Jballard2140

I am needing to do the TT bearing replacement and after speaking with Constantine at Black Sea I have been convinced to go with the 25mm driveshaft on my 89 S4 auto. Bearings got made and shipped but I found out that the email at Precision Motorwerks was not getting through. When I called them today, I was told that they are out of the 25mm shafts and wont have any for at least 6 months. Does anyone have one they could let go? I would also be interested in a factory 25mm driveshaft.

FredR

Have you spoken to Constantine the Great to see if he has one of his 25mm shafts available?

Tony

id assume that may have happened based on his first sentence...

Jballard2140

Constantine was as surprised as I that there were no shafts readily available. That was why he spent so much time convincing me to get rid of the 28mm original shaft and go with the 25mm.

mj1pate

I gave my old ‘25’ away a year ago after doing a TT swap. Keep this post bumped. Someone may hold one up and wave it if they see this post.

Constantine

Bump!

Yes, we had been making our own drive shafts but decided not to compete against GB since this is such a thin market. They are not inexpensive to make and must be made in batches to get the pricing down to an acceptable level for re-sale.

But due to the news from Jeff yesterday, will be dusting off the drawings and look into getting more made. However it will not be quick turnaround.

Cheers.

GregBBRD

We had an unplanned "run" on our custom 300M 25mm automatic shafts, which caught us without any.
And like Constantine mentions, getting more made takes time.
If you can use a stock 25mm shaft, we can easily provide that.
Sorry for the confusion.....Mary thought that you needed a 300M shaft, only

Jordan Thieme

I apologize if I am way off base, but I have the torque tube out of a 1984 Porsche 928 S that was an automatic if that helps at all.

FredR

Well you do pilot big aircraft for a living and you have been desert bound in Vegas for far too long!

Jballard2140

I am now curious as to how much of an advantage/improvement the 300M alloy shaft enjoys over the stock shaft. Is is 20% stronger? 30%? I can see the design advantage over the 28mm shaft by not have the conical “necked down” taper at the ends of the shaft, but I am genuinely curious what metallurgical advantage the 300m has over the stock material.

FredR

The stock 28mm shaft is perfectly adequate and the taper is an irrelevant canard that was kicked around 20 years ago when the shaft broke at the base of the taper- it failed at the weakest point but the reason for failure was not understood. As I am concerned the shaft was a victim of the system not the cause of the problem. The root cause of the problem appeared to be the flexplate clamp slipping inducing a compressive stress on the shaft and of course ultimately causing the thrust bearing to fail. This problem also caused premature failure of the torque converter bearings. Stop the clamp from slipping and these problems do not occur. Note the rear clamp can also cause issues and the rear pinch bolt also has to be checked and preferably changed out from time to time. We have seen examples of the 28mm shaft lasting 300k miles without any issues so any notion that the taper is a "problem" is "nonsense". The support of the shaft is another point of debate. If you fit three super bearings to a 28mm shaft Constantine belives the damper becomes superfluous. The damper is nothing more than a lump of iron supported in rubber mounts. Two stock bearings plus the damper are the same weight as three super bearings and that is by design intent. .

If there is nothing wrong with your 28mm shaft you could consider reusing that but then you would need three appropriately sized super bearings- grrh! I can well believe getting these things made will have a considerable lead time [6 months?] sad to say but well worth it in the long run.That or you could try sourcing a stock 25mm shaft 2nd hand to fit your super bearings to. ark at 928 International may be able to help you if GB has non.

That being said there is no logical reason known to any of us that the 28mm shaft with two support bearings was ever needed in the first place. It was only fitted into later automatics and invariably they are the models that generally but not exclusively suffer from TBF. Fit Constantine's wonderful clamp whatever you do- that or use one of the other supplementary techniques to reinforce the clamping force on the "Mickey Mouse" stock clamp design- that thing looks like the clamp that held the kick starter lever to the starter shaft on my 1957 BSA 650! A single bolt closing one side of a lump of steel. It should have had at least two clamping bolts to apply even force. I still use the Loctite 290 procedure and it has served me well since I first deployed it back in 1999 on my late S4 and then in my current GTS. I also have a Constantine clamp waiting to go in as and when I need to revamp the system.

Did you start this project because of a problemmatic drive shaft or was it a pre-emptive type of thing?

The shaft that GB provides looks like an excellent piece of kit and doubtless is superior to the stock item but whether such superiority is needed on a stock motor is most definitely questionable but nontheless nice to have. If the stock shaft is not serviceable it is a "no brainer".

Jballard2140

I first bought the car in March of this year. Four days after purchase, the engine stopped suddenly while going 75 in the left lane with the kids taking their first ride in it. The culprit was a bad relay/fuel pump. I replaced the relay and it ran, but whined. I replaced the fuel pump. Still whines. Noticing that the whine is proportional to engine speed, I got under the car and the sound was clearly coming from the transaxle area. After perusing many threads here and carefully going over the well documented service history on this machine (no mention of TT bearings), I decided I wanted to knock all of this out in one swoop. I did not enjoy getting stranded in a dangerous place and I am trying to get ahead of the next problem. Every piece of plastic and rubber on this car is 34 years old and every time I try to wrench on it, I realize I am more restoring the car than repairing it. The TT bearing job is a little more that I think I can successfully handle. While doing the service, I was hoping to take advantage of advancements in materials and engineering to mitigate a deficiency in the 928’s design.

Speedtoys

Eh..Constantines bearings are better, they are..but unless you're going big on HP, the OE shaft is just fine. There are not many places for "better" than OE.

While the transaxle is out, replace ALL rear suspension rubber pieces, all of them, including upper mounts.

So much can be done with this one big job that you dont wanna do again...

Of course, rebuilding the trans at Greg's shop should be on the list, too.

GregBBRD

Compared to the stock 928 torque tube shaft, a 300M shaft has over twice the shear strength and over twice the tensile strength (in both UTS and Yield.)
300M has become a very popular material used in very high performance axles, transmission input shafts, etc.

I wouldn't waste my time remaking stock torque tube shafts as the 25mm shafts are robust enough, for stock applications and fairly plentiful for the automatic version.
4340 would be almost as good as 300M and if I was making hundreds of these shafts, it would work well and be more economical.
However, since I make my torque tube shafts in relatively small batches, the price difference of the two steels is insignificant.