Things everyone should know about 928 Engine Thrust Bearing Failure
05-26-2010, 08:52 PM
#17
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Hi All,
Phew, everyone knew about the problem, except me of course, LOL!
Oh well, i have done much reading since my first post, and obviously my mouth was in first gear whilst, my brain was in nuetral for a bit there! Anyway, my over enthusiasm put aside........
I can understand the twist of torque on the drive shaft could possibly "wind it up" and make it shorter, and if that were the case the effect would apply with positive or negative torque and the shaft would be longest whilst off load. However, my engine has definitely had the crank pushed forward, so unless the coupling is being ratcheted forward with slip at the joint, that seems most unlikely.
Ballooning of the torque converter is also slightly puzzling, as the expansion (in length) of the torque converter would require it to sit hard against the gearbox to impart an opposing force toward the engine crank. I thought that the torque converter was effectivly "free floating" on its input into the gearbox, So either the oil pump drive takes the thrust load from the gearbox side, or the turbine on the input shaft would.
Is it, therefore,, not reasonable to just shorten the engine drive shaft slightly to leave room for a small amount expansion/movement at the gearbox torque converter interface?? Or is there another force, like hydraulic pressure trying to push the converter out of the gearbox?
I must say at this point, i have not had much chance to read any of the other posts in the forum, so please excuse me if i am going over old ground here.
Phew, everyone knew about the problem, except me of course, LOL!
Oh well, i have done much reading since my first post, and obviously my mouth was in first gear whilst, my brain was in nuetral for a bit there! Anyway, my over enthusiasm put aside........
I can understand the twist of torque on the drive shaft could possibly "wind it up" and make it shorter, and if that were the case the effect would apply with positive or negative torque and the shaft would be longest whilst off load. However, my engine has definitely had the crank pushed forward, so unless the coupling is being ratcheted forward with slip at the joint, that seems most unlikely.
Ballooning of the torque converter is also slightly puzzling, as the expansion (in length) of the torque converter would require it to sit hard against the gearbox to impart an opposing force toward the engine crank. I thought that the torque converter was effectivly "free floating" on its input into the gearbox, So either the oil pump drive takes the thrust load from the gearbox side, or the turbine on the input shaft would.
Is it, therefore,, not reasonable to just shorten the engine drive shaft slightly to leave room for a small amount expansion/movement at the gearbox torque converter interface?? Or is there another force, like hydraulic pressure trying to push the converter out of the gearbox?
I must say at this point, i have not had much chance to read any of the other posts in the forum, so please excuse me if i am going over old ground here.
if you look at the way the torque converter in held in place with the bearings it is very unlikely that converter has any effect on thrust bearing failure, unlike it does on a Chevy where it can and does happen.
05-26-2010, 09:30 PM
#19
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It only happens on automatics and mostly always S-4s > , like after Porsche stopped usiing the snap ring and shims on the end of the shaft which precludes the ratcheting ...... As noted the trans convertor bearings and spacer SHOULD prevent movement from convertor swelling being a problem and it does have its own "flexplate" on the convertor. The problem also seems to be often the result of a bad torque tube change where it is easy to preload the front flex plate with out really trying.
05-26-2010, 09:49 PM
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Hi m4martin. If you haven't figured out yet, you should soon learn that the real answer is NO!!!! NO ONE KNOWS THE ANSWER!!!! There are some who might think they know, but no one actually knows why. You should just follow the advise about checking the clamp on the flex plate frequently and put one of the proven preventive clamps in when you can get to it. And, Welcome. Jerry Feather
05-26-2010, 09:56 PM
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The sobering thing about this whole thing , is that it is becoming more prevalent , probably through lack of maintenance or ignorance , I have been in contact with 3 , that were serviced by reputable people that new 0 about this phenomenon , but now all these cars are writeoffs , (needing new engines ) of which the owners are unwilling to undertake , 3 in the last 6 months .
05-26-2010, 10:03 PM
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Yes John the vast majority of 928 owners have no idea that their engine even has a thrust bearing ....and many shops have yet to see and UNDERSTAND the issue. Porsche deleted the snap ring and shims and issued a tech bull stating that they no longer were needed. It seems they were wrong !
05-26-2010, 10:21 PM
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It only happens on automatics and mostly always S-4s > , like after Porsche stopped usiing the snap ring and shims on the end of the shaft which precludes the ratcheting ...... As noted the trans convertor bearings and spacer SHOULD prevent movement from convertor swelling being a problem and it does have its own "flexplate" on the convertor. The problem also seems to be often the result of a bad torque tube change where it is easy to preload the front flex plate with out really trying.
There have been cases when TBF happened to 5 speeds 928s as well. Very rare occurrences but it can happen.
As far as after TT R&Rs, we have been contacted by people who suffered TBF after 100K on the clock with the original TT in place. They didn't know about TBF and so did not check their front flex plate for forward pressure.
TBF is very preventable, it just takes awareness of the 928 owners/techs to stop it.
Kind regards,
05-26-2010, 10:28 PM
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Onthe 5 speed the clutch assembly is bolted to the flywheel and holding the clutch pedal to the floor pulls the crankshaft to the rear of the engine opposite of normal TBF....and in all my years I only knew of one such failure caused by a Tilton release bearing
05-26-2010, 10:32 PM
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05-26-2010, 10:35 PM
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05-26-2010, 10:43 PM
#28
TB failure is about torque (angle of twist), frictional co-efficients, moments of inertia, migration, excess resultant excess forward axial force, axial shuttling, tribology, boundry lubrication breaksown, wear,Newtons second law of motion and sometimes crankshaft thrust bearing failure, not necessarly in the order as written.
It dose happen, but as said it is preventable.
Tails 1990 928S4 auto
It dose happen, but as said it is preventable.
Tails 1990 928S4 auto
