Failed torque converter bearings or why do we have these flex plate issues???
 

Gents,
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 don’t 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.


That’s all…(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???

What causes the thrust bearing failures in your opinion?

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.

borland
90' S4, Slate Metallic

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...

LOL

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:
https://rennlist.com/forums/928-forum/222045-crankshaft-seized-after-trans-repair.html

http://members.rennlist.com/vilhuer/ClutchDiagram.gif

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.

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?

Borland,
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.

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.

Shocki:

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.

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.

Nicole,
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...

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.

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 :)

Doc

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.

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.

Docmirror: This is very interesting stuff, but again it does not explain why some people say that after a while there is no tolerance left for adjustment on the shaft. I think it was Bill Ball who told me that his flex plate clamp is quickly approaching the end of the adjustment range.

This can only happen when either
a. the shaft moves and stays forward
b. the shaft gets longer

How could you explain that?

Just a few thoughts on the forward migration of forward flexplate clamp assembly applying load on the after end of the thrust bearing via the crankshaft thrust collar.

A lot has been written on the subject including 10% extra torque on the forward flexplate clamp’s allen headed set bolt to prevent its migration, however, test have been reported to have been carried out on the frictional resistance of this extra clamping torque and the force need to overcome the frictional resistance of the clamp was nearly the same axial load applied to overcome the frictional resistance when the clamp was torqued up to the original specification.

Locktite 290 has been applied to the splines with claimed success on stopping this forward migration of the forward flexplate assembly on the drive shaft and others in the racing area have modified the clamp to a collet type arrangement.

I initially released the clamp on my 1990 928 S4 in February 2004 and it moved 3.8mm and the thrust bearing axial clearance was 0.008" or 0.20mm which was well within the tolerance given by Porsche AG of 0.110 to 0.312mm.

A week ago I released the clamp bolt and the flexplate sleeve moved approximately 2mm aft towards the transmission and the axial thrust bearing clearance is still 0.20mm measured with a dial indicator. I renewed the clamp set bolt and re-torqued the set bolt 10% over the specified torque. At this juncture no Locktite 290 had been applied, but I am considering it.

So why does this migration happen and what causes the forward flexplate clamp assembly to move forward along the drive shaft splines?

I suggest that the clamp does not move forward along the drive shaft splines due to ballooning of the torque converter or the increased length of the torque tube drive shaft as some have suggested, because if the drive shaft was to move forward via either of the two mentioned methods and the forward axial force acting on the forward flexplate assembly was strong enough to overcome the frictional resistance of the clamp then the flexplate assembly would move aft along the drive shaft splines towards the transmission. This does not happen, as all reports indicate that it moves forward toward the engine.

Let recap, the set up of the engine, torque tube, drive shaft and the associated flexplates fitted to the automatic transmission models (My 1987 to 1995) is basically one bolted up integral unit with the drive shaft connected to the engine flywheel and the torque converter by flexplates. The aft flexplate clamp should not move as is clamped and locked in position via the allen headed set bolt that is located in a groove through the splines of the drive shaft, so the forward flexplate is the only one that can move, as it is held in position via the clamp through frictional resistance.

However, it has also been reported that the after clamp set bolt is not a “fitted” bolt in the groove, so there is some room to move axially until the set bolt outside diameter abutts the surface of the groove if there is sufficient axial force to overcome the frictional resistance imparted by the clamp, so both clamp assemblies can move. It would be interesting to investigate whether the aft flexplate actually migrates in service (depending on the positioning of the aft flexplate in relation to the groove in the drive shaft, and whether it moves forward or aft along the drive shaft splines. If in fact it does move, its movement would be limited by the clearance of the groove around the set bolt.

The most plausible explanation, as to the cause of the migration of the forward flexplate clamp assembly is by the torque imparted into the drive shaft whilst the car is under acceleration.

However, any increase or decrease in the axial length of the drive shaft caused by ballooning of the torque converter, growth of the drive shaft or torquing up of the drive shaft during acceleration is contained by the crankshaft thrust bearing and the deep ball bearings just forward of the torque converter and in consequence any movement of the drive shaft should be taken up by the forward and aft flexplates, flexing.

It has always been reported the forward flexplate clamp migrates forward not aftwards, this therefore eliminates the growth of the drive shaft or the ballooning of the torque converter as the reason for the forward migration. If the drive shaft did increase in length or the torque converter expanded (ballooning) it would cause the forward flexplate to move aftwards if the axial force applied overcame the frictional resistance imparted by the clamp .

The forward movement of the forward flexplate assembly has been verified by, measuring the bowing of the flexplate towards the engine via a straight edge, the painting of the flexplate clamp assembly and the spline area of the drive shaft that gives a clear visual area at the rear of the flexplate sleeve where there is band of no paint on the drive shaft splines, and when the clamp bolt is slackened the flexplate splined sleeve moves aft toward the transmission.

From the above I believe that the migration forward is caused by torquing up of the drive shaft via the transmission of power reducing it length.

