I know a few shops (including myself) that used to use Bran Penn for everything, even the highest horsepower race cars. We stopped using it due to issues in cold weather, since it's paraffin based crude oil, people were finding it would almost gel up in the oil pan. For anyone in an always warm climate, I'd still use it but around these parts we all switched. Many of those shops are now exclusively using Schaeffer oil unless the customer requests something different.

 

I thought I read through all the oil posts and thought I’d picked “correct”. Lol. So I have Penn Grade synthetic 20w-50, so I’m guessing this is the Brad Penn/PennGrade you’re referring to? I’m in OH, so although it’ll never be exposed to extreme cold, it may get down into the 30s/40s in the barn. Paraffin does not sound optimal.

 

Roger, Notice I ordered 4 Of the bolts last week for my cars. Great little discussed maintenance item. Thank you Bruce Buchanan. Enlightening video.

 

One question had after watching the video surrounded the ZDDP levels as they interact with the catalytic converters. It was mentioned I believe the ‘86 and up were fine using high ZDDP levels but what about pre-86? Particularly 78/79?

 

Hi Fred R , we have never deviated from the original Torque spec 59 ft lbs or 60 ft lbs ( new bolt ) , they simply do not undo ( meaning turn / unwind ) they just stretch after many acceleration events , I have no real idea what was going on years ago about the vibration you mentioned , I would need to be with that 928 years ago to see what the hell was going on in regards to the vibration you mention , could of been lots of unrelated things , but there is no point me going through the possible causes of the said vibration
Regards
Bruce Buchanan

 

Hi Bruce,

There is no mystery about what happened 23 years ago- used my professional knowledge to fathom that one out with a bit of help from a couple of top notch mechanical engineering colleagues who also happened to be petrol heads.

The problem started just after I purchased my late 90S4 [end 1998] when the torque tube fractured at the base of the splines in the middle of nowhere on a beautiful road. At the time I knew nothing about the 928 and its idiosyncracies, no mobile phones, no internet and no access to external expertise like Rennlist- I was basically on my own with the local agents telling me there was nothing wrong with the 928 and with the customer laison representative telling me how he could go outside and do the same in 5 minutes trying to imply it was a driver thing. After being pursuaded not to punch his effing lights out by their chief [British] mechanic I was given some home truth insights about the 928. We removed the transaxle and in the process it was obvious that the front clamp had slipped and it was concluded that the excess length had in effect put the shaft into a situation where it had a compressive load that induced the vibration causing the shaft to fatigue and ultimately fail.

We fitted a new shaft but then the characteristic vibration would reappear after about two weeks. As soon as this happened we had the car up on the lift and sure enough the front clamp had slipped. Reset it- no vibration. The local agents reverted to Stuttgart and within 20 minutes they had a solution to a problem that "did not exist"-Ze Germans advised to fit a new bolt and over torque by 10% to 66ft lbs. We did this and it worked- for to weeks! During subsequent discussion with my colleagues we concluded that some kind of adhesive could be applied to the splines of the shaft and I was advised to apply for a domestic internet connection that had just be launched in Oman. 48 hous later I was online and immediately leaned that "Googling" was no longer a matter of watching Pamela Anderson on Baywatch! My first Google led me to Rennlist and my first post found a deluge of info about this problem but there was no definitive solution. However a fine gentleman by the name of Earl Gillstrom had concluded much the same thing as I had and was a step ahead of me in that he advised a procedure he was about to deploy wherein he intended to use Loctite 290 on a S4 front flexplate the following day. I immediately went to hardware shop I knew and they had Loctite 270 available which was more of less the same stuff and we Loctited the first two 928's within about 12 hours of each other and it was a total success. The first iteration lasted 6 years until I lost my S4 in a big crash due to some tw*t on a mobile phone who clipped me.

