Black Sea RD

Hi Bruce,

We have studied this problem and respectfully disagree with your findings.

The rear pinch bolt is secured into a groove at the rear of the drive shaft, which holds it in place. The rear pinch bolt is the same one at the front flex plate clamp, both of which are very sturdy and will not "stretch" out under normal tightening torque applied to them. We have measured them during our study of TBF from different 928s to include ones which have suffered TBF, no measurable differences.

The problem of 928 automatic drive shaft pullout from the front flex plate clamp is caused by wind up of the drive shaft under torque load. The front clamp was designed to work with other parts placed onto the front of the drive shaft before 1984 which helped to keep the drive shaft secured from being pulled rearward through the front flex plate clamp. They were discontinued in 1984 and the TBF problems started to surface. IMHO, Porsche should have changed the front clamp design at that time but didn't.`

Porsche tried to combat this drive shaft wind up effect by increasing the 1987 and later automatic 928 drive shafts from 25mm to 28mm. It didn't stop the problem and only added another problem of 28mm drive shafts shearing at their neck down areas.

TBF is a very real problem and is not just caused by poor oil flow. It is caused by the crank shaft being pushed into the engine block from the rear. It is a very real problem which is exacerbated by mechanics and owners who don't understand the root causes of TBF and what to check to ensure this does not happen to them.

Disclaimer: This is not an attempt to scare owners into buying our Super Clamp. In fact, we are contemplating the ceasing of it's production in the future and concentrating our resources on other products.

Best regards,

Bill Ball

Bruce :

Although your conjecture about the root cause of the problem differs from Constantine's, you have not seen any TBF following your procedure. Regardless of the cause, your service procedure of releasing the clamps, retorquing and replacing the bolts works and I wouldn't have expected otherwise. That's what I did periodically before installing the PKlamp. I do service the rear clamp periodically too, and the bolt remains fine/undamaged. I still release the front clamp every year or so even though there is no preload accumulated against the front flexplate and check crank endplay for my own information and reasurance. Although I haven't personally seen a problem with the rear clamp, others here, like Mrmerlin (Stan), have and recommend periodically servicing that as you do.

https://rennlist.com/forums/928-foru...xperience.html

https://rennlist.com/forums/928-foru...pictorial.html

JET951

Hi Constantine , I have the greatest respect for the Superclamp you have engineered & I know its superior to the standard "single " inhex bolt design of the front TT coupling ( engine bell housing area) , its just that we don't have any real problems in this area , or put another way we have a constant work load in maintaining the rear TT coupling ( again with just a single inhex bolt) , its quite manageable once we worked out a service schedule for their maintenance ( as mentioned on the previous post ) .

However if you could re engineer the rear coupling in some way ( with more bolts and two solid shafts ) , then this would be fantastic

Now , you made a tiny miscalculation ( as I did many years ago) in thinking that the rear TT coupling inhex bolt can not move( forward or back) within its corresponding single half moon cut out in the splined T Tube shaft ( Quill shaft) , but in fact they do move with some considerable force( and even more than you think) ,I know because I was shocked when I finally realised years ago , you can see in the second photo in this thread ( see post 29 ) it shows the marks on the ( once) smooth surface( non threaded part) of the inhex bolt , these marks ( gouges) are the contact damage as this bolt is being dragged up and onto the end of the splines of the TT quill shaft ,in doing so the splines ends that this inhex bolt is forced up onto are actually ground away in the process which in turn gives more room for the quill shaft to just keep moving forward . I kept an excellent example of a badly worn inhex bolt at work that show the teeth marks left behind on a coupling bolt we removed from one S4, its a ripper , even better than the one in this thread .If I find it I will put a photo up

Subnote } you will notice that the spline gouges on the bolt are in one plane , meaning the bolt is not turning over time , meaning it / they just don't loosen by turning they just stretch ( like all bolts can & do , particularly when there is just one of them), no big deal just replace on a regular basis