Hypothesis of flexplate forward movement.

When a rotational force is applied to a shaft the shaft twists and the angle of twist can he measured and this angle is directional proportional to the quantity of horsepower (torque) applied to the shaft (as a matter of fact this is the method used to determine the horsepower transmitted by the propeller shafts in large marine steam turbine installations in ships). As the shaft is elastic it will return to its original position and length once the driving force has been removed unless the elastic limit of the material is exceeded by excessive twist.

This angle of twist can he calculated by applying the maximum torque (rotational force), delivered by the engine (in ft lbs on N/m), the length of the drive shaft, its diameter and its modulus of elasticity. Once the angle of twist is known the shortening of the shaft can be calculated.

The problem of migration appears to be from MY 1987 with the introduction of the S4 automatic model until the 928 was discontinued in the GTS form in 1995. From the three dimensional view of the various torque tubes from the 3 speed auto and the introduction of the 4 speed auto transmission in the WSM and PET, I cannot determine whether there has been any major modification to the forward flexplate assembly, so could this phenomenon of migration been caused by the increase in horsepower of the S4 through to the GTS autos or a change in the elasticity or diameter or the drive shaft?

If we accept this forward migration of the forward flexplate assembly is due to the driveshaft shortening then the possible explanation/hypothesis as to why the forward flexplate migrates forward along the drive shaft splines is as follows:

The automatic transmissions fitted to all 928s are fitted with torque converters. Torque converters allowed for the removal of the clutch on automatic transmission automobiles, which allows for smoother take off, adjustable smoothness of gear changes and also for idling whilst the vehicle is stopped (probably not all the reasons, but sufficient for the argument).

When the throttle is applied, the take up of the driving force is initially by the torque converter due to the increasing of the sheering force of the oil circulating with the vanes of the torque converter and as the rotating speed of the forward rotating element of the torque converter is increased so does the velocity of the circulating oil and in consequence the force to continue sheering the oil increases until the aft rotating driven element speed nearly matches the speed of the forward driving element. realising that there is always a small amount of sheer. The force to sheer the oil at high rpm is adequately demonstrated when torque converter stall speed test are carried out as the S4 stalls out at around 2,400rpm from memory.

The action from the torque converter on the drive shaft is a gradual twisting up of the drive shaft as it absorbs the horsepower being applied until the reaction of the drive shaft, angle of twist, equals the horsepower applied (Newton’s 2nd Laws of motion (I believe) “For every action there is and equal and contrary reaction”). The drive shaft angle of twist reaches it equilibrium point when the car reaches it cruise speed on a flat consistent surface road with constant wind resistance etc. Unfortunately this point of equilibrium is never constant due to road surface conditions, bends in the road, hills, overtaking etc.

If throttle is depressed all the way (pedal to the metal) for fast acceleration from a standing start the torque developed through the gears follows the torque/speed curves and the angle of twist of the shaft follows these curves. These different loads are applied to the drive shaft as the transmission changes gears and, as there is also a ‘kick down’ switch installed, this causes more cyclic loading during aggressive acceleration. In addition, depending upon the setting of the pressure modulation valve within the transmission it is capable of varying the mode of change from an almost undetectable change (via slippage) to a harsh “kick in the back” type power change.

The various torque loading on the drive shaft are always more excessive during the ‘change up’ or accelerating mode than the ‘change down’ mode when slowing down mode while the engine is applying a braking force.

During the acceleration cycle assuming the end play clearance of the thrust bearing is around 0.20mm or 0.008” (as with my car) and assuming the aft flexplate is firmly locked in position by the groove in the drive shaft, then the forward flexpate is flexed in the aft direction during torque up. In addition there could even be a shock loading imparted momentarily when the thrust bearing collar is bumped into the forward end of the thrust bearing during acceleration. If the flexplate has a pre load (as initially required by Porsche AG or the flexplate has already started it migration) this shock loading will always be present.

The shock loading and the torquing up force could be of sufficient magnitude to over come the frictional resistance of the clamped flexplate sleeve journal to the drive shaft and the forward flexplate assembly moves a very small amount, a matter of 10th of a mm or thousands of an inch during each cycle of the above, that is at every gear change.

You can imagine the number of torque up cycles that occasions in a daily driver and the distance the forward flexplate sleeve migrates can soon mount up. Some posts have mentioned that when the flexplate clamp sleeve starts to move wear has started as well, so the frictional resistance will also start to decrease. This migration forward on the drive shaft can ultimately cause excessive load to be applied to the thrust bearing and in some case can result in the thrust bearing failure.

Please note that the collar type thrust bearing is possibly the least efficient type of thrust bearing and when the pressure gets too great the oil film will break down causing face to face contact, excessive heat generation and subsequent bearing failure especially if the faces are kept in continuous contact with excessive load. Reciprocating engines are not usually designed to accept continuous crankshaft axial loading.

The manual transmission 928 only have cyclic loads applied during clutch operation, which only load up the thrust bearing for short intervals and once the clutch is unloaded the oil will cool the thrust bearing, whilst maintaining the oil film.

