Lots of musings and thoughts being thrown on a topic which we've researched extensively. Our research was then used to come up with the 928 Super Clamp to stop the drive shaft pull out problem at the front flexplate which can lead to 928 engine thrust bearing failure (TBF). I will try to keep it short and hit the high points in order to explain this situation concisely.

1. When Porsche first devised the auto driveline they used washers, a bearing and a circlip at the front of the older 25mm drive shaft to help set the correct distance between the flywheel and flexplate. These parts can be seen in the work shop manuals (WSMs). The correct distance which was to be calculated by the mechanic in the field, would have the flexplate set a bit away from the flywheel and when the flexplate was clamped to it, there would be a bit of rearward pull on the flywheel.

2. Porsche learned of problems with their WSM instructions since mechanics were not correctly setting this distance and some customer 928s were returning with TBF. Porsche then sent out there driveline engineer on a world tour to teach Porsche techs how to properly set this distance.

3. Sometime in 1984 Porsche stopped using the bearing, washers and circlip arrangement at the front of the drive shaft and merely had the field techs just clamp the front flexplate clamp as the last step after a TT change out. This change is found in the WSMs. However the front flexplate clamp was not designed to hold onto the drive shaft by itself.
The front bearing, washers and circlip aided in this and when they were taken away it is our opinion Porsche should have changed the design of the front flexplate clamp.

4. Then Porsche came out with the 85-86 32 valve 928s which saw an increase in horsepower and torque. This further compounded the problem since the extra torque now twisted the drive shaft even more and the front clamp could not hold the drive shaft. Porsche started seeing a problem with this and instead of focusing on the clamp, they instead increased the diameter of the drive shaft from 25mm to 28mm sometime in 1987. I was told by the 928 Porsche drive line engineer this was done to handle the increased torque. However it's interesting that although the 5 speeds saw the same increase in torque, it's drive shaft remained at 25mm. It is our opinion that Porsche was trying to control the drive shaft pullout by increasing the drive shaft diameter.

5. Unfortunately this did not stop the problem of drive shaft pullout.
But it did give the 928 automatic drive line another failure point since Porsche, keeping it's old clamp design for the 25mm drive shaft spline, had to neck down the 28mm drive shaft back down to 25mm causing a stress riser at the neck down point. This has caused 28mm drive shafts to shear for owners in the field.

6. We looked at many different solutions for this problem and after testing a few, we came up with our Super Clamp design. It is *not* the one that Theo has shown. It is totally different and attaches at the front flexplate with no modifications, can be used with both the 25mm and 28mm drive shafts and clamps onto the drive shaft extremely well. This has stopped drive shaft pullout at the front flexplate.

Hope this helps with your discussions,
Black Sea R&D

Thought this was a understood point but I guess it's still in contention.

The drive shaft twists under load and shortens. It is due to it's design as a approximately 1 diameter inch bar which is pretty long. It's also called a "prop shaft." This is used in other applications to include the boat industry. That's why Porsche placed a flexpate at the flywheel to allow for this contractive force.

The front original flexplate clamp cannot alone stop the pullout of the drive shaft and it allows it to be pulled out a bit each time until it finds an equilibrium point. The contractive forces are much more than the subsequent pushing of the relaxed shaft and so gets hung up by the front flexplate clamp. The equilibrium point is usually seen as a 2mm-4mm rebound of the flexplate back unto the drive shaft after the front pinch bolt is loosened.

The only way this type of drive shaft will lengthen is when it deforms and just before it fails, just like when you bend a paper clip back and forth and break it. If you measure it just before the break it will be a bit longer. This however is not what causes most of the found forward pressure at the front flexplate in 928 automatics.

During our study we looked at the C-5 Corvette driveline to see what they were using. They have a hollow large diameter tube as their drive shaft and there is a rubber coupler between the drive shaft and flywheel which takes up rotational stresses but not contractive forces since there are none. They also experience TBF but due to heat expansion of their drive line. After a TT change they must run the car up to temperature then let it cool down and reposition the running assembly.

Last point, having any forward load unto the flexplate and ultimately the thrust bearing is bad. It will prematurely wear the engine thrust bearing and when that happens is any one's guess. That's why Porsche wanted *no* front load at all and devised the initial circlip, bearing and washer affair at the front of the drive shaft to have a slight tension pulling back on the flywheel. I have talked to many owners who have suffered TBF, a lot more than I care to have known about. Not everyone who owns a 928 is on Rennlist.



