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Schocki 09-22-2005 12:24 AM

Failed torque converter bearings or why do we have these flex plate issues???
My rear torque converter carrier bearings went bad on me last week. The car has 110.000 Miles on it and ran up to this point absolutely fine, it is my daily driver. I ordered all the parts and as a preventive maintenance also a rebuilt torque tube from Mark Anderson.

The rear axle and tranny are out and I removed the torque converter. One of the bearings on the torque converter housing is bad (most likely the converter side).
My rear crank seal is also leaking and will be fixed, the torque converter seal will be replaced as preventive maintenance.

Now here is the part that is interesting for me. I released the front flex plate clamping bolt about 6 months ago and it moved about 4mm. When I remove everything 2 days ago the flex plate was again slightly bend towards the engine. When I removed the bolt, the shaft again moved about 2mm.

I know there have been many discussions why the torque tube/ Flex plate moves towards the engine and ultimately if no pressure is released, the thrust bearing in the engine could fail and cause major engine damage. I donít think that the torque tube increases its length due to torsion forces. Why is this problem only present with automatics and does not happen (or really seldom) on manuals?

I asked in a previous post about the factory length of an automatic torque tube. If you have it please post it. As soon as I get the rebuilt torque tube from 928 Intl. I will measure the length.

What I think so far is that the carrier bearing closest to the converter starts to fail over time. This is caused due to ballooning of the converter itself and pushing against the bearing. Somebody in Germany posted that he had his carrier bearings replaced about 3000 miles ago and so far there was NO movement of the torque tube/ flex plate toward the engine. This makes perfect sense (to me) so far. But the one thing that surprised me is, that when I remove the carrier bearing housing form the tranny it looked absolutely normal. No signs that the shaft which holds the clamping bolt assembly ever moved forward. I have to see how the bearings look when I press them out. But one of the bearings inside the housing is definatly bad. The grinding noise can be easily heard when turning the flex plate assembly.

To my knowledge the rear flex plate assembly that hold the converter can not move forward unless a bearing gives up. The circlip that sits on the shaft prevents a movement towards the tranny.

Thatís allÖ(for now)

P.S. I just found the time to try to simulate the Torque converter ballooning against the Flex plate and the bearings. The defective bearings are still in the housing and I just put the whole thing into my press at home. I applied some pressure to the central shaft and there is no movement at all. That means that there is no possible way from the tranny side that pressure is applied towards the direction of the engine.

What is the reason for the need to release flexplate pressure???

Any thoughts???

Schocki 09-22-2005 12:42 AM

What causes the thrust bearing failures in your opinion?

borland 09-22-2005 12:56 AM

The torque converter bearings are deep groove bearings which can handle greater axial thrust than normal ball bearings. When these bearings wear, clearances allow vibrations to increase. It sounds like you detected these clearances and vibrations in the form of bearing noise.

When the bearings vibrate, they do so in two dimensions. One dimension is in the direction of the drive shaft or torque tube. The deep groove bearings thrust benefit, now is a detriment when considering this vibration. So, if you can imagine, the vibration would be like taking a hammer and tapping on the end of the shaft. Like you say, the shaft is constrained by the circlip and the bearings which are pressed on the torque converter flex plate shaft.

This hammering effect of worn converter bearing causes the forward flex plate splines to migrate forward very slowly. Sorta like driving a stake in the ground. The front flex plate probably moves forward, and not toward the rear of the car, because some of the wave energy of the vibration is more easily disipated by the movement the flex plate. The flex plate eventually finds an equilibrium position where the flex plate; which now under compression, resists any further forward movement.

Hope that helps.

90' S4, Slate Metallic

m21sniper 09-22-2005 04:11 AM

I really need to take apart this whole system on my 928 so i can get a grasp of what's going on in there.

Of course i won't...


Vilhuer 09-22-2005 08:38 AM

Originally Posted by Schocki
The rear axle and tranny are out and I removed the torque converter.

If you can, please take as many pictures as possible and try to figure out how it would be possible to install torque converter back in so that it would push TT center shaft forward. This could help Michael's case:

Why is this problem only present with automatics and does not happen (or really seldom) on manuals?

