Vilhuer

Hope these will we available for '80-95 5sp and 4sp automatic.

Black Sea RD

More interesting to us was why Porsche felt the need to increase the diameter of their automatic drive shafts in 1987 and up 928 models but never changed their 5-speed models with the 25mm drive shafts.

Our belief was that Porsche knew about the drive shaft pullout problem in the 928 automatics leading to thrust bearing failure (TBF). They then devised the 28mm drive shaft to help combat this twist in the later 928 models with increased torque. However they did not want to change the flex plate and transmission couplers to also accommodate the 28mm drive shaft, probably due to costs, so they necked it back down to 25mm at the splined areas. This then set up them up for the shear failures often seen with the 28mm drive shaft.

The stock Porsche 25mm drive shafts have held up really well, to include even in racing conditions, for years. I'm sure you've seen this over the many years you have been involved in your line of work, to include building and maintaining the most famous 928 racer of all time, Mark Anderson's "White Zombie."

But age is catching up to all 928 and 944 drive shafts and we are seeing shear failures and other problems even with the 25mm shafts, as I'm sure you have too. That's why we decided to R&D stronger drive shafts years ago.

As far as the OP's problems, I understand what you are saying about the parking pawl, but I always like to put everything out there just in case, especially when trying to diagnose a problem over the "internets."

Kind regards,

GregBBRD

Yes, both. And a 300M clutch shaft....since the original shafts do break, they wear out, and they have gotten really expensive, from Porsche.

GregBBRD

The loads on an automatic shaft are very different than the loads on a manual shaft.

The flywheel and clutch mass on a manual trransmission car are in front of the torque tube. The torque convertor is at the very back of the automatic shaft. The mass of the flywheel and clutch "absorb" a huge percentage of the "pulses" that an engine generates. Think of an engine as a giant impact, acting on one of those "wheel tightening extentions".

Way different "pulses" flow through the two shafts.

The original shafts are made from an alloy that is designed to twist and then return to its original position. Like any material, this material begins to deteriorate. This deterioration comes in stages.

The following applies mostly to stock engines....the problems get vastly worse, with higher output engines.

In the beginning, the shafts are at their "stiffiest" state. The shafts twist very little from the amont of torque that a stock engine puts through them....thus there is minimal "shortening" of the shaft. "Pull out" from the front flex plate is very minimal, if any.

As the shafts begin to deteriorate, they don't break....they simply allow more twist. This additional twist makes the shafts significantly shorter....thus they "pull out" from the front flex plate more than they originally did.

As the shafts deteriorate more, they twist more. The result is that the shafts shorten more and more....pulling out of/on the flexplate harder and harder.

Eventually, the shafts will deteriorate to the point where they will break.

Important to realize here.....adding a supplemental/different "clamp" to the flexplate doesn't reduce the amount of twist.....the shafts are still shortening the same amount....the additional/different clamp simply keeps the splines of the shaft from pulling out of the flexplate.

An additional clamp is a patch! The real problem is that the shaft is fatigued and is twisting more!!!! This is why you will never see "pull out" on a low mileage vehicle!

Is there a "downside" to an additional/different clamp? Well, the obvious answer is yes. Now when the shaft shortens, the shaft can't pull out of the flexplate....instead it pulls back on the entire crankshaft assembly. The result is very high friction on the front of the thrust bearing....resulting in a large loss of torque.

And the more torque that is generated, the more twist there is, which results in more pressure on the thrust bearing.

The more torque that is generated, the larger the torque loss...simply from the increase of friction! I'm not even discussing the torque loss from pulling a 65 pound crankshaft backwards, while the rod bearing have to "shuffle" back and forth!

SMTCapeCod

Would be nice to have some duty cycle/fatigue data - has the additional vulnerability to twisting and the shortening been measured?

Black Sea RD

Hi Greg,

We've studied this whole problem ourselves for years, but no one can quantify how much more "twist" older drive shafts have compared to new ones.

To do so would be very difficult and would also rely on what type of life the drive shaft being tested has lived during it's mileage, e.g. race car vs. street car driving.

What can be quantified is the thrust bearing failure (TBF) of engines in automatic equipped 928s when the drive shaft is allowed to move out of the front flex plate clamp. We have steered owners your way when they have suffered such an event and I know you have seen your share of TBF'd automatic 928s.

Our Super Clamp has now been in use for many years by many owners worldwide. So far so good, as far as we know.