So in other words the tri split female outer splined input shaft be it manual trans ( all year types) or the 4 speed Auto version is slightly flawed in design and if left unattended for too long the griping force ( spline on spline ) is reduced because the the single coupling bolt stretching , if it had the earlier 3 speed auto design ( sliding sleeve with 2 inhex bolts on two solid shafts coming together ) then this issue we see would of been greatly reduced .
The reason I say that is we modify Porsche 951's with modified 3.0L twin cam engines with Vitesse stage 5 running on E85 fuel with in excess of 450 BHP for the track & street & these 951's naturally have the 928 3 speed auto sliding sleeve couplings with its 2 in hex bolts clamping down on the two solid shafts ( standard Porsche for the 944/951), these cars cop an absolute caning with no stretching issues at all with their coupling bolts , but I can not say this for the 4 speed 928 auto version or the manual trans 928 ( all year types)with their single inhex bolt , these are a constant issue ( slight weak point ), but can be maintained, this is the reason why the manual trans input shaft ( tri split) break , we have had the 4 speed auto ones break ( rear flex plate assembly) on the new to us customer cars , plus you will often see the "Red" dust in this area , this is the wearing away of the male & female splines through lack of grip / force , due to a stretched inhex bolt .

Now onto what not to use on these single inhex bolts , try not to use loctite on the new bolt going in because when you go back in these to re-torque ( say 20,000kms later) a Loctite'd inhex bolt will give you a false torque reading because the threads are being held in place but the very important clamping force we need on the bolt head base to coupling will not be there ( simple really) , just use a slight smear of anti seize on the threads.

Now onto thrust bearings , I carefully used the words oil film strength , not oil flow , this is extremely important because Porsche engines from last century are very much in need of two things , oil pressure not being too low( hot) & just as important is oil film strength , meaning the way an oil can keep two metal surfaces apart when there is little or no oil pressure( like cam lobes , thrust bearings etc) .

We have see a lot of wear in last century Porsche engines & it was mostly due to low oil film strength oils

Subnote } there are two main ways of increasing oil film strength in engine oils }
A) increase oil viscosity , say from a 5w- 40 to a 20w-50 ( where the ZDDP levels are the same)
B) increase the ZDDP

If you add both you get high oil film strength .

We have had way too many of last century Porsche's with worn cams & lifter faces & worn twin cam chain sprockets , piston to cylinder wear , thrust bearing wear ,excessive valve guide wear etc , in every occasion these engines were on low oil film strength oils ( meaning this century so called synthetics )that just happened to be too low on viscosity & too low on ZDDP ( double whammy ) , don't get me wrong I am not complaining because it gives us a lot of avoidable work which I am most grateful .

However the Porsche's we see that have been on a high oil film strength oil ( like a good quality 15w-50 or 20w-50 oils with decent levels of ZDDP we see no wear at all , so the contrast we see is just amazing

Here is a link to other thrust bearing wear in another Porsche type from a thread on Rennlist where Steve Weiner from Rennsport Systems had the same issue years ago & switched customer Porsche's to a oil with higher oil film strength ( meaning different viscosity & probably more ZDDP )
https://rennlist.com/forums/993-foru...-front-24.html
Its post number 359

We had a new customer with a 1987 928S4 Auto a couple of years ago where the 928S4 of his was being serviced at general service( non Porsche specific) who was not familiar with the TT couplings and didn't go anywhere near these couplings for years and years , the good news was the engine was on a decent oil viscosity & decent ZDDP ( meaning 15w-50 & or 20w-50 ) and when we first serviced the car we found the rear coupling completely covered in the "Red rust Dust"( inhex bolt with no tension) & the front flex plate ( engine bell housing) was deflected in to the absolute max , probably the worst we have ever seen .
In all cases ( this one in particular) we installed the special tool we made to hold a dial gauge to measure the crankshaft axial play ( thrust bearing play) , both Sean & I were a little amazed to see the wear of the trust bearing was well within tolerance , in fact it was still within the new spec range & not the max spec range , that was impressive because I knew the thrust bearing had been under a constant excess forward load for year & years & lots of Kms.( oil film strength at work)

We have found also ( just plain experience over time ) that even the ones that have near max thrust bearing wear limit measurements( been on a low oil film strength oil) , that if we switch these ( new to us Porsche's from last century) to a 20w-50 with very high oil film strength we can ( in most cases) stabilise the thrust bearing wear at that point and no further , which is nice to see , this is the stuff we are now very used to .

