So, Im considering swapping the 28mm shaft in the torque tube of my 92’ GTS automatic for a 25mm. I have a 25mm from a 88’ S4 automatic. Are these the exact same length? No other issues?

 

If you do this (and I think it is a good idea - I did on my 1990 S4) then you can get the greatest benefit by using a version of the 25mm shaft with the circlip groove at the front. With circlip fitted the shaft cannot migrate through the flex plate, hence preventing flexplate distortion and thrust bearing failure. I think my 25mm shaft was from an 85 model.

 

Yeah, agreed. But I don't have an early shaft available. Only the 88’.

 

If your gTS has covered 77k km what makes you think you need to do anything with the shaft- is it making funny noises?

 

I put one of Greg Brown's drive shaft and Constantine's bearings in my TT and they work great.

 

No. But Im having the engine out to change engine mounts + various gaskets. And, it would be a shame not to change the torque tube bearings whilst I have the engine out. (Even though they have only 77tkm on them, they are almost 30 years old.)

So, I already have the 25mm bearings and sleeves on the shelf. And a 25mm shaft / complete torque tube from an 88’ on the shelf.

Does it make sense?

 

Bought the ‘contantine’ bearings for one of my cars. Nice design, but completely over-engineered. Not necessary. The original design works for 100’s of thousand of kilometers and decades. No issue.

 

If you have the parts and the inclination it makes sense as one cannot really predict the aged condition of the rubber bearing sleeves. Personally I do not buy into the commonly perceived notion that the 28mm shaft is a weak link- rather they are the victim of clamp slippage as may the bearings if they fail prematurely, but then I do not understand why Porsche went to the 28mm shaft design when there was seemingly nothing wrong with the 25mm design in the first place and more so the original version.

Whatever you do ensure you use Constantine's clamp on the rebuild- that or one of the other secondary securing devices on the market or like myself use the Loctite approach on the stock clamp. If my engine comes out or the gearbox for sure I will go with Constantine's clamp.

 

I use the original clamp. I drive very little. Have 9 cars. Tend to release the clamp every year when I service the cars. No issue.

 

Hi,

Above is a picture showing the difference between the 28mm (bottom) and 25mm (top) automatic drive shafts. All 25mm automatic drive shafts, regardless of year, have the groove at the front of them. Make sure your 1988 model has the 25mm drive shaft since some also came with 28mm drive shafts.

The 928 WSMs have a section on how to correctly set the proper distance between the flex plate and flywheel using the circlip, bearing and washers if using these items.

You might also want to check with 928 International since they have stood up a cheaper line of TT bearings and rubber holders for the vibration dampener. The OE rubber holders will come apart and the vibration dampener will slide up and down the TT causing more problems.

Good luck with the rebuild.

 

Thanks for the info. Yes, indeed, my 88’ is a 25mm. If it has the groove, even better.

 

If the engine is out, it takes a little bit of measuring with a pair of calipers and a straight edge, and a tiny bit of math to figure out how many "shims" (and what thickness) you will need to get the flex plate in the perfect position.

By doing this, you can install all the pieces with the circlip, before the engine goes back into the car, and slide the flexplate to the rear, until the engine is in the chassis. And the cost won't run your 993 Turbo for more than a few minutes.

Before Constantine made his clamp, this was virtually the only way we had to prevent thrust bearing damage, other than torquing the bejeebers out of the pinch bolt.

 

As Greg says.

shims can also be added / deleted and the circlip installed and removed with the shaft and engine both in situ. In this case the required at rest rear thrust (is it 0.3mm by memory?) can be set by ensuring that, with the shims and circlip in place, rear bolt tightened (so shaft is in correct fore / aft position) and with front clamp tight and holding flex plate hard up as forward as possible against shim arrangement and with flexplate UNBOLTED from flywheel, that there is the required rear thrust clearance between flex blare bolt position and flywheel - feeler gauges can be used for this. Thus when flex plate is then bolted to flywheel the required rearward thrust us exerted on the crank thrust bearing as per spec - to yield the same outcome as per Greg’s comment above.

not sure now in retrospect why we adopted the above method foe one car we did - maybe because it was altogether more simple - much like adjusting tappets with feeler gauges (no maths required or opportunity for bearing destroying mathematical errors).

 

Getting that circlip onto the torque tube shaft, with an S4 flywheel and the engine installed, is a tough job.
That's why I prefer to do it, beforehand.

 

Great info. Cheers.