Why do some cars suffer failure and others do not?

Some cars may suffer from a double loading effect especially after a torque tube change. As previously stated, the aft flexplate assembly is held in position by a clamping set bolt via friction and also by the groove location. If the set bolt, when assembled, was located with its outside diameter abutted to the aft end of the drive shaft groove and the above mentioned migration also occasioned with the aft flexplate, until it takes up the clearance, this could possible give an initial double load on the thrust bearing.

The cyclic torquing during acceleration gear changes causes the drive shaft to be drawn further out from the flexplate assembly until a resident state is achieved. This resident state could happen when the flexplate moves forward sufficiently that the flexplate sleeve follows the shortening of the drive shaft aft without any further migration forward. This could occasion when the aft axial force from the bowed flexplate matches the residual frictional resistance remaining via the clamp. This reciprocating movement should occur mainly during acceleration and would cause a fluctuating load on the thrust bearing and when the car has reached it cruise state it will maintain a continuous load on the aft face of the thrust bearing via the collar on the crankshaft.

This residual state seems to occur around 4mm load up of the forward flexplate and this load may be sufficient to cause failure of the thrust bearing in some engines over time, as the load could cause the oil film to fail with face to face contact, heat generation and bearing failure. The failure of the oil film could also be dependent on the type of oil used in the engine and the type of driving, aggressive compared to milder acceleration.

If this hypothesis is indeed correct then the flexplate clamp should be released at least at each service period ( every12 months) and re-clamped to the specified torque plus 10%. More frequent observation is possible by painting the forward flexplate clamp sleeve and drive shaft and at regular intervals checked for migration by viewing through the hole at the bottom of the bell housing using a small 12 volt globe on the end of a probe (this can be made up very simply). I would suggest that any migration detected should be released immediately.

As a further precaution some owners may wish to apply the Locktite 290 to the clamp sleeve splines and splines of the drive shaft to stop this forward migration. If this is done and is successful in stopping the migration, then the thrust bearing will only suffer fluctuating loads during acceleration and when there is no migration there will be no residual axial load whilst cruising. This state will actually emulate the manual gear box operation.

Tails 1990 928S4 Auto.

First of all thank you for very good arguments. Unfortunatelly I have to disagree a little. Since it seems about 4mm is max. amount of flex on front plate, in order of any theory to successfully explain TBF phenomenon it needs to be able to explain this 4 mm flex also. I haven't done calculations on how much TT center shaft would contract but I find 4mm quite a lot. Also we must keep in mind that flex plate will resist clamps movement and flex somewhat before clamp movement happens. This means center shaft would need to contract at least few mm's more than 4mm to be able to create 4mm offset on clamp. I would expect this movement also to happen at once, not little by little over longer time period.

I'll throw in one more theory I read once. Not sure if this would solve the mystery or not. It could be one more piece of the puzzle where many different phenomenon work together and create TBF as end result.

If we think crank and torque converter as end points and static we have two separate systems that will expand and contract when temperate of the parts change, one inside other. On outside there is aluminium clutch bellhousing, (at least partly cast) steel TT outer tube and aluminium torque converter housing. Inside are flywheel, front flex plate, TT center shaft and rear flex plate, all made of different steels. When both front and rear clamp are tightened all parts have their respective dimensions on temperate of the workshop. This locks them together from front and rear ends. TT bearing in the middle don't really play major part in this as center shaft moves inside them quite easily compared to what it takes to seriously flex flex plate.

When car is driven, parts will heat up and/or cool down as result of outside temperature, heat from engine cooling system, warm exhaust, etc. What effect these temperature changes have to above mentioned parts? They can be seen as two rods. One consisting of aluminium-steel-aluminiun and other steel-steel-steel-steel. Will temperature changes into these seven parts be enough to cause one set to expand or contract more than other?

I don't know what effect if any temperature has to all this. Just one more thing to consider when calculating what's going on.

I don't see manual having quite same loads as clutch plate bore can move very easily on clutch shaft. It is always position of least resistance by far and should never require any real pressure to move.

Just wanted to add-on my latest findings:
I replaced the torque converter bearings last night ad they had quite some play. Each bearing had a 0.4mm for/aft movement. This adds up to a maximum of 0.8mm movement.
For my understanding the situation is clear:
With brand new bearings and no axial movement the front flex plate does not move. Just because there are no forward/rearward vibrations present. As soon as the bearings start to wear the flex plate is slowlly pushed forward. Finally when there the infamous "rear axle" vibrations are present at 3000 rpm or higher speeds, the bearings are close to be worn out ad need to be replaced.

I think Porsche understood the problem and allowed to torque the pinch bolt to 110% in an attempt to stop or minimze the flexplate movement. It is to bad that the pinch clamp assembly can't be bought seperately. Otherwise I would have tried to use to clamps @ 110% torque to stop the movement. I will use Loctite 290 and painted markings on the shaft. Below some pictures of my bearing measurements the a feeler gauge.