Later that year I acquired my current 928- a 1993 GTS with a TBF'd motor and we fitted my S4 motor into it with a few tweaks. We deployed the Loctite solution again and that remains intact as of today. Crank end float remains 0.2mm [last I checked]. I usually do a good service every couple of years and a major service every 4 years. For the front flexplate I check the alignment against the paint marks to ensure the front clamp has not slipped and I check the torque - I have never found a need to tighten that bolt further- it has held rock solid. I do not change the bolt for fear that it might induce the loctite to shatter- it is very fragile under tension.

There is no Loctite on my bolt threads and no signs of bolt stretch whatsoever. For the rear bolt I check the torque and again that has never moved. I have changed the rear bolt several times but have never seen any signs of bolt stretch whatsoever despite measuring the shank diameter and the overall length of the removed bolt. Thus I am a bit puzzled as to why you consistently [?] see them "stretching" both front and rear. It is interesting that you appear to find this a bit of a mystery when I do not see ths happen and have never seen anyone report such on Rennlist. I seem to recall a post many moons ago where some chap noticed some damage to the rear bolt but that is about it..

Similarly I have no recollection of anyone advising that they have loctited any of these clamping bolts - like you I see no reason to do so. I also find it a bit strange that you perceive that the bolt stretches in service and yet does not lose any tension- that to me is mechanically impossible but maybe I am missing something?

The other thing I have difficulty with is understanding how the rear clamp can possibly let go when [as we agree] the rear bolt does not lossen itself. My understanding is that your perception is that TBF happens when the rear clamp slips but that must be nigh on impossible. The shaft has a significant detente on it- the boss the shaft pushes into engaged about helf the cross sectional area of the bolt as does the floating clamp. When fitting the bolt everything has to be aligned perfectly to even get the bolt in thus how on earth can the thing possibly slip from the rear? I certainly need educating on that one! For any forwards movement to be possible one would have to lose a chunck of the bolt and what was left would then have to wedge the extended shaft into a locked position. I would not like to say that is impossible but it would seem highly unlikely- again maybe I am missing something.

That the 928's you look after never see TBF for sure is a credit to your efforts. Presumably the failures you have seen were on examples that the owners brought to you in distress.

Considering my own experience with the slipping front clamp and how I managed to stop this from happening by increasing the effective area of contact [aka with the Loctite] hopefully you can understand how I am mystified as to how setting the clamp at the specified torque can possibly solve the issue.when so many examples have perished with the correct bolt set at the correct torque. Then I read your most interesting post [No 9?] again and a huge smile erupted on my face as I missed something the first time I read it. The clamping bolts are 10 x 1.5mm affairs grade 12.9. When torqued to factory spec somewhat surprisingly they only tighten to approximately 60% of yield as per my calculations. This a bit unusual as high tensile bolts are usually designed to stress at about 80% of yield. Thus it was no surprise to me that Porsche were able to advise me to increase the torque by 10% and even then it did not work. What produced the smile on my face was when i spotted the bit about how you "lubricate every bolt". As I am aware all these bolts have to be installed clean and dry at the torque specified.. At the end of the day tightening a bolt using a torque wrench is inferential in that the design stress is typically achieved by fastening to the specified torque but in reality the stress achieved is not actually measured- it is inferred. As per my bolt stress calculator. it would seem you may have unwittingly tightened the bolts to yield stress levels. If so this may well explain why you consistently see these bolts "stretching" . When a bolt yields initially the stress drops a little and stays more or less consistent as strain increases and then the stress increases once more until UTS is achieved. When tightening bolts in the elastic range the gradient on the stress strain curve is very steep thus why it is difficult to achieve a given stress. Thus why TTY bolting [torque to yield] came about because this point of the stress strain curve could be targetted more reliably. with minimal bolting noting that bolt tension is rather critical on some joints like cylinder head gaskets. The ultimate advantage of course is that you have in effect installed the bolts at optimal tension levels and thus were in fact tight enough to prevent slippage. what you have done is in effect like torquing a dry clean bolt to 100 ft lbs. Ironcally maybe all this kit ever needed was to tighten the bolts like this and all the TBF issues might never have happened.


Trust you find the above of interest- feel free to rip it to pieces if you feel it appropriate.