We'll see what the OP finds, hopefully something not too awful.

Have a Happy Fourth!

GregBBRD

Yes, I agree that your clamp has "saved" many engines from failures....and you know that I'm a huge proponent of this "additional insurance." You also know that I suggest/use your clamp whenever an engine or transmission is already removed.

Because of what I do (and the shear volume of what I do), I get to "see" more (in months) than the "average" 928 owner/mechanic gets to see (in their entire lifetimes.)

The wear on the front of the thrust bearings, on automatic 928's equipped with a clamp on the flexplate, is significant enough for me to "re-think" what is happening. This problem of wear on the front of the thrust bearing gets progressively worse, as the torque output increases.

My above post ^^^^^^^, is my current theory......my theory alone. It's something that is tough to measure, in real time. It's something that changes as the temperatures changes. It is something that changes as the torque and tractiion of the vehicle changes. It is something that would cost multiple thousands of dollars to absolutely verify.

I can tell you this....

1. Engineers and materials people...way smarter than I....have considered this problem, at length. Pieces have been studied. Materials have been tested. Driveshafts have been cut up, for testing. The "resulting" pieces that I'm making are the results of what these people have figured out.

2. My findings and the logic of what is happening ^^^^^^^ is very difficult to dispute.

Black Sea RD

Hi Greg,

It seems we both have a respect for each others work. I have always held you in the highest regard when it comes to anything Porsche.

We also conferred with engineers and cut up drive shafts for metallurgical testing when we went down the same road of making new drive shafts. We wanted to find that material that would be better than what Porsche used. We finally settled on a specific material that seems to be working very well in both our 944 5-speed (some being used in race cars) and 928 25mm automatic drive shafts.

We can also make them out of 300M, but the costs to do so is significantly more than our upgraded drive shafts.

From what we have found most owners want to keep their costs down and don't always want the new drive shafts when a used one in serviceable condition can be sourced.

Best regards,

GregBBRD

Oh, there is no doubt about that.....you might even be understating the "issue".

However, a tiny bit of my market is building high output engines.....and I believe that a limiting factor in output is the torque tube shaft....I believe that my analogy about the "torque limitiing wheel lugnut extention" is literally what is happening.

the flyin' scotsman

Given Gregs statements and that are our cars are all 'old' the TTs are twisted to the point of robbing torque and despite best efforts all shall snap?

Now what? we need a heavy duty shaft with super brgs and clamp?

My S4 is about to implode

GregBBRD

The material that the original shafts were made from is really a wonderful material....to be able to absorb all of that torque, without breaking.

However, one needs to be realistic.....it is a 25mm, 4+ foot long shaft.....with a lot of power going through it. Add more power.....and things get ugly, quick.

In my theory, the clamp is a patch for a weakening, twisting shaft. If the shaft doesn't twist as much, it should not be required.

All of "my" automatic transmission shafts will be of the "early design", anyway. This will allow the "early" circlip and spacer washers to be used to limit the movement inside the front stock clamp. (This is what I did before there were clamps, to prevent the TBF failure.)

FredR

Some interesting posts on this old but very pertinent chestnut. My problems/experience date back to 1999 when my S4 drive shaft failed just behind the front splined section- this really pissed me off as such kind of failure was the last thing I expected from a marque like Porsche- how naieve I was!

After I was able to implement a solution [new torque tube assembly] my [engineering] attention was focussed on the logic behind what had happened that should not have. What the Rennlist enabled me to conclude was that the failure did not seem to happen on new machines but rather sometime into the life span and for some reason more often than not when the motors reached about 100k km [60k miles].

It was at that time I learnt about TBF and that really put the ******* up me. Pop the engine and to a large extent all you are left with is a wonderful looking garden ornament. At the time I postulated that the torque had to be winding up the shaft, shortening its effective length and thus creating an enivronment wherein the shaft wanted to extract itself from the clamp. As to why the clamp would let go was the dilemma but the underlying consensus was that there was insufficent grip. Why it should let go at that point in its life cycle remained to be seen.