It seems very unlikely that the shaft shortens that much. 4mm flexplate offset into engine direction means that the shaft may be shortening as much as 8 mm before pulling out more into the transmission direction. no no... it must be something else.
A wobble in the flex plates fits the ideas I've accepted -- it would affect the clamp at between about 10Hz and 100Hz; it would affect the forward end of the shaft much more than the rear end; it would not involve special loads or movement on other parts of the car; it would be addressed by a strong clamp such as Theo's or Constantine's; and it might generate a force in one direction (towards the flex plates, or towards the un-torqued portion of the splined fitting) -- but I can't think of a way to verify this one.

Actually I thought of a way to verify this, or something very much like it.  One problem with the test setup that is being suggested is that for the amount of movement needed to be shown there will need to be in the millions of  revolutions. Another problem is that with the small amount of shaft that is being suggested the amount of movement will likely be much less than in real  life mainly because of the weight of the shaft, which I think plays into the  equation.

What I did a few days ago while this discussion was going hot and heavy was set up an experiment in my lab (shop) where I found a piece of aluminum round bar stock about 3 and a half inches long and I drilled a hole through it long ways to just under 3/4 inch. Then I intended to ream it to exactly 0.750 inch, but my reamer was too big to go in my tail stock chuck on the lathe, so I put the work piece in my mill vise and chucked the reamer in the mill and reamed the hole there.

Unfortunately (or maybe not) the hole did not come out quite precision in that is is centered on one end but about 3 or 4 thousandths off center at the other end. I put it in my lathe that way and proceeded.

Next I found a piece of stainless steel heavy wall tubing about 18 inches long that was laying there and slipped it in the hole in the "body" piece I now had in the lathe. I put it about midway on its length in the body piece. I turned the lathe on at about 400 rpm with the lathe turning toward me--as if making a conventional cut of the front of the body. That was with the lathe turning counterclockwise when viewed from the work position, just as if looking forward at the flex plate from behind the engine, and with rotation the same as the 928 engine.

At that rate of rotation the stainless tube gravitated out of the hole in the body piece about a half inch in about a minute. Then I reversed the rotation and the tube gravitated back the other way into the body about the same rate.

I reversed the body piece in the lathe and did the same thing and had just about the same result. This time however. I think I had the centered hole facing me and I think the rate of migration back into the lathe with reverse rotation was slower.

I was excited about that test and the results and tried to post it but I don't think it showed up. If it did I apologize for the redundancy.

What I was looking for was evidence of a kind of force that would put endways pressure on a rotating object such as a shaft. Oh, I forgot that I also did the rotation test as described in part 2 at a higher speed, about 625 rpm and the result was more pronounced.

The thing about the first test that troubled me was that with the body piece hole being off center there was a slight but noticeable wobble to the end of the shaft that was visible on my side, even though it seemed less pronounced when the experiment was reversed in the lathe.

What I was perceiving was that the body piece was playing the part of the flexplate and clamp assembly, although with no clamping, and that the tube was the playing the part of the drive shaft and also not clamped.

Then I thought that since in the 928 it seems to be the flex plate gravitating on the shaft rather than the shaft gravitating in the hole in the body, I should reverse the process. So, I chucked the tube in the lathe and put the body on it and turned it in the lathe. It didn't do much, but there was some discernable gravitation outward, but not much when the rotation was reversed.

Of course there was much less wobble to the rotating body than to the rotating tube when the body was chucked.

What I did next was decide that I needed to clean up my act and put a little more precision into the test, so I first chucked my reamer in the lathe and turned the end of it down so I could get it into the drill chuck in the lathe tailstock. Then I drilled another piece of 2 inch aluminum round bar, this time a little shorter, because that was what I had, and then reamed it while still in the lathe. Then I polished the stainless steel tube with croacus cloth to be sure it had no nicks or burrs and tried the experiment again.

This time to my surprise there was very little movement of the shaft in the body hole when rotating CCW and almost none the other way; and this was at just about any speed between 400 and 650 rpm. The experiment was turning very true in all phases.

I tried also to reverse the test as before with the tube chucked and the body floating and got essentially no migration either way on the tube in either direction of rotation and at any speeds as before.

I was disappointed by this test experience until earlier today when one of you put up the excellent piece about vibration. Now, when you have suggested this similar kind of test for wobble I am beginning to think that my first sloppy test has some validity, and more so than the more refined test.