Some additional thoughts based on what I saw/read:

ZDDP has no impact on lube oil film strength- as I understand it creates a localised film of high strength in highly stressed areas and does so by means of a chemical reaction temperature induced by local hot spots on the flat tappets and the cam lobes. It has remained in service for so long because there is nothing to compete with it.

My favourite oil when I could get it was synthetic Redline 20W50. I used to have a good rapport with their chief chemist many years ago. Unfortunately I went up to Dubai one day many years ago and the shop that sold it disappeared without trace. I then reverted to Castrol GTX dino oil as it was available in Oman and even that disappeared so I reverted to Shell Helix HX5 - a 20W50 dino oil blended to API grade SL and that should contain 800 ppm of ZDDP. Modern witch **** oils appeared because they could deliver acceptably high film strengths with low viscosities. Porsche told the local agents that Mobil 1 0w40 was good enough for our 928's and unbeknown to me stuck some in my 928 after we rebuilt it.17 years ago. I took the car for a test drive and after it warmed up the idle oil pressure struggled to make 1 barg. Took it straight back gingerly- asked them what was going on and they recovered the oil and we filled with Shell Helix- oil pressure 1.5 barg in summer heat.I also ran for a while with a jug of STP thrown in in the hope of increasing ZDDP level a bit and hot idle increased to 2.0 barg. Nowadays I do not run it in full midday summer heat and stick with tt=unadulterated Shell HX5 and check the cam lobes from time to time. If I was going to track a 928 with a modified or high performance engine I would run with the Redline synthetic with ZDDP around 1300 ppm or something equivalent.

The other and possibly even bigger problem with running low viscosity oils is that oil p[ressure will drop noticeably and I suspect it will not drop evenly across all passage ways. I suspect but do not know for sure that given we have positive displacement oil pumps there is a real danger that more oil will escape via the big ends and mains and thus possibly cause some degree of oil flow starvation to the cam lube system.

Finally, thrust bearing loads on an engine running correctly are minimal. If we had to rely on engine oil to prevent TBF we would be in real trouble. Stop the flexplates from migrating and the problem simply does not exist.

 

Hi Fred , the first part of your thread ( regarding the broken Torque Tube shaft at the base of the splines ) this is a result to the said 928S4 suffering the dreaded Exhaust Temperature Monoriting System ( ETM ) dropping 4 cylinders intermittently( they do this in old age ) while the human driving car ( not you , the previous owner ) just put their foot down and kept driving to get to where they needed to go , this in turn damages ( minute cracks forming ) inside the T T shaft & interestingly it takes quite a few of these ETM playing up events ( meaning running on 4 cylinders ) before the T Tube will snap & its always snaps in the same place where you saw it broken .

None of the above is a mystery , seen plenty of those ( 1989 onwards ) & of course the 1989 year model was the first to receive the ETM system & hence many years later with the ETM playing up ( always out of the blue while driving ) & the T T shaft is weakened at every one of these events & its then only a matter of time when that ( now internally cracked ) shafts just fails , remember that is just induced metal fatigue

The Mystery part his the vibration are referring to , could be lots of things

----------------------------------------

When checking the crankshaft end float with a dial gauge , I would strongly suggest the removal of theTorque Tube ( front & rear ) coupling bolts & when doing so , note the tightness of the front & rear bolts as you undo them and then discard both of them , mount the dial gauge to the crankcase and measure the flywheel forward and back movement ( with the flex plate coupling still mounted you may get an incorrect measurement )

If the front flex plate is pushed in ( like it was on Stuarts's 928S in the video ) then the T Tube shaft has moved forward , and hence you need to push the shaft back a bit & you will see the rear hole alignment look a lot better as you push it back a little & then install two new genuine Porsche coupling bolts and torque up ( no Loctite )

