Mrmerlin

sidehatch if you bring your car to me i will show/help/check the flex plate tension.
And i will let you see how the system works .
Since you dont really have too much of an idea on what your commenting on it would be best if you took some time to disassemble the parts in question to see whats really going on.
When you pull out an engine thats got TBF and see the crank throw ground into the rear side of the thrust face you might have a better understanding of the forces involved and what improper maintenence can cause to an otherwise good running engine/driveline

sidehatch

So doing this procedure will stop tbf and prevent it from happening,i thought i read that someone released the pressure on the flex plate,and tightened everything up only to find out after driving it a short while it went rite back to were it was originally.Mabe over time this is something that creeps up and is a design fault of the car,and there really is no way of preventing it. If this procedure works i want to have it done,but when people say it takes a hour to do,and a master mechanic says it takes atleast 3 hours to do,im not sure what to do.Can new parts be put in to prevent tbf? Is something getting loose that needs to be re torqed?

Mrmerlin

sidehatch, see post 50, if this is followed then the highest chances of success using factory parts will be achieved.
That said Constantines clamp is an upgraded part however the TT must be removed to fit it.
Most owners have had succesful results using the factory clamps.
Checking for crank end play is an essential operation to be performed on any S4 or newer auto equipped 928.

As far as a master mech taking 3 hours to do this operation I would find another mechanic, if you have a lift it should take no more than 1 hour to properly check both of the clamp bolts, if your working on your back then add about 1/2 hour

Schocki

Earl,

Thank you for the reply, my experience is so far very good with Loctite 290. Drove the car until fall 2006 in TX and now it's back in Germany. I drive the car on a regular basis and it has seen speeds up to 160 MPH on the Autobahn. No flexplate movement at all
132000 miles total and no issues so far.

Daniel Dudley

I have read about this problem many times, and I still have a hard time understanding what causes it. Does some kind of twist in the drive shaft while it is under load cause the shaft to get shorter and pull against the clamp ?

RyanPerrella

so i have a question, and no this doesnt relate to anything in my current car but i was wondering this when i rebuilt the GT engine.

So if you have a car that is on the high side of spec, maybe out of spec but still hasnt gone to the point where the crank grinds into the block. Your problem here would be the main bearing is worn out, but not so much so that the block is damaged. Do you resolve this issue then by replacing the main bearings. There is no other issue with the engine correct? I mean the thrust bearing wear is limited ONLY to the thrust bearing itself until which point its so worn that it then tears up the block and ruins the block and crank. Am i understanding this failure correctly?

Black Sea RD

There could be a few scenarios that could cause forward pressure at the front flexplate. The most commonly seen and the one we found to be most prevalent is what you describe. The drive shaft twists under load and gets shorter momentarily. There is enough difference that the front flexplate's movement cannot compensate for the difference and the drive shaft is pulled out a bit. This will continue to happen until the differences are negated. This usually ends up with forward pressure on the front flexplate pushing the crank into the thrust bearing surfaces.

Black Sea RD

The best person to ask on this list would be Greg Brown.

From what we have seen, during the grinding of the crank into the thrust bearing there is heat being generated that sometimes can crack the block. Or the thrust bearing will start rotating around with the crank due to the pressure and the block is ruined that way. That's with the thrust bearing not yet being worn through.

Best one can do is open the engine and carefully inspect the block and other internals for any damage and proceed from there.

Cheers,
Constantine

Earl Gillstrom

I did fix an '88 with .040" crankshaft end play. Porsche spec is .016" maximum. The crankshaft was too worn to reuse.
I used a crank that needed regrinding to use .010" undersize bearings. The .010" undersize bearings have wider thrust bearing surfaces, so the crank thrust surface has to be ground. At .040 end play the crank was not hitting the block.

I am not sure what happened to this car but the driveshaft was falling out of the car (loose bolts) and broken rear driveshaft coupling and there was a broken wire going into the EZK so it was running on 4 cylinders.

morganabowen

Hey Dr. Bob, you need to hurry up and get your lift so that we can have a "Check your Torque Tube" clinic

RyanPerrella

I see, that makes sense

Yes if the thrust bearing is moving in the block then yes the block would need work or just be replaced. Gotcha

OrionKhan

Kinda hard to remember the exact symptoms.

It happened over 5 years ago. I had never heard of a TBF. I noticed something wrong while driving to a family function. Unfortunately, I was about 30 miles from home. But the car was still ran. But there was sort of grinding sound. I had a little trouble getting it started. But she fought on valiantly. I limped her home and then had it towed to a be repaired. That was a whole different lesson to be learned about regarding mechanics that know 928s. See this thread for details: https://rennlist.com/forums/928-forum/38231-shock-the-munckey.html

In retrospect, I should have towed it home. I might have been able to save the engine. I think driving it home caused most of the damage. Now, as soon as anything sounds unusual, I stop and inspect. And I tow her if can't figure out what is causing the problem. I'm freakishly paranoid about it now. But to be honest I haven't had any problems since the new motor. BTW, believe all of the positive things you hear about Greg Brown.

sidehatch

Whos greg brown

OrionKhan

Search is your friend, if you use it.

Black Sea RD

Thank you Wade! Very sorry to hear about your incident and then the further frustration with the chosen mechanic afterward. Great that Greg Brown and crew came to your rescue at the end.

Thanks again,
Constantine

P.S. Sidehatch, click on Wade's imbedded thread and you'll read about Greg Brown, probably "the" engine builder/mechanic of choice for us 928ers.