Oh, one additional consideration I had was that perhaps the migration was somehow related to the lathe and whether or not it was level. I found that it is not level, but that most of the migration out of the hole was also uphill.

I think that this test or some of the results together with the discussion about vibration could lead us in the direction of finding the answer to the problem that so many have already solved for us, but without knowing what is causing it.

Any further ideas would be welcome.

Jerry Feather

From: Wally Plumley [mailto:wplumley@bellsouth.net]
Sent: zaterdag 10 oktober 2009 5:09
To: 928
Subject: [928] Re: TBF discussion

One suggestion that has been made is that the drive shaft is twisting and shortening upon hard acceleration. This shortening is very powerful, and is causing the driveshaft to slip back in the clamp. When the shaft has pulled back, it then puts constant force on the thrust bearing. The force is especially destructive during dry starts. Eventually this destroys the thrust bearing, then the block.

I suspect that what we have here is a failure to communicate, and that the flexplate actually moves to the rear on the shaft when released, rather than the shaft moving to the rear.

Wally Plumley
928 Specialists

Hi Wally and others,
if you remove the clamp form the assembly that holds the TT, you see that there was a lot of internal movement. It is easily seen on the pictures on my web. To me it shows that there is a micro movement under the clamp, and that combined with a pulling force makes the clamp creep. Imagine that the surface of the clamp and the surface of the part that holds the TT constantly move a little bit over each other when the rotating forces of the engine is applied. The metals shift and slide, If you pull a little, the slide is into one direction, the pulling direction. When the forces, stop there is no sliding back as the clamp starts holding again.
At some point the forces start to balance and there is no more creeping. It doesn't move back as there is no reverse force. If there would have been an identical reverse force the clamp would creep back into place.

The TT is pulled out of the clamp and stays there when the clamp gets grip again. That moves the clamp towards the engine on the TT, causing the flexplate to bend in the engine direction. When you release the clamp bolt, the clamp moves onto the spline into the transmission direction. The protruding spline bit that you see gets 2-4mm shorter. (Wally said this already, just restating)
If someone thinks driving in reverse would help, that needs some explaining to me :))

It is assumed (!) that the force pulling on the TT is caused by twisting. Also temperature expansion has been mentioned. There is no real proof yet.

Another very good solution to this problem would be a more flexible flexplate. We tried that and came up with a design that would work, but looking at the strong design of our flexplate, it was not possible to keep that strength. It failed badly in our test. Another option was make a loose coupler, basically a rubber spider link between two fixtures. But space, difficult assembly, durability and cost were an issue. So we came up with a proper and very strong clamp, and that works :)
I am convinced that this is what Porsche actually intended with the original design but failed to achieve.

My advice: take Constantines or our stronger clamp, or keep loosening the clamp on a very regular basis. (I hate loctite to glue it in place but that works too)

To Johnny: dear friend, we agreed not to agree... right?
I will put the Porsche statment online on my web later today. Not that is any good to anyone. It looks to me like a disclaimer.

1992 928gts Midnight Blue (2006-)
1988 928s4 Cherry Red (1999-2006)
The Netherlands

From: Johnny Billquist [mailto:bqt@softjar.se]
Sent: zaterdag 10 oktober 2009 10:52
To: 928
Subject: [928] Re: TBF discussion

Of course, an equal force in the other direction will also appear upon
no acceleration. So why don't that force count?

The clamp slips when you apply force in one direction, but don't slip
when you apply the same force in the other direction... Impressive.

Also, how much do you think the shaft will shorten because of this twisting?

We are bedating again...:)

**Assuming** the shortening of the TT because of TT-twist is the actual cause of the pulling force, you will see that twist effect on both accelerating and decelerating when the rear wheels drive the engine. The Accelerating force will be stronger obviously. But there will not be a pushing force to complement the pulling force. Now, when decelerating, the rotational forces working on the clamp will be far less, allowing the clamp to hold by friction. No micro movement anymore.
The clamp only moves when under serious load, that is my believe.

The TT twisting will be very small, not 2 or 4 mm I think. It is the creep effect that makes this large relocation. There was a debate long ago about this twist effect. Don't remember any numbers or calculations though.

