006
Posts: 259
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From: Brisbane, Australia
Tough crowd here....
OK, OK, OK......Grant. 
[I]Grant [I]the the author some metophoric licence, please.
This was my one chance to actually sound like I know what I'm talking about - now all credibility is lost.
(I guess it does sound somewhat tautologious).
[I]Grant [I]the the author some metophoric licence, please.
This was my one chance to actually sound like I know what I'm talking about - now all credibility is lost.
(I guess it does sound somewhat tautologious).
Rennlist Member
Joined: Jun 2006
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From: Sydney Australia
Now just wait your turn!
Rennlist Member
Joined: Jun 2006
Posts: 331
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From: Sydney Australia
NO!!!!
That capped it.......now we are all sitting around phraseologically battered and beaten!
Rennlist Member
Joined: Sep 2005
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From: Still here...
Hmm... I recently received some email regarding a certain medication that purportedly reduces "gravitational drainage".
Let me know if you want me to forward it
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RL Technical Advisor
Joined: May 2001
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From: Portland Oregon
The crank journals are hardened but the counterweights and thrust surfaces are not.
When the thrust surfaces on the main bearing become sufficently worn, the crank can move axially and that creates wear on both bearings and that part of the crankshaft. In some cases, all one needs is to replace the thrust bearing to restore proper end play but if this has gone on for some time, the crank may be worn sufficently that simple bearing replacement is not an option for a proper repair.
Seared
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From: Ellicott City, MD
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From: NJ
George,
The crank journals are hardened but the counterweights and thrust surfaces are not.
When the thrust surfaces on the main bearing become sufficently worn, the crank can move axially and that creates wear on both bearings and that part of the crankshaft. In some cases, all one needs is to replace the thrust bearing to restore proper end play but if this has gone on for some time, the crank may be worn sufficently that simple bearing replacement is not an option for a proper repair.
The crank journals are hardened but the counterweights and thrust surfaces are not.
When the thrust surfaces on the main bearing become sufficently worn, the crank can move axially and that creates wear on both bearings and that part of the crankshaft. In some cases, all one needs is to replace the thrust bearing to restore proper end play but if this has gone on for some time, the crank may be worn sufficently that simple bearing replacement is not an option for a proper repair.
Ok... I do understand your explanation as you speak from experience. it just doesn't quite seem reasonable but I guess any ferrous material can wear when it comes in contact with softer crank bearing surfaces - just at a much lesser rate.
Instructor
Joined: Feb 2007
Posts: 163
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From: Cypress, California
Malcom,
Any driveability issues with this crank problem? I ask because I too have that squeek you speak of but how does one tell if it is actually a belt tension (or sensor) issue or this much more expensive crankshaft issue?
Any driveability issues with this crank problem? I ask because I too have that squeek you speak of but how does one tell if it is actually a belt tension (or sensor) issue or this much more expensive crankshaft issue?
Thread Starter
Racer
Joined: Nov 2006
Posts: 259
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From: Brisbane, Australia
Aperalta, absolutely none at all. The engine was strong and smooth as was the drivetrain. The only sign was that 3 to 5 second squeal on cold start. Not there with a hot or warm start up. I had had the belt tension sensor replaced twice I think. The only change over the 6 months that I was aware of this, was that of late the sound was lasting just a little bit longer - say until I had reversed out the garage. It was a listed item for investigation during the installation of the upgrade. Others would need to speculate but I wonder if it would have been identified if I had not decided to do a clutch upgrade and drop the engine. It was during the clutch installation that the excessive North/South crank movement was identifiable. I doubt that this problem was even "on the list" so to speak, in the mind of my "wrench" who is very experienced and does Porsche only work on all manner of vehicles from track GT3s, Targa Tasmania cars. to road cars. Now we know of 2 or 3 others (in Melbourne) who have had this problem. Its just that there aren't many 993tts here in Australia. Steve Weiner clearly knows of this. Kevin (UMW) likewise. It would be interesting to know how it could be diagnosed without dropping the engine. You, in particular, might like to know....(Hope your noise is innocent!)
Instructor
Joined: Feb 2007
Posts: 163
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From: Cypress, California
Thanks Malcom. I just had my clutch and flywheel replaced less than a thousand miles ago. I hope this is something my wrench would have detected while the engine was dropped. Cheers--Aaron