Way to go Schocki !

The WSM procedure includes heating the bearing housing in an oven. The bearings will fall right in with that heat applied. But you still need a press to remove the old bearings and put the shaft back in place. That's the way I did mine, per the WSM.

During the assembly, I lubed my splines with moly grease (per WSM) and torqued the pinch bolts to the original WSM spec. With the new bearings, I had zero movement of the forward flex plate when I checked it after 2000 miles.

borland

Borland,
I used an electric heat gun and heated the housing with it. Worked very well, no problems.
If you don’t have this there is als an opposite way.

Put the bearing in the freezer!!!


Update,
Took the torque tube today and measured my old torque tube against the rbuilt unit frim Mark Anderson.
I tried to be very precise and here is what I got:

Rebuilt: 136.9 cm
Old: 137.08 cm

That is a total difference of 1.08 mm or 0.0425197 inches.

After 110.000 miles this a neglectible differce!

I do not believe that the torque tube shaft extends in length over the years. If it is getting “longer”, it is due to worn torque converter bearings and a little bid of movement due to tolerances in the rear clamping assembly.

I also found that the front flexplate is NOT made out of a bi-metal! It is simply made of to high grade steele metal sheets. These are bolted togther in the center by the clamping shaft bolts and later when the flexplate is mounted by the 6 flywheel bolts. Other than this the sheet metal is not coected or welded together. This is the reason why the flexplate ALWAYS comes back to it’s original completely straight form. If it would be a single sheet, it would bend over the years and would not completely release all tension. Very simple but a very effective design.

I also checked my crank for thrust bearing wear and measured 0.3mm. The factory maximum is 0.4mm. I blame this primarily on my previous owner(s) who most likely never released any tension for the first 100.000 miles. I think I was lucky…

My rear crank seal is also leaking badly, finally it is good that I do this job now. Because it would have been a big disappontmet if a new oil pan gasket wouldn’t have resolved my oil leaking engine…

Your rebuilt is missing the guide sleeves on the rear mating surface. Make sure you transfer them before returning your old tube.

They will pound out, but they are spring loaded, so will fly upon being dislodged.

Borland,
Thanks for the heads up!

Certainly a discussion as old as the mountains. And though very plausible, this is the first time I've read that particular theory Borland puts forth.

Is it correct? Possibly, possibly not. Is it important? Possibly, possibly not. What is important, is that it happens.

I believe as has been discussed over and over again that the torque tube acts as a very large elastic band. if you twist it, it shortens. If you release twist, it lengthens again. And what happens inbetween, is that the front clamp on the flex plate (and maybe rear clamp too) slips (or more precisely, the tube slips out of the clamp). When the tube lengthens again, the clamp is pushed forward.

Personally I believe the way to solve this is to design soemthing that does not clamp, but slides, or U-joints.

Am I correct? Possibly, possibly not.

Is it important that I be right or wrong? Not so much. Problem exists. Here is the more interesting question: Why does it happen almost exclusively in S4+ cars ... torque?

Hmm ... and right under that flex plate and that pinch clamp sits a device that is usually glowing red hot .... the cats. Interesting thought.

This is just an attempt to verify what's going on inside the transaxle.

If the shaft twists and shrinks or if vibrations are the reason, the outcome is the same. The flexplate is pushed against the flywheel

Question: If twisting causes the sahft to shrink, why is there no movement in the flexplate with NEW Torque converter bearings? The shaft is still twisting and the torque converter bearings have nothing to do with this. Why do so many people have bad vibrations in the rear end of the car and new bearings make everything smooth again? Just asking...


Crank oil seal findings: Removed the old seal and after the flywheel was off, the reason for the leak was visible. The seal was installed tilted by the factory The upper portion of the seal was pushed in further than the lower portion. This caused more wear on the lower part of the seal and finally a leak.
Pretty amazing how good the seal looks overall and how much material is gone due to the increase in pressure at the bottom of the seal.

Yes this is how I understand it. Schocki if it is a fact that new bearings == zero tbf then I am happy with your explanation but is that certain?

The shaft does not change it's length under normal use (elastic deformation). It's just not a consideration when designing rotating machinery. Here's some engineering discussion on the topic:

http://www.eng-tips.com/viewthread.c...=106945&page=1

Schocki.... Did the flywheel come free without unbolting the clutch housing?

borland

No, the upper part of the bellhousing had to be removed too. A 19 mm wrench with an extension did the job. Little tricky setup, let me know if you want to see a picture. The bolts are super tight and hard to reach.

Thanks for the link!

As Tails argues, I've always accepted that winding up of the TT shaft during accelertion shortened it a bit and with the help of vibration, pulled the shaft back through the clamp, accumulating the 3MM of preload most people find whenever they check and release the clamp. Others have done the calculations and say the twisting and shortening just is too small to be the explanation. Also, on my 89, the exposed spline has gradually gotten less and less after each release, suggesting the shaft is moving forward or stretching. I really don't know what is going on here. Since my torque converter bearings are starting to whine, I'm headed towards a converter bearing and TT job soon. I plan to install the earlier type shaft with the shims and circlip to eliminate shaft migration back through the clamp and then just check the flexplate for flatness every so often.