As I have mentioned at the beginning of this post , I have the greatest respect of what you guys make its just that I know what we seen at work over many many years & I just hope this input will shed some light on these quite easy to handle issues once understood fully .

Kind Regards
Bruce Buchanan
Buchanan Automotive
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Dictys

So guys to summarise:

1) use a good oil 20w-50 with a good ZDDP level.
2) check, retorque or renew pinch bolts forward AND aft at regular intervals.
3) fit a good clamp either the super clamp or pklamp if you wish extra assurance.

is this about right?

JET951

Perfectly summarised

Dave928S

I'll save Bruce the trouble of posting pics that show the damage that occurs to the rear of a drive shaft when the rear bolt stretches, and the damage that occurs to the bolt as the splines sink into it. The movement occurs on both autos and manuals. The shaft and bolt pics are from my manual 82 when I pulled it apart some time back, and the bolt head still had the paint marks showing no rotational movement ... it had definitely stretched. The stretched bolt had allowed the shaft to migrate forwards and it had ridden up onto the splines, as can be clearly seen. The pic of the bolts is a brand new one below the damaged one, which measured longer than three Porsche new ones. You can clearly see the spline marks on the shank of the damaged one, and the red dust (rusty powdered steel).

As soon as I got my manual GTS I checked the rear pinch bolt and found exactly the same thing had occurred to the bolt. Paint reference marks were still showing the bolt had defintely not rotated, but the bolt was loose, and therefore must have streched. Luckily the shaft hadn't been damaged, but the bolt was difficult to remove as it was no longer central, but against the rear of the shaft half moon 'waist', which illustrated that the shaft had moved forward.

I also think that Constantines product is vastly superior to the original front connection ... but without maintaining the health of the rear connection, shaft movement can still occur.

I wish Porsche had put more than one pinch bolt to clamp the rear of shaft, and I wish the previous Porsche dealer mechanics, working on both of my cars from new, had learnt to carry out the simple checks that Bruce does.

Edit: I obviously share his views on this issue, but also his view of the importance of correct viscosity/oil film strength/high ZDDP oil.

JET951

Thanks Dave , that saves me a bit of time ( particularly at work ).

Regards
Bruce
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Black Sea RD

Hi Bruce,

We appreciate your comments about our Super Clamp, thank you!

We also appreciate your years of experience doing this work and your observations.

We have also seen some damaged rear splines on drive shafts as you've mentioned. However most of what we've seen was from rear pinch bolts being forced through misaligned drive shaft grooves relative to the rear coupler. We have seen some damage as you describe too, but not that often.

We have gotten in drive shafts from 928s which suffered TBF and there was no damage to the rear splines or pinch bolt.

What you are saying contradicts what has been observed by many owners who have kept a vigilant eye on their front flex plate coupler. That being the drive shaft moving rearward within it.

We have witnessed the running of a severely TBF'd 928 and there were loud knocking sounds coming from the engine. This was due to the crank shaft being forced into the engine internal structures. When the front flex plate was released the knocking sounds stopped. No damage was seen at the rear drive shaft splines of this 928.

What is missing with your theory is what is keeping the drive shaft under constant tension if the rear pinch bolt is being damaged and allowing the drive shaft to move within the rear coupler. There would be "slop" at the rear coupler, but not a condition where the drive shaft is getting hung up as the front flex plate pull out.

We agree that owners should have their drive line inspected at both ends for any signs of damage or movement at these couplers. As far as oil viscosity problems, we would also agree that poor oil film strength is bad for the engine, but that is not the root cause of TBF in 928s from our observations.