Now , you mention the cut-out in the T Tube shaft at the rear , its not a snug fit for the bolt , the shaft can move quite a few mm forward or back with the bolt in and loose ( of course ) , the migration of the shaft forward causes the flank of the coupling bolt ( which has not turned at all ) to "Ride Up" onto and into the male splines of the shaft , a few years ago I put up a photo on Rennlist showing the teeth marks from this very thing happening on an old T Tube coupling bolt , we have seen quite of few of these old rear coupling bolts with these teeth marks , but the one I put up on Rennlist years ago was the cleanest and most defined one I had ever seen

In fact , on Stuarts 928s ( video car ) , the very old rear coupling bolt had the beginning of the teeth marks forming on the flank of the coupling bolt , meaning the bolt flank had started to engage against the male teeth of the shaft ( very simple stuff )

----------------------------------------

The rear T Tube coupling bolt always stretches , its does not unwind & as a result the clamp looses its necessary clamping force ( it keep saying that over and over again ), a good example was on Stuart's 928S in the video , the front flex plate ( engine end ) was pushed in but the front coupling bolt was bloody tight ( quite normal ) , but his rear Torque Tube coupling bolt was not tight enough, or put another way ,was not as tight as the front coupling bolt ,the rear coupling bolt had lost a bit of its clamping for by being stretched over hundreds / thousands of acceleration events & hence his Torque Tube shaft had Migrated Forward , pushing the front flex plate in against / towards the flywheel

If the Front of Torque Tube coupling had ( as you put it ) Slipped , then the male splines of the T T shaft would of just gone into the female splines of the front flex plate and you would then see NO flex plate deflection , but that is not the case and hence the front flex plate is pushed in ( very simple physics )

In the past 30 years ( in my own business ) or even before that at the Porsche dealership I worked at , we have never ever lost a 928 engine to thrust bearing failure ( not one ) and we most likely work on more 928's then anyone else in Australia , thats because of 2 important reasons }

A ) We realised 30 + odd years ago that the rear coupling was often not as tight as it should be after X number of years , but the front coupling was not an issue , meaning we could see flex plate deflection , by maintaining the T Tube during major services , meaning making sure the flex plate is not deflected and the front and rear couplings are tight ( new bolts ) , this was a good first step

B ) The use of a good quality 20w-50 engine oil , but because we never varied from this at any stage ( even going back 47-48 years ) & because the 20w-50 has good oil film strength( helping to protect the thrust bearings ) , we had this going for us as well and hence no thrust bearing wear & as it turned out no cam shaft wear as well ,because of the high oil film strength engine oil

Note } We have seen / and heard of destroyed thrust bearings on 928's here in Australia ( none of these 928's were out customers cars ), the poor things had been running for some time on a lower viscosity synthetic oil and with the deflection of the flex plate ( mild push in of the crankshaft ) the lower viscosity oil/s ( all were so called synthetics ) , be it 5w-40 or 10w-40 oils , these low oil film strength synthetic oils could not help protect the thrust bearings , we had only become involved in these engines long after the damage had been detected and we could only give advice on how to avoid it in the future

You mentioned ZDDP , yes I know what you mean , but put simply the ZDDP comes into its own with with metal to metal contact / stress , so this may explain why with 20w-50 ( high oil film strength + ZDDP ) we see little or no wear of things like thrust bearings , cam lobes et etc etc , BUT on lower viscosity oils , like 5w-40 or even 10w-40 oils we see a lot of wear of the said components

Regards
Bruce Buchanan

 

I use here in North Germany Valvoline VR 1 Racing 5 W 50. It has high level of ZDDP, 1500 ppm and the oil consumption of the S engine is significant lower.
The car has no cats so the highly with additives loaded racing oil can’t do any harm.
The first 10.000 km after the engine overhaul we use a 10 W 40 to break in the rings but you have lot of oil steam.

 

Bruce,

Much appreciate the response- the intent of my post is to test how much detail we can clarify and more to the point what evidence supports the perceived position. It seems there are some points where you have not quite grasped the concept so herein I will try to open the discussion a little further based on the points I have captured.

Regarding the broken Torque Tube shaft at the base of the splines ) this is a result to the said 928S4 suffering the dreaded Exhaust Temperature Monoriting System ( ETM ) dropping 4 cylinders intermittently.

