Tom928

I hear alot about Thrust Bearing Failure (TBF) and how on most pre-85 models there is a clip that prevents the shaft from being pushed forward into the flex plate and causing TBF.

I removed the flex plate cover and everything LOOKS normal - flex plate is flat; however, I don't see the clip. Should it be visible or is it buried in the TT?

It looks like one could be right behind the coupler; however, their is nothing there?

Here is a pic.

edco

The clip, bushing & shims are located on the other side of the flex plate. So it’s the clamp, flex plate and shims located on the engine side of the flex plate. The torque tub still uses the clamp but runs completely through the flex plate and then the clip and shims keep it from moving forward.

Tom928

Ahhh..OK. I guess I was expecting something on the TT side.
Thanks Matt

edco

You should be able to look up through one of the holes in the flywheel and see it but maybe not, I can't remember now I did remember to snap a photo...

Tom928

Yes, I see it. The clip is there with the bushing and shims.

Scott M.

How about this shot??

Tom928

OUCH!!

What caused it to shear like that?

Tom928

While I was there I got a halfway decent shot of the inner TT bearing. Looks like it is not too bad.
Opinions?

Also not sure what the other material is all over inside?
Looks like paper.

PorKen

Matt,

In your pic, the flexplate is unbolted, correct?

edco

Yes-good point, the flex plate is unbolted in my photo and just for comparison sake the end of a late automatic tube you can see how much longer the early tube is in the 2nd photo.

I think what you should look for on the bearing is if it has moved back in the tube. I believe my rebuilt tube on the 91 and my 88 5 speed the bearing is around 9 or 10" back, if the bearing starts to move back it could cause problems.

Scott M.

My guess would be the massive torque that 78 4.5L Euro automatic puts out.

Other than that, I'm guessing either a migrated TT bearing or an engine that was missing time badly causing some bad harmonics.

Black Sea RD

IIRC, Porsche put out a TSB concerning the fracture of driveshafts after one bank of cylinders quit firing and the 928 runs on only 4 cylinders. The vibrations and harmonics during the rough 4 cylinder operation can cause a complete fracture of the driveshaft to happen later down the line, even after the problem is corrected.

Interesting picture showing that driveshaft fractures can also happen to the older style constant diameter driveshafts that are thought to be somewhat impervious to this happening. Seems that prior service life of a driveshaft should also be a concern for those rebuilding their 928 TTs.

Cheers,
Constantine

atb

Constantine wrote:
Your being kind with "interesting" Constantine, "disheartening" is more what I had in mind.

Scott, do you know what the sheered driveshaft come out of and what were the underlying facts causing the failure?

Scott M.

Adamn;

Sheared shaft sourced from a soured 78 Euro automatic. The underlying facts is that I found it in the back seat of the car I plan on chop-toppin'. Other than taking measurements for a rear deck fab, I ain't none clue.

PorKen

In contrast to Loctite, has anyone tried putting something like Moly-60 (motorcycle spline lube) on the splines? After cleaning/buffing. Perhaps the reduced heat and longitudinal friction would help the clamp recenter itself?