daveo90s4

My 1990 928 S4 auto has a 28mm central shaft (that tapers down to 25mm before the front clamp). I also have in my shed a 25mm central shaft and torque tube from an earlier auto, with the circlip groove at the front end of the shaft.

I'm planning on replacing my central shaft bearings this year, so was wondering about reconditioning the 25mm shaft and installing that instead since I could then also install the circlip to help prevent thrust bearing failure.

Any views on running the smaller diameter shaft on a later model S4?

Thanks

Dave

Landseer

Have you talked with Constantine about this?

Send him a PM, he really has this stuff figured out.

I eliminated the circlip in favor of his big clamp on an 84 running a 25mm 84 TT to an 89 S4 trans.
Buy his bearings, his clamp, use your late tube, eliminate the heavy counterbalance weight inside.
(I would have also used the later TT, but it had bad bearings and I couldn't allocate the funds to the Superbearings at the time)

James Bailey

The small shaft works just fine and is LESS likely to break as the taper down concentrates all the load at the taper. Using the clip correctly shimed should eliminate any tendancy for the clamp to creep. It was only after Porsche decided not to use the clips that all the thrust bearing issues began to happen......sometimes Porsche makes bad decisions.

Landseer

Interesting, so it simply comes down to the matter of having a set of shims and the process for shimming the circlip?

daveo90s4

Hi Gents, thanks for the comments.

Landseer, yip I'm up to date with Constantine's clamp and better bearings - I'll need to talk to him about 25mm v 28mm ID availability.

Jim, yes that was my thinking too. But as a person who has experienced central shaft failure due to faulty ignition monitoring circuit tripping, I was wondering if the problems experienced with the 28mm shaft might be related to the advent of cats and that circuit faillure / 4 cylinder mode / harmonics as per factory bulletin, rather than to the shaft diameter itself?

In any event, unless I hear compelling reasons to the contrary, my thinking is to go with the 25mm shaft, with circlip,with improved bearings, and with vibration damper steel 'slug' removed.

Am also tempted to install small diameter machine screws into torque tube fore and aft of (not into) new bearings to prevent new bearings from migrating.

Happy New Year to all,

Dave

jon928se

Dave O

The 28mm shaft brings only one slight benefit and it's really only perceptible to 5sp owners - there is less transmission wind up as the 28mm shaft doesn't twist as much *** the 25mm.

It brings a lot of disbenefits.

1/ 28 shafts necks down to 25mm spline sizes so statically it is only as strong as a 25mm shaft
2/ The neck down generates a stress raiser
3/ The 28mm shaft being substantially stiffer has far less shock absorbing properties (not a good eng description but you get picture) than the 25mm shaft thus all the shock loading ends up applied destructively to the 25mm neck down portions and more violently than if it was a 25mm shaft.

The best thing you could do with a 28mm shaft is to machine it down to 25mm.

daveo90s4

Hi Jon,

Makes sense to me.

Pub at 3-Lamps, Ponsonby, Akl - how many years ago now?? !!

Cheers & hi to Rhonda,

Dave

Vilhuer

28mm for 5sp do not exist. All three stock versions are 25mm.

James Bailey

Erkka as usual is correct the 5 speeds are all the small diameter....

Black Sea RD

To add a little bit more about the 928 25mm and 28mm drive shafts:

1. The 28mm drive shaft has a slightly larger spline diameter too. We believe the reason was to make it a tighter fit into the OE front flex plate clamp to help stop the pullout problem.

2. From our research and in our opinion, the 28mm drive shaft, which was put into service sometime in MY 1987 for all 928 automatics, was Porsches further attempt to control the drive shaft pullout from the front flex plate. Inexplicably they only put in two bearing units into these TTs which did not support the drive shaft as well as three and further exacerbated the pullout problem. This increase in diameter did not help anything and plenty of 87 and later 928s still have forward thrust on their front flex plates and suffer TBF.

3. Why did Porsche stop using the circlip, bearing and washers at the front of the automatic 25mm drive shafts? They were having problems with field techs understanding and implementing the WSMs instructions on how to correctly set the proper distance between the flex plate and flywheel. This was causing "issues" with customer cars to the point of Porsche sending out their chief driveline engineer on a world tour to hold classes on the 928 driveline and these procedures. Later Porsche also sent out a TSB that any 928 TBF problem is the cause of improper mechanical technique and not a Porsche warranty problem.

4. Sometime in MY 1984 Porsche stopped using the circlip, bearing and washers and just had the mechanics tighten the front pinch bolt as one of the last steps in a TT change out.

5. We contacted the Porsche engineer by email who was sent out on this road trip and asked why Porsche changed to the 28mm drive shaft. His response was for the increased torque of the 87 and later 928s. When we asked why then the 5 speed model drive shafts were not also increased, he did not respond back with an answer. This change only for automatics doesn't make sense given his response.

6. It is true that the neck down portion of the 28mm drive shaft is a stress riser and some have been known to fracture at this point. However we have also seen 25mm drive shafts fracture and twist off as well. These drive shafts are getting old and their life cycle is not infinite so there should be expected failures every now and again. Not too mention the damages that are done to drive shafts during improper mechanical service of the TT, transmission or clutch.

HTH,

James Bailey

Constantine ".......3. Why did Porsche stop using the circlip, bearing and washers at the front of the automatic 25mm drive shafts? They were having problems with field techs understanding and implementing the WSMs instructions on how to correctly set the proper distance between the flex plate and flywheel. This was causing "issues" with customer cars to the point of Porsche sending out their chief driveline engineer on a world tour to hold classes on the 928 driveline and these procedures. Later Porsche also sent out a TSB that any 928 TBF problem is the cause of improper mechanical technique and not a Porsche warranty problem......"

Same can be said about setting the timing belt tension correctly anytime you have flat rate mechanics who get paid a set amount per job they often do it quickly after all it is not their car ! So perhaps it can be said that the design using the clip was fine but implementation during repairs was NOT. With so many on this Forum doing their own work one might expect more attention to details since it is their car. I too agree that the added torque comment was a bogus excuse for the increased diameter. My experience over some 16 years was that the vast majority of broken driveshafts were the large diameter. Porsche maintained that driving on 4 cylinders was a cause of this as the rough running caused the shaft to twist and untwist. The 944s used a big rubber disc to absorb these loads in the 4 cylinder car.

dprantl

And engine balance shafts too I have a feeling that this circlip went away for the same reason the DD clutch did.

Dan
'91 928GT S/C 475hp/460lb.ft

Landseer

Be thankful we don't need to replace the big rubber flexplate piece in the 944.
I've got two here, dry rotted and very expensive
Its cheaper to install a clutch on the automatic car instead of the flexplate ( and retain the automatic).

Black Sea RD

Hi Jim,

We are both in agreement.

The circlip, washers and bearing were designed to work together with the original flex plate clamp. Once they removed them from the drive line and did not change the clamp to adjust for the extra load it was to see, then more problems happened. The 28mm drive shaft was another stop gap measure to stop the pull out problem to no avail.

Glad to see you on the Forum by the way!

Kind regards,

daveo90s4

Hi Constantine, you note that the 28mm shaft has a slightly larger spline than the 25mm shaft.

1. To all, does this slight size difference preclude me using a 25mm shaft (with circlip) in the original clamp?

2. Constantine, do you offer the 'super bearings' for the 25mm shaft?

Thanks all,

Dave