Update:
I worked on the car all day today and everyhting is back in the car except the exhaust. I'm waiting for the O2 Sensor from 928 Specialist which should be here tomorrow.
Everything went bak in the car with no problems. The flexplate was adjusted to factory sepc. and glued with Loctite 290 on the shaft. I also put some white markings on the shaft for easier checking in the future. As you see in the pictures, I used an old washer and bolt to have a closer visual mark on the shaft itself. Because I want to know if the shaft actually moves or stretches over time.

Borland,
I used a 19 mm wrench and a 3/4 inch breaker bar extension to remove the upper bellhousing bolts. It was no problem with this setup, but with the wrench only it is impossible!

Can anyone tell me what distance "A" is in frame 7? I don't think it is too critical as there is lots of room on the splines on either end. Anyone differ with that asessment? Also what is the 110% torque value on the pinch bolts?

That front shaft looks like its set too deep, you said you changed the torque converter bearings, did you install and tighten the rear pinch collar and shaft first, before you tighten the front one.

Answer for "A" is 0.4mm

Look at the pinch bolt value and give it an additional 10%. This was aproved by the factory and made in an attempt to control the movement of the flexplate.

???

PICT0117.JPG

I thought the early models had a circlip and never moved but Porsche went away from this simple solution.
Why would they invent a more complex and costly solution?

Marton

I think what Steve is indicating is that there is almost no spline showing on the front of the TT shaft. Usually 3/4 inch or so shows behind the clamp.

That's how it was with the old transaxle too. Everything was adjusted to spec. It is a year now after my repairs and everything is working well. The flexplate has not move since then (loctite 289) and sits exactly at the same spot. The car drives great and is my daily driver.

Below is a picture from Pirtle's homepage, mine is not a lot different...

Could be, but why would the clamp slip in one direction and yet hold in the other?
Also this sort of ratcheting effect would in theory continue until the centre of the flex plate is pressed against the flwheel and can not physically move any further; does this happen?

Marton

We have studied this problem and concluded the forward pressure found on the forward flexplates comes from the driveshaft twisting under load, shortening it which pulls it out a bit from the front flexplates. The front flexplates do not have a groove to hold the driveshaft as the rear trans coupler does. Porsche stopped using a circlip, shim and bearing design at the very front of the driveshaft sometime in 1984 which located the flexplate to the driveshaft at the front, sort of like the grooved area of the rear driveshaft and bolt design.

In 1985 and onward the HP/TQ of the cars were increased which further compounded the driveshaft twist problem. Shortly thereafter Porsche increased the diameter of the driveshafts from 25mm to 28mm to handle this increase better. This is from a Porsche engineer.

The driveshaft acts just like a torsion bar in a torsion bar suspension set up found in 911s, 944s and other Porsches and cars. The concept behind this suspension is that the torsion bar twists and acts like a spring. These bars are not as long as our driveshaft but still twist. When a steel bar twists it shortens. The shortening effect is finite and when the parameters are reached the driveshaft is no longer pulled from the front flexplates.

The theory of the torque converter (TC) bearings wearing out and causing the forward push is not plausible. There would have to be constant forward pressure being exerted to the driveshaft from the rear. The design of the TC housing would not allow this just as Shocki found when he tested this theory using a press on the TC housing. If this theory were true then the Loctite, circlip/washer/bearing, and our clamp would not work. But they do.

As far as using loctite there are drawbacks. Once Loctite is used it is near impossible to release the driveshaft from the front coupler and people have had to unbolt the front clamp from the flexplates to remove the torque tube from the car. There was also an instance when the driveshaft did move after loctite was introduced and the owner could not release the forward pressure he found at the front flexplates.

We have also conducted tests on the OE Porcshe clamp and found that it loses clamping force when its temperature increases. Its design also does not lend itself to much re-use and using a new clamp bolt and torquing it to a higher torque setting had no effect in our tests.

The clamp we designed was tested over two years in the 1986.5 track car. The movement was stopped. The clamp was released and re-clamped a few times to test the clamps ability to be re-used which it did well. Its clamping ability was tested against the OE clamps and our clamp held fast past 4500 PSI, even after being heated in an oven to 200 degress F. The OE clamps were found to let the driveshaft move at about 1700-2100 PSI cold and about 1400 PSI when heated to the same temperature. This is important since the front flexplate lives near the engine and catalytic converters which generate a lot of heat.

Why did Porsche stop using their original design at the front flexplate area to locate the driveshaft to the flexplates? From our gathered information from a few different sources we have concluded this set up was difficult to install correctly by Porsche techs in the field and sometimes caused TBF to happen to customer cars. It is our opinion that Porsche should have re-designed the front clamp when they took these parts away.

I posted our findings in a more detailed fashion a while ago and this post can be found by doing a search using my name and TBF as the search parameters if anyone is interested.