Clearly you did not know that both my 928's are/were non cat models as the local refinery [where I was the chief engineer] did not start making unleaded gasoline until 1994. My 90S4 never ran on 4 cylinders as per the original Dutch owner - when the shaft snapped the local agents gave me a copy of the Porsche TSB concerning how torque tubes have been known to fail when running on 4 cylinders with those relay controlled drop outs but who on earth would run a 928 like that? They are bad enough with 1 cylinder down yet alone 4 cylinders. The local agents had no credible answer as to why the torque tube snapped and they told me that thus my analysis- it was a very clear fatigue failure but without ever running on 4 cylinders.

What the post failure running told me clearly was that when the clamp was in the extended position the vibration was present and when it was corrected it was as smooth as silk. When I corrected the slippage issue with loctite on the splines the vibration disappeared. The front clamp has never slipped since securing it and the crank float has not changed at all. When checking the crank end float I release the rear pinch bolt. I do not not undo the front bolt as I suspect that might fracture the loctite bond. I do check occasionally to ensure it is fully torqued and has not moved relative to the index marks I placed on the shaft. I have no problems telling when the crank is at a hard stop both rearwards and forwards.

Now , you mention the cut-out in the T Tube shaft at the rear , its not a snug fit for the bolt , the shaft can move quite a few mm forward or back with the bolt in and loose ( of course )

With respect it is a snug fit- I have a spare transaxle unit in my garage and today cross checked what I said- there is a slight movement that I would estimate to be about 0.5mm but that is not going to cause TBF. I can well believe if the bolt de-tensions it can cause the bolt to dig into the splines a bit [I seem to remember a case of this being listed] and even if the movement was increased to one or two mm by those indents that would not trouble the thrust bearing as the flexplates would take care of that.

In fact , on Stuarts 928s ( video car ) , the very old rear coupling bolt had the beginning of the teeth marks forming on the flank of the coupling bolt , meaning the bolt flank had started to engage against the male teeth of the shaft ( very simple stuff )

There is no diasagreement about what you saw on Stuart's 928 but his motor did not exhibit TBF- Please correct me if I am wrong. My entire point is that it is mechanically impossible to cuase TBF by the rear clamp slipping. I have seen 5 cases of TBF over here and all of them were undisputably caused by the front flexplate clamp slipping.. Beyond that I have lost count of the number of TBF examples tabled on Rennlist over the last 23 years- scientific wild *** guess says about 50 such examples? All the same and not one of them driven from the rear. When TBF occurs the flex plate is not distorted it is mangled and the front clamp has typically migrated in the region of 10mm to 15mm forwards- the exposed length of splines tells its own story. It would be interesting to see a case of such happening from the rear clamp but that is just not mechanically possible- not without losing half of the bolt cross sectional area. Whether or not a "loose" rear bolt can set up vibrations that can subsequently cause the front clamp to slip is something I have wondered about.

If the Front of Torque Tube coupling had ( as you put it ) Slipped , then the male splines of the T T shaft would of just gone into the female splines of the front flex plate and you would then see NO flex plate deflection , but that is not the case and hence the front flex plate is pushed in ( very simple physics )

Your understanding of the word "slipped" appears to be somewhat different to mine- slip is not a vector quantity [i.e. has no directional component].- slip is the movement of one piece relative to another that should by design have been held together. . You described this problem quite nicely in the video wherein the shaft wind up causes the effective length of the shaft to decrease and thus create a rearwards axial shock load that for reasons never completely identified manages to pull the shaft out of the splines in tiny increments. This movement cannot be reversed but once it starts can be replicated over and over again. The flexplate couplings can handle a small amount of axial movement [a couple of mm?] but then they give up the ghost and in effect become a hard coupling and that mangles the front flexplate.- as the attrition grows the rear face of the crank becomes a sort of disc brake, the heat melts the white metal, this eventually causes the thrust bearing to spin, the crankcase webs crack and that is game, set and match.. Stop the front flex plate from slipping this way and TBF is a thing of the past. Loctiting the splines stops this from happening- it is not a perfect solution but 23 years ago it was brilliant in that it was the only solution available at the time. The clamp design is rediculous- if nothing else it should have had at least two bolts pulling down evenly on either side of the sleeve. Constantine's clamp is the perfect solution.

