Things everyone should know about 928 Engine Thrust Bearing Failure
#1
Things everyone should know about 928 Engine Thrust Bearing Failure
Hi Everyone,
I am new to this forum but not the 928.
I have just bought a '87 928 S4 with a dead engine and here is what has happened:
Problem= The crankshaft has a failed thrust bearing.
Cause= It appears that virtually all 928's, particularly automatics, have the driveshaft within the torque tube put excessive pressure on the flywheel/drive plate and this wears away the thust bearings inside the engine. In my case the wear was so excessive the crankshaft has worn into the internal faces of the block casting, and wrecked the engine! Please read this: http://members.rennlist.com/v1uhoh/cranksha.htm
My question is WHY does the driveshaft have a force pushing it into the engine, or WHAT IS GOING ON WITHIN THE GEARBOX THAT CREATES THIS??
Can anyone elaborate on the exact sequence of events that cause this problem??
PLEASE if you want to keep your engine safe CHECK THE PRESSURE ON YOUR DRIVE PLATE, it's a quick check and it may save you a new engine!!!
I am new to this forum but not the 928.
I have just bought a '87 928 S4 with a dead engine and here is what has happened:
Problem= The crankshaft has a failed thrust bearing.
Cause= It appears that virtually all 928's, particularly automatics, have the driveshaft within the torque tube put excessive pressure on the flywheel/drive plate and this wears away the thust bearings inside the engine. In my case the wear was so excessive the crankshaft has worn into the internal faces of the block casting, and wrecked the engine! Please read this: http://members.rennlist.com/v1uhoh/cranksha.htm
My question is WHY does the driveshaft have a force pushing it into the engine, or WHAT IS GOING ON WITHIN THE GEARBOX THAT CREATES THIS??
Can anyone elaborate on the exact sequence of events that cause this problem??
PLEASE if you want to keep your engine safe CHECK THE PRESSURE ON YOUR DRIVE PLATE, it's a quick check and it may save you a new engine!!!
#3
Drifting
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Question of the decade.
Being new (welcome aboard!) we're always open to one more opinion on the cause so give it your best shot. IIRC current theory is that the torque converter balloons along the longitudinal axis due to high temperature under heavy load.
Being new (welcome aboard!) we're always open to one more opinion on the cause so give it your best shot. IIRC current theory is that the torque converter balloons along the longitudinal axis due to high temperature under heavy load.
#5
Nordschleife Master
Martin,
Nice first post...might want to start by reading the "BEFORE YOU POST" thread at the top of the forum. Thanks for letting us all know, but we have been preaching this FOR YEARS AND YEARS HERE!
Welcome to Rennlist! We know things about the 928 that Porsche has not even figured out yet!
Nice first post...might want to start by reading the "BEFORE YOU POST" thread at the top of the forum. Thanks for letting us all know, but we have been preaching this FOR YEARS AND YEARS HERE!
Welcome to Rennlist! We know things about the 928 that Porsche has not even figured out yet!
#6
Team Owner
from what i have found usually the rear pinch bolt isnt as tight or checked when the thrust clearance is checked.
So in essence the driveshaft can still move in the rear clamp even though there is a cutout in the shaft for the bolt the shaft can chew the bolt shank up while its moving.
Most owners that have checked both pinch bolts front and rear and then used blue loctite on the bolts then torqued to 66ft/lbs have found little to no movment.
The are other suppliers that have made new , better front clamps the hold the shaft in the front , but this still doesnt address checking the rear pinch bolt.
Martin BTW since your new here it would do you well to read the FAQ at the top of the page as most of your questions may be answered and additional info gleaned.
Goodluck with your engine replacement
So in essence the driveshaft can still move in the rear clamp even though there is a cutout in the shaft for the bolt the shaft can chew the bolt shank up while its moving.
Most owners that have checked both pinch bolts front and rear and then used blue loctite on the bolts then torqued to 66ft/lbs have found little to no movment.
The are other suppliers that have made new , better front clamps the hold the shaft in the front , but this still doesnt address checking the rear pinch bolt.
Martin BTW since your new here it would do you well to read the FAQ at the top of the page as most of your questions may be answered and additional info gleaned.
Goodluck with your engine replacement
#7
Rest in Peace
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Martin,
Nice first post...might want to start by reading the "BEFORE YOU POST" thread at the top of the forum. Thanks for letting us all know, but we have been preaching this FOR YEARS AND YEARS HERE!
Welcome to Rennlist! We know things about the 928 that Porsche has not even figured out yet!
Nice first post...might want to start by reading the "BEFORE YOU POST" thread at the top of the forum. Thanks for letting us all know, but we have been preaching this FOR YEARS AND YEARS HERE!
Welcome to Rennlist! We know things about the 928 that Porsche has not even figured out yet!
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#9
Thank you for the heads up.
Now, did you buy the car knowing it had TBF or did you buy it thinking it could be a cheap fix?
Now, did you buy the car knowing it had TBF or did you buy it thinking it could be a cheap fix?
#10
Shameful Thread Killer
Rennlist Member
Rennlist Member
The reason is due to an action called 'toque coupling'. This occurs when the drive shaft twists in response to the torque applied to it and it actually shrinks slightly. Then, it untorques and unshrinks and the repeated flex on the flex plate causes the splined clamp to migrate rearward, thus putting forward pressure on the flex plate, and grinding the crank into the cheek of the bearing and block.
Yes, we know it well.
Yes, we know it well.
#12
The reason is due to an action called 'toque coupling'. This occurs when the drive shaft twists in response to the torque applied to it and it actually shrinks slightly. Then, it untorques and unshrinks and the repeated flex on the flex plate causes the splined clamp to migrate rearward, thus putting forward pressure on the flex plate, and grinding the crank into the cheek of the bearing and block.
Yes, we know it well.
Yes, we know it well.
This action accounts for most of the forward bowed flex plates found in automatic 928s which can lead to TBF over time.
There are other reasons which cause sudden TBF failure to include techs and owners who work on the 928 drive line with no knowledge of the TBF problem with the 928 and how critical it is not to have any forward pressure on the flywheel after a transmission or TT R&R.
Cheers,
#14
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.
#15
Rennlist Member
Welcome. You can save yourself a lot of trauma and drama. Accept that it happens. Don't worry about the PhD level finite element analysis proof. It doesn't matter.
We have preventative measures to stop it on your next engine.
Where are you located? That might expedite your access to the replacement engine.
We have preventative measures to stop it on your next engine.
Where are you located? That might expedite your access to the replacement engine.