Cheers,
Constantine

Hi Constantine,

Your theory is that under certain conditions the drive shaft shortens. Then when the bar returns to its "normal" length and so flexes the flex plate..
OK, but after this happens for the first time then next time when the bar shortens again and lengthens - why does it repeat the cycle to cause greater flexing when it lengthens again. It should be simply taking the flex out of the plate when it shortens after the first time and be putting the same flex back as the first timke when it lengthens again.

Marton

Hi Marton,

The OE front clamp allows movement of the driveshaft until the driveshaft and flexplate reach an equilibrium. The OE clamp allows the driveshaft to be pulled out since it was intially designed to work with the circlip, bearing and washers. Porsche never re-designed the clamp.

A stronger clamp at the front allows the flexplates to work as intended, that being to flex to compensate for the driveshaft shortening.

Hope that helps,
Constantine

Constantine is correct, I have said this same thing for years.

Marton, yes when the shaft shortens and releases pressure again, it is only while torque is being applied. Undewr low-torque the shaft is long and untwisted, which maintains pressure on the flex plate and therefore crank shaft.

That Constantine is correct becomes luminously clear when one looks at a broken torque shaft. I have owned TWO TBF engines.

Steve...I havn't installed the transaxle and TT yet. I lost the notes I wrote with the measurements of how far the driveshaft sticks out at either end. The notch in the transaxle end that is viewed through the hole in the tt gives me a rough idea. Just line the notch up with the hole.

Schocki...thanks for the info , it will address my problem above. Having said that , it looks to me that as long as you can get at the pinch bolt to tighten it that any variance in length at the transaxle end can be dealt with at the engine flexplate end. Would you agree?

I agree with Constantine and all his theories. I have no idea why Porsche went to the non-circlip design. It is harder to setup, but when it's adjusted it's maintenance free and there is no thrust bearing failure. There is a lot of discussion about TBF and I heard all kinds of theories. Mostly about the increased torque and power of S4's and younger models. Flexplate movement has nothing to do with the power output of your engine. It is only caused by the missing circlip design and a clamp design that was never improved.

Why did Porsche go this way? My theory is that they wanted to safe costs. Not production costs in material but man hour costs. It takes a couple of attempts to get the circlip and the washer adjusted to spec even when everthing is out of the car. Try this with the engine still in the car if you have to change the transaxle.... :)

I went the Loctite 290 way, because people tried it and it worked well and is cost effective. The only time that I heard about movement after application of Loctite was when it was not left long enough to cure. Releasing the pressure annually is OK but if you drive your car every day there is a problem. When is the pressure high enough to cause increased friction on your TB? Nobody knows! If you release the flexplate annually you are NOT eliminating the pressure completely, you're releasing the pressure temporarily.

Can anyone provide me with the pinch bolt torque? I don't have a manual although I'm starting to think it might be a good idea...

You need a manual if you are going to work on these cars, period!

Jim Morehouse has them on CD, money well spent!

I have Mitchell on demand at the shop which is a pretty good source of information on car repair, but it is no where close to what the manuals are.

I think that Constantine has the right info on this subject, for the price of a TBF on one of these cars, it makes his clamp look cheap as compared to what it will cost if you have a TBF.

Constantine's clamp really is a very nice looking piece, BTW.
I feel this is another area where my money was put to good use, along with his recommendations on having three bearings put into my rebuilt TT.

Hi Steen,

There are two, one that is in the manuals, 59 ft lbs., and the other is the one now used by many and has become the new standard, 65 ft lbs.

Interesting how this new rating came to be really, it morphed from a long ago post from a gentleman in the UAE (?) who was one of the first to publicly talk about the driveshaft pullout at the front flexplate on the Rennlist e-mail list. This was before we had this wonderful 928 Forum in addition to the list.

If I remember correctly the fellow had his mechanics call to Porsche AG to find out more about this pullout problem that kept manifesting itself in his 928S4. His mechanics were told by Porsche AG mechanics they recommended to change the flexplate and coupler assembly, change the pinch bolt and torque it 10% higher than in the manual.

Over time these instructions have morphed into only torquing the pinch bolt 10% higher and maybe changing out the pinch bolt. I guess people just dis-regarded the new flexplate/coupler assembly due to its cost and the cost of installing it. Again, our testing did not find using a new pinch bolt and torquing it to the higher setting does any good to stop the driveshaft pullout. That Porsche suggested (if it's true) to change out the flexplate assembly also tells us the clamp is not meant to be re-clamped much.

Hope that helps,
Constantine
P.S. I second the recommendation that you should have the 928 manuals in some form if you are going to work on these cars or want to learn more about it.

I still think there is more to it that twist-shortening. My 89's TT shaft has less spline reveal than it did 6 years ago. I release it every 3-6 months. Something has moved. TC ballooning is a concern with other cars. Why not ours?

Is the problem more evident with 87-up, or the other way 'round?

IE, would the larger torque converter contribute more or less to ballooning? (IIRC, larger diameter = lower stall)

Disclaimer: I am not a mechanic.