We know that this issue does not happen on brand new examples. It typically sets in after about 50k miles. Something causes the front clamp to slip- in my opinion that something is a corrosion concept known as "fretting". This is where two surfaces having a lot of energy transmitted through them experience at micro level slight vibratory movements that cause very small amounts of steel not visible to the eye to shear off the interlacing splines. The shaft spinning at high speeds causes these particles to be thrown out radially and thus embed themselves on the soft alloy of the casing where they eventually rust- hence the "red particles" you refer in your video- ferric oxide. Sufficient clamping force stops this from happening, there is no more fretting and theoretically no more "red particles".

To summarise- your successful management of this issue is down to the excellent practices you deploy. As I stated yesteday your technique is stressing the bolts at 100% of yield - whether or not you were aware of this remains to be seen- had your followed the factory instructions I am pretty sure your clients would not be so fortunate.

On the ZDDP and oil fronts I think we view this matter in pretty much the same way. I do not buy into the notion that optimising the oil will prevent TBF but it may well buy some time before destruction takes place- how much time I have no clue.

 

Hi Fred , the 1990 928S4 you had ( years ago ) , you mention a Dutch owner , was the said 928S4 delivered new in Holland ?
Regards
Bruce B

 

Hi Fred , here is the photo I put up some years ago on Rennlist of a 928S4 ( Auto Trans ) Torque Tube rear coupling inhex bolt , where you can clearly see the flank of the bolt riding up onto the male threads of the T Tube shaft as the T Tube shaft is migrating forward , would of taken a few years of the rear coupling not being tight enough ( stretched ) , so you can clearly see that the male splines are no obstacle at all , the bolt's flank is cutting into them or put another way , the male splines are cutting into the bolt flank , meaning as the shaft is migrating forward its not only making more room to move forward in cutting into the bolts flank , & the bolt is destroying / wearing away the male splines its contacting make even more room for the shaft to move forward

You can also see that the bolt has not turned at all over a long time , otherwise the Teeth marks from the T Tube male splines would not be visible in this way

All very simple stuff , but because we work on so many 928 models over such a long time we get to see stuff you do not , its just experience and nothing else

Regards
Bruce B

 

Hi Bruce,

My late 90S4 as I am aware was delivered in Oman. The Dutch chap was the 2nd owner but as I can remember he purchased it when it was less than 1 year old as I vaguely recall. Thus the car was a non cat version as per Mid East spec. I have known quite a few Dutch chaps over here because Shell supports the national oil company and as I recall about three of them owned 928's.. One of them owned an imported black GTS that suffered TBF and he fitted a new engine he personally imported from Germany..

 

Again many thanks for posting the photo- as I can see it is confirming what I perceive can and does happen- the rear bolt digs into the splines and some [limited] axial movement is the result. What I am trying to find hard evidence for is whether this movement can be of such magnitude that it actually leads to a "confirmed kill" TBF wise and my aim in this line of enquiry is to understand whether you with all your many years of 928 experience have ever seen a motor that has succumbed to TBF as a consequence of this action on the rear bolt. My take is that the motor that bolt came from did not suffer TBF- is that perception true or false?

As I stated earlier it is my opinion that to cause the attrition needed to cause TBF there has to be in the region of 10mm to 15mm of axial travel of the clamp along the splines. We see this time and again on the front clamp but I have never seen this degree of motor attrition happen from the rear clamp- I have seen the odd bolt or two showing something similar to your photo. It is of course disconcerting but that is not capable of causing terminal damage or so I believe.

Considering the above the next obvious question if you would so kind as to indulge me, is how did you conclude that the rear pinch bolt stretches? I suspect your conclusion is inferential based on the -premise that [quite understandably] it was the only contingency you knew of that could explain what you saw. I suspect there was no positive verification of such- aka measurement- kindly correct me if my intuition is incorrect.

