m4martin

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!!!

M. Requin

Search is your friend. Covered on this forum ad infinitum.

DonS

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.

76FJ55

There have been many threads on TBF. If you do a search you will find several theories on the cause, but the root cause is still under debate.

pcar928fan

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!

Mrmerlin

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

blown 87

Very true.

AO

Believe me... Porsche has zero interest in the 928 anymore. That ship sailed a LONG time ago.

SeanR

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?

docmirror

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.

Imo000

If the engine block isn't cracked from the worn bearing, then it can be saves and it doesn't involve welding.

Black Sea RD

A slight correction on this point. The shrinking of the drive shaft under load causes it to be pulled out of the front flex plate clamp and in effect the front flex plate clamp migrates forward on the drive shaft. Which then results in a forward bow of the front flex plate toward the engine.

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,

robot808

Seriously Constantine, what the hell do you know about it?




















I'm kidding.

m4martin

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.

Landseer

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.