Manual cars have clutch that separates TT center shaft and crankshaft from each other. Clutch shaft (E) is mounted to TT center shaft but it's not in direct contact with crankshaft (X). There is pilot bearing (Y) in between. Clutch shaft (E) goes inside pilot bearing (Y) but there is free distance (A) between wider part of the clutch shaft (E) and pilot bearing (Y). If TT center shaft moves forward enough and distance (A) is removed all together manual gearbox car can suffer from same TBF symptoms as automatic cars. This is rare though and probably requires something to be seriously wrong elsewhere in TT structure to happen.

On my GTS engine, 3rd main bearing was worn similarly as what starting thrust bearing failure must look like. Rear side was almost totally through top coating on the the bearing. Luckily crankshaft had not suffered at all. It's possible that clamp at gearbox input shaft had been loose as center shaft bores were worn somewhat.

Nicole 09-22-2005 06:15 PM

Thank you for posting this here, Schocki! It seems we (or borland) might be getting closer to figuring out what causes this "cancer of the 928" and how it can be prevented.

It would still be interesting to do a measurement project, where we would verify that the shaft indeed stays at factory length, regardless of mileage and use. Has anybody done that?

Schocki 09-22-2005 11:09 PM

3 Attachment(s)
Thank you for explaining the vibration issue. What it comes down to is that slightly worn torque converter bearings cause an increase in vibration. These vibrations act on the torque tube just like a power shizzle that is held against the rear of the rotating shaft.

As you said the torque converter bearing assembly is held in place by the circlip. The rear clamping bolt that connects the torque converter to the shaft sits in a groove and can't move either.

But the front flex plate sits on a "smooth" shaft and has no groove that secures the position. The reason why there is no groove is simple: TOLERANCE. The clamping action is good enough to hold the flywheel flexplate in place and it does not move until the very high frequency vibrations start. That is the point where the front flex plate start to move toward the engine. It finally rest when the flex plate created enough counter pressure. But this pressure is pushing against the crank and ultimatly can cause thrust bearing failure.

It makes know perfect sense that Porsche attempted to "fix" the problem by increaing the torque on the clamping bolt. But the fix does not work it slows the flexplate movement down, that's it.

What is the permanent fix? Increaing the torque on the bolt was a good idea but not enough. I know some used Loctite to prevent the movement of the flexplate. Or the re-designed clamping bolt?

I think the only real fix would have been if Porsche would have adopted a clutch like flexplate assembly. Where the torque tube shaft would sit in a pilot bearing and the flexplate (reinforced) would have a chance to move, just like a clutch.

I think I will go for the Loctite solution. I want a fix that serves me well for years and somewhat cost effective. I'm installing a rebuilt torque tube, new torque converter bearings and a new crank seal. If the Loctite will prevent the flexplate from moving and my torque converter bearings last another 110.000 miles it is an OK solution.

The pictures below show the different setups (automatic and manual) I circled the significant differences.

Nicole 09-23-2005 12:06 AM

Either I'm missing something, or we stil have not explained the following:

Owners have reported that over time the shaft moves forward to the point where there is lttle adjustment left on the flex plate side (front) of the shaft. So, ulsing loctite to "clue" shaft and clam together would still reslut in pressure on the crank, because the shaft moves.

But if the torque converter can't move, the shaft is clamped in the rear, and vibration is the only force on the shaft, then why does the shaft move closer to the engine?

I think we still need to measure the lengths of new and old shafts.

Bill Ball 09-23-2005 12:52 AM


If you get a rebuilt TT, you can get the earlier shaft with the shim and retainer system and set it up to prevent the TT from moving back through the clamp - no Loctite needed.

Vilhuer 09-23-2005 06:15 AM

How about this. Drill suitable sized hole from crankshaft direction towards gearbox to front end of TT center shaft. Lets say 2cm (3/4") deep. Make thread to it, maybe M8 or M10 if metric size. Put 2cm long bolt to it and under bolt head, add few mm's thick washer where center hole is just larger than bolts thread. Stack up suitable thickness shims with 25mm large center holes between this support washer and flex plate. This would simulate factory '83-84 ('84 only in ROW) model year 4sp tranny tubes retainer system. Probably Loctite solution is good enough also though.