If the shaft were twisting more with repeated torque cycling - effectively softening the resistance to torque wind up then was it a simpe case of the clamp being overwhelmed? I theorised that when the clamp slipped the effective length created an imbalance in the shaft of some kind. My thinking in this regard was srengthenged because when the new shaft was installed, I noticed soon afterwards a vibration at exactly 3050 rpm- normally a good cruising point in 4th gear. We checked the clamp and sure enough it was pre-tensioned by about 3mm. Resetting it would see things ok for a couple of weeks and the problem would re-emerge. This was being discussed intensely on Rennlist with folks like Earl Gilstrom and Constantine into this not to mention myself and my issues. This is when Earl's solution was floated and my S4 was probably the second 928 to use this approach. The problem with the slipping clamp ceased to be an issue and I have not had a repeat problem since [touching my wooden head] on both my late S4 and my current GTS.

whether or not I am sitting on another ticking time bomb I do not know but if my current drive shaft fails I am at least gratified to know that there are options from Constantine and Greg available. I still use the loctite procedure to this day but should my torque tube fail I and I had to rip out the thing, I would very much moe towards Constantine's clamp- an elegant solution.

I also believe the automatic torque loading profile is quite different and have often wondered why Porsche fitted only two intermediate bearings in the auto TT when they fitted 3 on the manuals.

Finally, what would be interesting to learn from the experiences of both Greg and Constantine is whether shaft failures occur on auto 928's that have had the clamp well locked or whether they occurred on examples still using the stock clamp.

That shafts fail with extended age is one thing but I also not folks like bill Ball having extended useage with higher outputs and do not recall Bill having had a TT failure [or TBF]. This may a classic case of "chicken and egg" in that i believe shaft failure, clamp slip and TBF are all interconnected on auto models.

Regards

Fred

Bill Ball

I assume that if the twisting/shortening of the old TT shaft is significantly wearing the front face of the thrust bearing when something like the PKlamp or Constantine's Super Clamp is installed, the crank endfloat will increase over time. Mine hasn't changed a thou (or even half a thou) in all the years I've been measuring it. We will recheck George's endfloat soon.

Black Sea RD

After studying and rebuilding these drive shafts for years now, I don't come to the same conclusions as Greg.

The 25mm constant diameter drive shafts we have seen really do not show any deformation marks along their length to suggest they are twisting more with age. It would stand to reason that after they become more "twisty" the metal would start to exhibit signs of metal fatigue, as in a paper clip that has been bent over and over where the metal first becomes more brittle then finally snaps.

The 28mm drive shafts with the neck down portions are more prone to failure because of their inherent design where the twisting forces are concentrated in the necked down areas. This is where they always shear as seen in my above pictures. We have seen some 25mm drive shafts shear too, but no where near the frequency of the 28mm drive shafts. Usually these 25mm drive shafts also had signs of rust that had permeated into the drive shaft substrate metal from prior damage like a nick or dent.

But we also know that there are many 928s with 28mm drive shafts with pretty high mileage and doing fine.

What we do see is many drive shafts coming in with deformed splines due mainly to poor maintenance procedures such as not lining up the pinch bolt grooves correctly or not tightening down the pinch bolts enough to secure the shaft without slop. Or the shaft itself has been damaged due to the bearing units or vibration dampener coming apart and causing metal to metal contact with the drive shaft.

The reason we went through the trouble to replicate the 928 25mm auto drive shaft was so owners with damaged drive shafts, or those wanting to replace their problem prone 28mm drive shaft, would have an option available to them which would be even stronger than the original Porsche shaft.

I also respectfully disagree that a front flex plate clamp would not be needed if new stronger shafts were used. Porsche designed the automatic drive line for clamps to be used at either end to secure the drive shaft. This design is set in stone due to the drive shaft design used.

If you want to use the old style clamp with the circlip, bearing and washers arrangement which needs to be carefully measured according to the WSMs, and which Porsche themselves stopped using due to the problems of correctly setting it up, or other products such as our Super Clamp, that decision is the owner's. But there should be a clamp at both ends.

The only way to alleviate the clamp at the front is to replicate a sliding coupler, much like a clutch design of a 5-speed. We were doing this but the costs to the owner would have been very expensive, much more so than our Super Clamp, so we shelved the idea.

By the way, our automatic 25mm drive shafts are exact duplicates of the original one and have the groove in them if someone wants to use the circlip, bearing and washers method to secure the drive shaft.

If Greg's theories were true, there would be many automatic 928s sitting on the side of the road or in junkyards by now with broken drive shafts or bad engines. That is not what we are seeing with the torque tubes coming in for our rebuilds or in the cores we purchase. The only bad engines we hear about due to drive line issues is when they suffer TBF, not that the front of the thrust bearing has been worn away to cause a failure.

Guessing this thread will last a long while...

Apologies to the OP of this thread

Cheers,