Bottom line, all this discussion is great since it keeps this topic fresh and will hopefully keep more 928s on the road.

With respect,

JET951

Hi Constantine , there is no pull back & forwards its just forwards and stays there its very simple , this is because the tension on the quill shaft via the black nylon TT bearing ( inner ) insulators are just enough to keep the quill shaft forward( once moved) under a constant tension , remember the flex plate can be pushed by a human thumb , its not a huge force , but its constant , just look at the photo's from Dave , look carefully its very simple once it clicks in the mind , look at the cuts in the coupling bolt , they align perfectly with the TT quill shaft splines , you are very correct about the forces that make the TT change its length under wind up effect , so in effect we are both getting to the conclusion of this thing .

Regards
Bruce Buchanan
Buchanan Automotive

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Black Sea RD

Hi Bruce,

There is simply no way the nylon bearing inserts have enough grip on the drive shaft to keep it under a constant state of tension to be able to force the crankshaft into the engine and cause TBF. The force of the front flex plate with a 3-4mm load, which is the norm when drive shaft pullout is found, has a very significant force rearward, more than the nylon inserts can hold onto the drive shaft.

And as I said, we have not seen rear drive shaft damage in the cases when we inspected the drive shaft from TBF'd 928s.

We have looked at this TBF problem for years and are comfortable with our thesis as to the root causes.

Best regards,

elgreco

I have a 85 auto in my shop that I'm doing an engine swap due to a tbf. I'll pull the rear bolt tommorow and see what it look like

jeff spahn

Yep. I have never touched my rear clamp. Greg Brown installed my transmission so I figured that it is done right. I have a Constantine Clamp at front. No movement, flex plate is flat.

Bill Ball

I have tried to move my TT shaft in the bearing holders with a 3 lb sledge and was completely unable, even after 230K miles. HOWEVER, with every other TT I have seen, including freshly rebuilt ones, the shaft moved quite easily with nothing more than a soft "plastic hammer" blow, and it was easy to change the position to get it to the WSM-specified end clearance, as indicated. So, I'm not quite following Bruce's theory either. Still, the rear bolt and shaft have been found damaged and/or loose, as Dave documents and Stan has mentioned numerous times, although I have not personally observed that. Other than terminal damage to the shaft and rear clamp splines, requiring replacement of the torque converter (in an auto) and TT shaft, I'm not sure what other problems this could cause.

Vilhuer

I have seen rear spline damage on both 5sp and automatics without any TT center shaft movement. I think rear bolt stretches and allows center shaft and gearbox input splines to deastroy each other. This is almost totally disconnected to any TBF problems. So there is basically two separate problems, automatic front clamp weakness and rear bolt weakness in all cars. Root cause in both cases is probably low RPM TQ.

Dave928S

Maybe there are a few possible case scenarios when 'wind up' causes the shaft to temporarily shorten, which would explain different observations.

1. Only the rear coupler is loose or is the most loose, and allows movement which, with shaft shortening on wind up, results in the shaft jamming forward on the splines at the back only.

2. Only the front coupling is loose or is the most loose, and allows movement which, with shaft shortening on wind up, results in the shaft pulling out of the front coupler and jamming at that pull out.

3. Both are equally loose, and the shaft jams after pulling out of the front and rear couplers.

Both my cars were the first alternative with the shaft forward relative to the TT case. My 82 was jammed hard forward on the splines, with the front coupler still tight, and the stub shaft rammed into the pilot bearing and giving constant forward pressure (luckily no TB wear). Mine were definitely forward movement of the shaft, as both bolts were hard against the rear of the half moon 'waist', and the bolts were hard to turn as I removed them, as they rotated against the shaft. Which is what the damage to the bolt in the OP's case seems to indicate.

Regardless of how we individually interpret what goes on ...the bottom line is that if both front and rear are maintained the problem is avoided.