Regarding other problem of center shaft moving forward. What exactly moves? Does rear flex plate stay in it's right place and only TT center shaft moves or do they both move? If only center shaft moves Loctite at rear flex plate clamp should stop this. If center shaft lenghtens, only sliding connection front flex plate will ever solve that. I would think whoever does (for example 928 International's) TT rebuild tubes would know the answer if shafts lenghten or not. Another matter is if they are going to tell what they have learned over the years.

Schocki 09-23-2005 09:19 AM

Yes the tube itself can move to but only up to a certain point and it is stopped.
The front flex plate movement is stopped by the flywheel.

I checked this out yesterday: I took a large screwdriver clamped it in a vise and put a nut over the shaft. I used a small hammer and tapped against the rear portion of the handel. If you find the right frequencey the nut starts to move forward or rearward. This depends on the frequency you tap with...

Try it! That's how it works..

Schocki the wannabe Myth Buster...

tammons 09-23-2005 09:38 AM

All this sounds really bizarre. I have never heard of a car that suffered from drive shaft creep like these do. Actually i have never heard of any cars with drive shaft creep.

I know this has been discussed before, but what year cars does this affect. I have an 83S that i am thinking I should check.

Also what actually causes the creep. Is some part of the torque converter moving around.

docmirror 09-23-2005 10:29 AM

Well, I have the answer to the 'why' questions from above. Unfortunately it's long, boring and not worth the electrons to fully explain. Let's see if I can use a simile and then apply that to the system to see where Porsche went wrong;

Think about it this way - You take a towel, get it fully wet, then lay it down and measure the length. Now, in an effort to dry the towel, twist it from both ends into a spiral by applying TORQUE to it in opposite directions. Once twisted into a spiral, set it back down and measure the length again. It's shunk by quite a bit right? Take the towel, untwist it, and lay it back down and measure it again. It should be as long, or longer than the first measurement of the wet towel.

That is called torsion modal shrink. Now, if you can think of the towel as your torque shaft, inside a torque tube, where the ends are rigidly fixed, I think you can see the problem when torque is applied to the shaft repeatedly within an undamped system.

What they could have done differently; Most engineers would use a coupling at one end to relieve not only the coupling moments, but also that slight modal shrinkage that occurs. A Lovejoy or other type of elastomeric element at either end would have absorbed this torque and allowed the shaft to shrink and grow as the torque is applied then removed. The "flexplate" performs this function to a certain degree, but apparently they did not allow for enough longitudinal compression to stop the clamps from moving.

It appears that the clamps on one end allow the shaft to shrink, and then when torque is unapplied, that shrinkage is not taken up by the clamped area, but forces the flexplate to bow in as viewed from the crankshaft. This then preloads the crankshaft, and the whole process is started all over again when more torque is applied. Do this a few thousand times, and it looks like the take up exceeds the play in the shaft, and starts eating into the bearing by too much preload.

Why autos more than stick? The answer is left as an exercise for the community. Hint; think about how an auto car is used on occasion. How much torque twist does the shaft encounter? I don't have the specs for it, but I would estimate under full load, worst case, short moment twists of probably 150-200 degress end-to-end. The air cooled VW trans input shaft experiences this, and it's much shorter. Of course, VW put splines on the shaft, and torsion springs on the clutch plate for billions of hours of good service. That, or the 40 HP just doesn't put much stress on it :)


borland 09-23-2005 12:20 PM

The earlier shaft is a smaller diameter.

Under elastic deformation, the shaft does not change it's length. The S4's max. torque is only about 300 ft-lbs.

As preventive maintenance on the later automatics, replace the torque converter bearings and rebuild the torque tube every 75K miles. No worries, no locktite.

tammons 09-23-2005 05:33 PM

Okay, I got it now. Another bad design issue on a otherwise superb car. I am suprised they did not address this problem in the later cars as it is pretty serious.

The 944 automatic does not have these problems, but they have some sort of rubber flex disc at the flywheel end. Unfortunately it is prone to failure.

I ran into someone a while back that replaced the flexdisc with a pressure plate and clutch disc (non operational) and that worked for him and no more flexdisc failure.

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