Stupid Question: It seems the "twist-shortening" theory is the most widely accepted. Wouldn't it be easy to test this? Seems like if you made a tiny permanent mark - say with a small file or even a hacksaw blade - in one or two of the shaft splines just as they meet the clamp, then re-check after a year or so, wouldn't the mark then be a few mm farther back from the clamp?

Edit: In fact, wouldn't it be exactly the same distance as the flex plate change?

Ballooning is not a factor because the TC sits in it's own housing. Ballooning forces push against the TC bearings, not against the transaxle shaft.

Ballooning is definitely a factor if the tranny is connected directly to the engine and the TC sits on the flywheel. I replaced my TC bearings and there is no way for the TC to push against the transaxle shaft.

Maybe I'm not understanding things... (not exactly a news flash! :roflmao: )

But if twist-shortening pulls the shaft ever so slightly rearward out of the front clamp, wouldn't that be measurable over time?

Some info in this thread. Especially see post #43.

If I ever buy '85-87 automatic one of the first tasks is to check front clamp position and add shims into it. '88-95 version would require glue or preferably custom clamp from Constantine.

Constantine...thanks for the spec

I have bid on a set of manuals a number of times on ebay but have never been able to close the deal. I have had 2 928's over the last 15 years and don't plan on ever not having one. The first one was an 83 and aside from brakes and an alternator , it was trouble free for the 5 yrs I had it. I replaced it with my current S4 and I have let it go somewhat. Hence the BIG PROJECT. So far I've replaced the TC seal , TC brgs , trans filter , tt brgs , rear wheel brgs , rear shocks , O2 sensor, gas tank strap and fuel filter. I'm going to remove the air pump and maybe the CATS and am thinking about an OTT X-pipe. I also have to tend to a nasty pwr steering leak somewhere in the resevoir area.

All this work has been a challenge for a first timer , especially without manuals , but rennlisters have eased the way considerably. My thanks to those who have responded to my queries.

I'm still on the lookout for a set of manuals...

The theory of the torque converter (TC) bearings wearing out and causing the forward push is not plausible. There would have to be constant forward pressure being exerted to the driveshaft from the rear. The design of the TC housing would not allow this just as Shocki found when he tested this theory using a press on the TC housing. If this theory were true then the Loctite, circlip/washer/bearing, and our clamp would not work. But they do.

As far as using loctite there are drawbacks. Once Loctite is used it is near impossible to release the driveshaft from the front coupler and people have had to unbolt the front clamp from the flexplates to remove the torque tube from the car. There was also an instance when the driveshaft did move after loctite was introduced and the owner could not release the forward pressure he found at the front flexplates.



If this theory were true then the Loctite, circlip/washer/bearing, and our clamp would not work. But they do.


There was also an instance when the driveshaft did move after loctite was introduced and the owner could not release the forward pressure he found at the front flexplates.


There is a contradiction here.





Constantine...




How did you verifier/ confirm this twisting of the drive shaft under load which method did you use to come to that conclusion.

There is constant forward pressure being exerted to the driveshaft from the rear, the rotating mass (torque converter) and fluid pressure within causes the converter to expand/ balloons some much it distorts the converter. Also the bell housing is made of aluminum; toque converter bearings are steel, heat expansion is at different rates creating an unknown amount of end play where the bearing seats in the bell housing, there must to be some movement.





You can’t compare a torsion bar to a rotating shaft, In a car one end of the torsion bar is attached firmly to the vehicle chassis; the opposite end is attached to the suspension arm or A- frame. Vertical motions of the wheel cause the bar to rotate along its axis and are resisted by the bars torsion resistant.


We have different opinions as to the cause.

In drag racing it has been common knowledge for years that torque converters drive the front pump which creates line pressure, which tries to drive the converter & everything in front of it forward, hence creating force against the bearing. It's been proven through tests. It's no secret to drag teams.

Hammer

Hi Steve,

There is no contradiction about the instance when the driveshaft did move after loctite was introduced. It moved for some reason then was frozen in place in a tensioned state. Others who have used this method have not had anymore movement, although I personally don't advise using loctite as I mentioned before.

I understand there is pressure in the trans under operation, but if you are saying the torque converters (TC) are ballooning under this pressure and pushing the driveshaft forward then the following would be seen by the owners:

1. A lot of aluminum shavings coming out of the little grate of the TC housing.

2. No effect of the methods currently used to stop this movement.

People have reported 3-4mm or more of backward movement of the front flexplates on the driveshaft when the front pinch bolt was released. That is too much for bearings to be displaced by heat. Also, the driveshaft has not been known to move forward when it is marked in relation to the front bellhousing.

I can compare a torsion bar to our driveshaft since it does wind up under load. A long steel rod about 1" in diameter that is our driveshaft will twist and was designed that way. That is why Porsche placed a groove in the rear of the driveshaft that a bolt goes through, the circlip bearing and washers up front and placed flexplates on either end to allow for this contractive movement. Not because there would be a forward push from the rear. It is our opinion Porsche should have re-designed the front clamp when they took away the circlip, bearing and washers from the front.