My take on what happens is that the fretting corrosion concept takes place between the teeth of the splines.- this is nothing that one can really see or measure but it is enough that over the course of time degrades the interface to the point where the compressive forces applied by the clamp are no longer sufficient to hold the joint together as per design intent. This in turn needs more force to be applied. This is precisely what Constantine's clamp does, it is what the Ritech clamp does, it is what Ken's PKLAMP does and in the case of the Loctite approach, flooding the splined area with the goo fills in all the nooks and crannies and restores/improves the contact area. Needless to say I have never come across any TBF failure on any 928 that has deployed these solutions.

Then of course in post No 9 you articulated that you have been deploying new bolts front and rear for the last 30 years and do so lubricating the bolt threads. Whether wittingly or unwittingly you came up with what may well be a "perfect solution" because in effect this approach deployed the bolt at 100% [maybe more] of yield stress. You advise that by doing this you have never lost a 928 motor under your care- fantastic- you deserve a knighthood!

Now a teaser for you Sir Bruce! In all the years you have been doing this on your customer's 928's have you ever seen any signs of the rear clamp slipping ? My guess is "not once". So, if installing the rear pinch bolt at or close to yield does not permit slippage presumably the bolt is not stretching due to the oscillating loading concept- ergo did the bolt ever stretch at all? You may well argue that the bolt does not stretch because you change it out regularly - I would argue that it does not stretch period and you have solved the fretting issue by application of superior clamping force. I also suspect that by doing what you have done there is probably no need to keep on changing out the bolts at every service but of course needless to say there is no harm in doing so other than relatively minor financial attrition.

 

Hi Fred,
I think you have been misrepresenting post 9 a little here, Bruce states that importantly it is the oil that is protecting the thrust bearing as its the oil film strength, hence the reason for a 20w 50 (in the correct climate) with a good amount of ZDDP.
His indication of the flex plate pushing forwards, as they always are when we release them and cause some perceived wear upon the thrust bearing as there is a constant force being applied to the crankshaft at all times, apply this with a thinner oil, and say one with even as little as 600ppm as were the case with some of the top brand oils back in the era surrounding 1996 and he can say goodbye to your thrust bearing.

"Now a teaser for you Sir Bruce! In all the years you have been doing this on your customer's 928's have you ever seen any signs of the rear clamp slipping ? My guess is "not once"."

Absolutey we have, and on so many occasions it would be impossible to recall them all. I would say we have seen at least 50 occasions where the torque tube shaft has slipped forward causing the indents you see on that rear torque tube bolt, and many more times where once you remove the bolt you can see that the cutout in the torque tube is not aligned with the clamp. on every occasion we centralise the shaft and install new bolts.
Regards
Sean

 

Hi Fred
We conclude that the bolt stretches from the following.
We have found in the past that when a bolt was loctited in place and used for a predetermined amount of time we were able to record the angle of the bolt, crack the bolt loose (the locktite makes it feel very tight) and when we were to torque it back up we go significantly past the recorded "tight" angle. signalling to us that there had been a significant loss on the tension of the bolt and loosening of the clamp( even though the bolt feels tight because of the loctite). This is particularly true of the later manuals with the hollow input shaft/5 gear assembly. We have found that its extremely common for them to break due to the use of loctite as the owner/user thinks its tight all the time but because the bolt stretches (even though the loctite makes it feel like it is torqued to spec) the clamp looses its clamping force and the input shaft breaks as you can imagine this is a costly repair and we have greatly reduced this from occurring by using a greased bolt done to torque spec and checked regularly. Porsche have even run out of specific S4 year model input shafts due to them breaking and only a limited number of spares manufactured

We also found that if we were to get access to the area after a predetermined period of time with a bolt with grease on it we would find that the bolt has lost tension and we were able to apply torque back to "spec". We could do this a few times over a few services and continue to see the loss in torque spec. We always check these at every major service and change them when we feel the need on the circumstance

Regards
Sean