Thrust bearing

Many of our listers have probably read the threads on torque tube failures and thrust bearing problems wrecking motors. I have contributed a few thoughts on the subject and have been most concerned [alarmed] about some of the conclusions I have formed.

As some of you may remember my drive shaft broke almost two years ago. I have also complained of a resonant vibration at around 3000 rpm which I have previously listed to seek opinions.

We posted a suggestion that it may be prudent to regularly check the tension of the flex plate to ensure that the thrust bearing is not overloaded. I took my S4 into the local dealers this afternoon and we carried out this check. When the pinch bolt was released the flex plate popped back by about 4 to 5mm! I shuddered when I saw this.

We also checked the crank end float which was approximately 0.5mm or on the outer limit of the tolerance range. Two years ago we checked this and it was half the amount. We are not entirely sure about the gauge that was used so we are going to recheck this in a day or two with another dial gauge.

After the adjustment was made a trial run indicated that the vibration at 3000 rpm had been significantly attenuated albeit there was still a little roughness. I will comment on this when I have done a bit more mileage.

What does all this mean?

First of all I am confident my drive shaft was replaced correctly two years ago. So, one of two possibilities has occurred, either the drive shaft has stretched or the splined central boss has moved forward. Whether or not there is damage to my crank thrust bearing remains to be seen but without doubt it has been under more load than it might otherwise have been and, subject to further checks, may well be at the back end of its theoretical service life.

Given the apparently unreasonable incidence of torque tube failures on S4 automatics and/or destruction of the main thrust bearing I am rapidly coming to the personal conclusion that for some reason our cars may be prone to some kind of design related failure. In Oman we have seen 4 torque tube failures in a vehicle population of around 30 machines in the last two years. Interestingly all of them were S4's or GTS's, none were earlier models [of which we have plenty here] and they typically had around 50 to 60k miles on the clock. I note that the recent Rennlist failures also had this type of mileage on them.

This begats the question why does it affect the automatics? Not sure on this one but I have a theory that it may relate to the way in which the gearbox is set up. The gearbox has a vacuum modulator. Set to one extreme the change is soft but sloppy. On the other extreme it is quick but a little thumpy. My gearbox has always been quick on shifting. Remember, even though the auto box is a Mercedes Benz design, the valve body was modified by Porsche to give a sportier performance. I am starting to wonder if sharks with their box modulated [intentionally or unintentionally] on the sporty side become more vulnerable to shaft failure because of the torque impulses exceeding the fatigue life criteria the machine was designed for.
If this is the cause then it may be unfair to describe the failure as being "design related".

The torque tube, besides transmitting the power, also acts as a transmission shock absorber. The shaft continually winds up and under constant load twists proportionately to the torque applied [a simple application of Young's law]. Excessive shock loading [during gear changes for instance] may well have an impact on the fatigue life of the material.

Hence, if the shaft is exposed to more torque than it can absorb with the elastic limit of the material or if the torque cycles applied to it mean that the fatigue life is significantly reduced, one can reasonably predict that the torque tube will stretch and ultimately fail. As it stretches, it thrusts in a forward direction and la voila- the thrust bearing saga.

If you are lucky, the torque tube goes pop and if not the whole engine goes due to thrust bearing failure.

It only takes half an hour for the mechanic to carry out the check for the tension on the flex plate and about an hour if you want to check the end float on the crank while you are at it [recommended that you record this].

If you own an S4 or GTS automatic and you have not had this checked I strongly recommend that you consider doing this immediately and ask for the check to be built into your services. If there is a design weakness we cannot prevent the torque tube failure but we can protect the thrust bearing.
There are other reasons as to why the clamp may have moved on the splines. Incorrect torque being the obvious one, maybe vibration, but this would not explain the torque tube failures.

Boys [and gals] I think we may well have been shafted!


Fred R
1990 S4 auto- questionable end float!


The autos are affected only because they are hard connected to the driveshaft. Manuals have the clutch and pressure plate saving them from this problem. Also it is true the autos shift under load which may impact the driveshaft stress. But driveshafts are apt to fail as often on manuals, just less cars are manuals, so it may be a numbers game.

The thrust bearing can shoulder a load up to some calculated value, the surface area, construction of the bearing determines this, just as with other bearings in the car.  E.g Simplistic example - wheel bearings carry 4000/4 1000 lbs each.  I am not a mech eng, but obviously increasing the load on the bearing will shorten its life, but with minimal load, it should
easily last into high mileage.

I have posted a tech service bulletin about this issue in addition to the information we posted earlier in the week at (select 928 etc.)

New bearing crankshaft axial play is .110 - .312 mm the wear limit is .40 mm (note this is metric mm )
89 s4 at 58k
did the driveshaft /flexplate thing with capt earl this past weekend (thanks earl), and the flexplate was quite bowed.   moved 0.130 inches when the collar bolt was released (quite scary, visions of a new motor)  thankfully, the play in the driveshaft was only 0.005 inches (within tolerance) whew!