Chevrolet uses a hollow large diameter tube for their driveshaft found in the C-5 and up Corvettes which also have a torque tube set up. Chevrolet uses a rubber piece in between the engine input shaft, the driveshaft and the trans input shaft to let there be a bit of a twist, or torsion effect, in their driveline with no appreciable contraction. However, I was told they also suffer TBF issues due to heating and expansion due to this different set up and they need to follow certain procedures when replacing a TT, trans or engine.

As far as us having different opinions as to the cause, I would agree. ;)

Great to see you back on the list by the way, a lot of people are having issues with their transmissions!

Kind regards,
Constantine

Hi Constatine,

I think we have 2 different topics here, I am not questioning that your stronger clamp fixes the problem. I am questioning your theory about the cause.

BTW, I am not sure why Porsche put a clamp on the front, I understand the TT is located at the back so why does the TT not float in the flex plate at the front? In the manual the clutch floats on a similar shaft?

I understand people usually release the pressure on a flex plate by releasing the clamp bolt then the flex plate returns to normal and so the clamp slides down the shaft by 2mm or 4 or whatever; the clamp does not need to be levered or pulled - for me that is then equilibrium.

So take the example of an auto in normal use with a flex plate that is buckled by say 2mm and wants to slide down the shaft to the correct position but is prevented by the clamp. Then a condition arises from your theory where the TT shortens by say 0.1mm (I do not imagine that this temporary shortening would be 2 or 4 or more mm?) - why then does the shortened TT pull out of the clamp by this 0.1mm to make the buckle 2.1mm? I would think the flex plate would happily reduce the buckle from 2mm to 1.9mm to accomodate this temporary TT shortening?


Cheers

Marton

Erkka-

I looked into this thread and couldn't find the details about your description here. So '85'-'87 has the 25 mm TT, and '88 up has the 28mm. So I'm keenly interested here, but didn't see mention of the shims? Is this within the clamp? Any details are appreciated, and advance apologies for not catching it in the other thread.

Chris, my response was to Steen Jensen's question about front-back position of center shaft inside outer tube. It must be correct for rear clamp bolt to align properly in to the shaft groove. Specs I could find from WSM are in post #43.

How to determine shim for '78-'84 TT front end is also in WSM but not in said thread. I have understood '85-'87 have exact same TT center shaft and front flex plate as earlier 4sp cars. This means its possible to install same shims to these cars as well and virtually eliminate possiblity of TBF. In my opinion very cheap way to buy some peace of mind.

Thanks, I'll take a look at the WSM.

Hi Marton,

I have sent you a PM to answer your questions.

Constantine



[QUOTE=marton]Hi Constatine,

I think we have 2 different topics here, I am not questioning that your stronger clamp fixes the problem. I am questioning your theory about the cause.

My 88 has the circlip grove on the shaft. I installed the washers and circlip, pain in the butt. Not very much room to work.

Cheers,
Glenn

Is it also 28 mm thick (other than front and rear 5" of it)? Looking for time when shaft was changed or if in fact 28 mm has also grove.

The 28 mm shaft is the neck downed, shorter version that does not have the area needed to place the circlip, bearing and washers.

One thing to remember is torque tubes (TTs) have been changed and one should visually inspect their driveshaft to confirm which style they have before ordering these parts.

If one wants to install these parts the manuals have a formula to calculate the shim thickness needed to set the correct distance between the flexplate and flywheel.

Porsche stopped using this method for a reason. We believe the reason is the difficulty in setting up the correct distances and the problems Porsche techs in the field were having in doing so, sometimes causing TBF to befall customer cars. Porsche even sent a Porsche engineer to the U.S. and other countries to teach the field techs on the proper procedures of installing a TT.

For this reason we believe Porsche stopped using these pieces and changed the procedure to just leveraging the crank to the back of the car and then clamping the front pinch bolt as being one of the last steps of doing a TT change. This would ensure no forward pressure is being exerted onto the thrust bearing. It is our opinion Porsche did not take into full account the weakness of the front flexplate clamp in not being able to hold onto the driveshaft during operation.

Constantine

When we replaced Matt's (Edco) TT in his 91, we used the early shaft and the shim setup. The shims appear to be a challenge due to limted space to use a circlip plier. With a small plier, it is easy to place the clip on the shft from in front of the flywheel, through the cutouts. Utlimately, it only took a minute to get the clip on the end of the shaft, then prod it down the shaft into its groove.

The instructions for setting the clearance are confusing. My interpretation was there should be 0.3mm of play between the shim and the back of the flywheel. My further interpretation is that the idea of the small amount of clearance is to not exert any rear preload and to prevent the shaft from moving back much through the clamp by having such very little clearance. This is hard to measre directly, although it could be done with a long feeler gauge.

I suspected that to be case also but its possible Porsche did two versions of 28 mm. Probably not though as someone would have mentioned it before already.