Porsche says that fixing the clamp to 85Nm and after that, fixing the flexplate to the flywheel will cause pressure on the TBF which is not covered by warranty. The clamp should be the last to tighten. They are absolutely right. It also makes it hard to prove that something else caused a TBF as obviously some mechanic must have screwed up or did not read the service bulletin at all. And do all mechanics know all the service bulletins? Do we....?

This discussion can go on and on for ages.... I'd fix the problem without exactly knowing the root cause.

1992 928gts Midnight Blue (2006-)
1988 928s4 Cherry Red (1999-2006)
The Netherlands


Agreed, both acceleration and decelleration will cause twisting forces. I thought it was said differently, like decell would reverse it. Obviously not.

At some point all forces are in balance if you count the friction of the clamp in too. Yes, friction under the clamp plays an important role here.

The real question is where does that force originate from.

Think of the frictional forces like this: Imagine the clamp not from steel but from plastic. A plastic clamp on a broomstick. At forces the steel deformes pretty much. If you twist the plastic clamp with force, and pull out the stick with a axial force, you will see tiny movements every time you do that. Turning forward/reverse is a good practice to remove the plastic clamp from a stick. The clamp moves bit by bit until it is off. Just pulling would not do it.

Did you notice this:
Look at the movement marks. I did not do this. These are traces of internal movement under the clamp. The whole thing twists and deforms under load.

Imagine this, please follow the thought:
- we start at zero pre-load
- we drive and for some reason the TT shortens (assume, we know there is a force), pulls on the clamp
- the flexplate bends to the rear
- we pull harder
- the rotational forces cause micro movements under the clamp, and the TT is pulled out a 1mm
- the rotational forces stop, and the system goes in rest.
- the clamp does not move but holds since the cause of loosing grip is gone (rotational)
- the flexplate is now 1mm pushed towards the engine (TT back to normal)
- next time we drive the TT pulls again, but needs to compensate the 1mm preload
- it needs to shorten 1 mm to reach the zero preload point
- TT shortening pull continues and forces reach a point where the clamp slips again, lets say 1mm extra
- rotational forces stop and the system settles again, but now at a pre-load of 2mm
- the force from the flexplate is less that the friction force that holds the clamp on the TT
- the flexplate is now bent 2mm into the engine direction. Without rotational force the clamp holds that position
- and so on. The max movement I heard is about 4mm.
- maybe at some point the the flexplate makes the clamp move back a little until forces reach a equilibrium, who knows

Imagine shortenening of the TT of lets say 5mm. That seems like extreme to me.
One other option is that the crank itself causes this, basically moving the flexplate. It would pull the flexplate towards the engine. I don't see that happen and the crank play is just 0.2mm or so. One more option is the thermal effects, where the expansion of the tube is different than the expansion of the central shaft. But 5mm?

So.... So I'm still puzzled. I'm an electronics engineer, mechanical is not my game.

Anyway, my big concern is that 2 or 4mm pre-load on the flexplate is achieved at 300-500kg force. And this is so much that you canít expect the flimsy thrust bearing to cope with it. But worst is that this is not a momentary force, but a structural one that is at its highest point when the engine is not under load, and it decreases as the engine is under heavy load. Even reverses direction I think. The severe load is when the engine is poorly lubricated, just after starting. That is why all those thrust bearing failures happen. Not because some lunatic tightens the rear TT clamp, front TT clamp and then starts to bolt down the tranny by pushing it into position. Same goes for putting in the engine, causing stress on the flexplate when bolting it down to the flywheel, thus pulling things together with M10 bolts. Porsche makes it very clear that there should be a stress-free situation when bolting down the clamp.

In my opinion, and you don't have to agree, it would be best to have either a sliding clamp at the flexplate, a better flexplate that absorbs the 5mm or.... Or.. a clamp that at least prevents a structural preload when there is no severe pull from the central shaft. The better clamp makes it a momentary force at a point when the engine is lubricated at its best (hot thin oil under pressure at higher rpm's) and settles back at zero pre-load when shut down.

And by far: Porsche said themselves not to have any stress in the central shaft when tightening the clamp. This preload is what they said *not* to have and repairs resulting from ignoring the procedure will not be under warranty. So if you still think it is ok to have flexplate pre-load you must think again :)

Damn long story, I know, it also helps making up my own mind :)

Ok, lets rest his issue until a *new* bright idea pops up.
1992 928gts Midnight Blue (2006->)
1988 928s4 Cherry Red (1999-2006)
The Netherlands