Been reading the thrust bearing failure (TBF) conversation and thought to chime in with what I found when I studied the problem for a bit to come up with a mechanical solution to the front flexplate movement found on many 928 automatics. In many 928 automatic cars the front flexplate was found to be pressed forward and when the pinch bolt was released the front flexplate moved back onto the driveshaft a few millimeters, between 2-4mm usually.
I bought a 1986.5 928S that was track prepared and was campaigned in Texas by Tim Westby for a few years before I flew out to Houston to buy it. I was buying it to use as my DE car and as a test mule for a new front flexplate coupler I came up with to arrest the front flexplate movement.
Before buying the car I had the mechanic do a simple check to find out how healthy the thrust bearing/engine was. This is how I had the mechanic check it which any competent mechanic can do to any 928 automatic within 30 minutes:
1. Raise the car on a lift which one should do for a PPI anyway.
2. Locate the flywheel/front flexplate cover and remove it.
3. Before undoing the pinch bolt of the front flexplate collar, put a small straight edge against the front flexplate to see if it is bowed toward the engine. If it is, most will be, then there is some pressure being exerted on the thrust bearing. Mark the location of the rear edge of the front flexplate collar at the rear on the driveshaft.
4. Undo the pinch bolt and watch the flexplate move back on the driveshaft. Measure the amount it moved using your first mark. It should move about 2-4mm, maybe a bit more.
5. Leverage the flywheel/crankshaft forward, toward the front of the car, as far as it will go. Attach a dial indicator to the car so the dial indicator probe is against the flywheel somewhere to record the movement of it when it's leveraged back, toward the rear of the car. Make sure the dial indicator is zero'd.
6. Leverage the flywheel/crankshaft backward and read the movement shown on the indicator. Do this a few times to get an average reading. NOTE:
The pinch bolt must be released at the front flexplate to get accurate readings. The front flexplate will move along the splined area of the driveshaft with no problem, unless someone has put loctite in the splines. If that's the case, might as well pack it in since you will not be able to get a correct reading. You will never separate the front flexplate coupler from the driveshaft during this PPI.
7. On page 13-8 of the 928 workshop manuals checking end play of the crankshaft is discussed when replacing the crankshaft bearings. The manual states the play of new bearings should be within 0.110 to 0.312 mm and the wear limit is 0.40 mm. Check and see where you readings fall.
Mine fell within acceptable limits. When I got the car home I also took off the oil filter and tore it open to check for any metal. None found.
If you want to do this do not cut it open since you will introduce metal shavings into the oil filter.
Fast forward about 1.5 years ago when I decided to replace my connecting rod bearings as preventative maintenance in this same 1986.5 928S. The bearings almost looked brand new, no appreciable wear seen on the bearing surfaces facing the rear of the car. No weird marks in the crankcase which would have been there if the crankshaft would have moved in the case and it's counterweights would have smacked into the internal crankcase webbing surfaces.
I hope this somewhat long winded explanation will be of help to some wanting to buy a 928 automatic. I strongly believe that most of the catastrophic TBF stories we've heard involve driveline work just before the event with the transmission being removed from the car and the work done by mechanics/owners not knowing how crucial it is to not to have any forward pressure on the front flexplate after the job is complete. Of course there is a bit more to all of this but I will save it before all of you nod off. ;^)
On January 18th Constantine wrote a procedure for checking for crankshaft
thrust bearing failure (TBF). The procedure is good except for the conclusions
in step 6.
>6. Leverage the flywheel/crankshaft backward and read the movement shown >on the indicator. Do this a few times to get an average reading. NOTE:
>The pinch bolt must be released at the front flexplate to get accurate >readings. The front flexplate will move along the splined area of the >driveshaft with no problem, unless someone has put loctite in the splines.
>If that's the case, might as well pack it in since you will not be able >to get a correct reading. You will never separate the front flexplate >coupler from the driveshaft during this PPI.
If the "Loctite Procedure" was performed before the TBF check, you will have no problem doing the check, since the clamp will not have moved on the splines. That is the purpose of doing the "Loctite Procedure".
The purpose of the "Loctite Procedure" is to stop hub movement on the splines. With no hub movement, the flexplate gives the flexibility needed to pry the flywheel back and forth to check crankshaft end play as well as the reduction in constant pressure against the thrust bearing that can cause TBF.
For more than you ever wanted to know about TBF, check my site.
http://members.rennlist.com/captearlg/ The only thing missing is conclusion of what causes TBF. Maybe Porsche is correct and TBF is caused by mechanics error. The "Loctite Fix" seems to work, since there has never been a TBF on a car that had the "Loctite Fix" Performed.
If you have a 5 speed 928, don't worry about TBF. I don't think there has ever been TBF in a 5 Speed 928
Earl Gillstrom '91 GT
Hi Captain Earl,
It is my opinion that a good check should be performed with the flexplate un-clamped from the driveshaft to take away any un-needed pressure since one is trying to measure in the thousandths. The crank should be first placed all the way forward in the engine then leveraged back against the dial indicator. Depending on where the prior owner loctited the flexplate to the shaft could artificially stop the forward movement of the crank a bit, especially if the travel exceeds the flexplate's travel.
I have personally talked to one 928 owner who used the loctite procedure on his 928S4 auto. Upon a later inspection he found forward pressure on his front flexplate characterized by a forward bow of the flexplate. He then tried to reposition the flexplate on the driveshaft and could not since the loctite worked very well and froze the flexplate coupler to the driveshaft. I believe he finally solved this problem but with some difficulty.
I personally know of another owner here in my hometown who had TBF issues with his 928S4. He kept repositioning the flexplate only to have the forward thrust appear within a short time on the front flexplate. This also started the deep knocking sounds coming from the engine which was the crankshaft counterweights hitting the internal block webbing structures!
I then told him of your loctie procedure and he tried it. He squirted the loctite in the splines of the front flexplate and when he tried to move the flexplate/flywheel backward he only got a certain way before the loctite took hold. He said he couldn't budge it to position it as far back as he wanted. I drove the car after the procedure and you would never know you had a problem, unless you drove the car for a while and kept looking at the oil pressure.
I have also heard of another owner who could not un-do the flexplate from the driveshaft during a torque tube R&R. The reason was he used loctite in the splines of the flexplate coupler earlier during his ownership. He and his assistant used heat and force trying to separate the components and finally ended up un-bolting the flexplate coupler from the flexplates to remove the torque tube assembly. I read about this on this list in fact. This would then have precluded them from repositioning the flexplate to the driveshaft upon reassembly of the driveline. This repositioning should always be done after a torque tube R&R.
I am not trying to start a debate about the use of loctite or any other procedure over another. I just am stating what in "my opinion" can be done by prospective owners before their purchase. The procedure I discussed is not written in the 928 workshop manuals but comes from my own research into the 928 TBF problem. In my instance this procedure worked for me by the inspection of the rod bearings later confirming there were no TBF problems existing in my 1986.5 928S.
Your status amongst our 928 family is legendary and I have enjoyed my few talks with you. I am sure many have used your loctite procedure with great success and your method has probably saved a few 928s from TBF. But as with any procedure, care must be taken during it's execution and the consequences of using it should also be discussed.
I also believe that someone just recently responded that they had a earlier model 928 engine, 79 maybe, that was a five speed and suffered TBF. It is correct that TBF is much more common in a 928 automatic, more so with the later S4 models.
Here's a great writeup from Tony Harkin's site: