FredR

Sean,

Good to see you joining in as it were.

Your comment is interesting in that [unintentionally] in and of itself it is a tad misleading.

The thrust bearing is a plain white metal bearing interfacing with the steel journal and thrust faces of the crankshaft. The entire point of the ZDDP content is to make steel bearing surfaces more resilient to wear when in contact with one another and not as in bearings of this kind. I am assuming you know and understand this. The thrust bearing is simply not designed to support superimposed loads due to the clamp slipping irrespective of whether it is the front clamp or the rear clamp. If the thrust bearing can support such it is fortuitous and that is all.

As with any bearing there will be a margin between normal operational requirement and wipeout but if one's expectation is that the thrust bearing should support the superimposed loads and the lube oil is primarily responsible for doing this then I would politely suggest that would be a somewhat naive school of thought..

If when you remove the rear bolt the cut out is misaligned that would tell me that the front flexplate was under tension and upon releasing the rear bolt that tension caused the shaft to move relative to the previously clamped rear sleeve- ergo it was the front clamp that had in fact slipped. That you released the rear bolt first suggests to me that you were expecting the rear bolt to have a problem when it was [evidently] the front clamp that had in fact slipped.

Trust I am not missing anything here..

JET951

Hi Fred
It's not really, My comment is based on the correct viscosity of oil, in the case above you can see that its 20w50 for our climate in Australia as represented in the owners manual. The addition of my comments about ZDDP is that its needed upon more components in the 928 engine such as the camshafts which as you know have been overly represented in wearing out/pitting when on the incorrect oil. I guess thankfully for us for staying with the correct oil all these years none of our customers, which have always been on the correct viscosity oil with the correct amount of ZDDP , have had to replace any of their camshafts or lifters.

I think I would not quite agree with you in regards to the thrust bearing not being designed to support superimposed loads and by this I mean the entire function of both the thrust bearing, the oil and the pressure. The reason I say this is that on manual cars we have a clutch which is operated in either a push or pull force upon the pressure plate (this force is then applied to the flywheel which is attached to the crankshaft). This would be much more force then what the flex plate would be applying when distorted but it would also be constant rather then intermittent of the clutch force. I would hazard a guess that the application of the thrust bearing/ oil and oil pressure was taken into account when someone designed it.

I do believe it is the lube's responsibility for protecting the thrust bearing/ crankshaft as its the only compound in that area at time of operation capable of doing so, on top of that we have seen when an inferior oil is used then thrust bearing/ crank is able to be destroyed. this is why I say the correct oil must be used and I do not believe its naive to think that certain oils were designed for this function.


"If when you remove the rear bolt the cut out is misaligned that would tell me that the front flexplate was under tension and upon releasing the rear bolt that tension caused the shaft to move relative to the previously clamped rear sleeve- ergo it was the front clamp that had in fact slipped. That you released the rear bolt first suggests to me that you were expecting the rear bolt to have a problem when it was [evidently] the front clamp that had in fact slipped."

I never said I released the rear bolt first, in most cases we actually go to the front bolt as we perform a thrust bearing check first up to see the health of the thrust bearing. If I may I will even throw another spanner in the works for you. We have only a few customers that have aftermarket clamps on the front coupling, I would say 6-7 out of the few hundred 928's that we look after. Even with these clamps we have seen where the cutout on the back have moved over time. How could that be since there is now superior clamp up front. one more another thing. Even if you were to release the rear coupling first the torque tube shaft on 99% of 928's will not move because of the rubberised vulcanised material on the torque tube bearings hold the torque tube shaft in place and it takes much more force to move the shaft then just the pressure to the shaft being applied by the distorted flex plate. I say 99% because every so often we get shafts that can move freely on the bearings because they get oil on them or they wear due to age etc.
Regards
Sean

FredR

Sean,

Your first paragraph does not appear to have any relevance to the point being discussed and that is that ZDDP has no relevance when talking about the performance of the plain bearings in the crank and rods and that includes the thrust bearing. Using a recommended oil is a no brainer and we have well covered that already.

With regards to superimposed loads you appear to have missed the point that I am referring a specific load- that superimposed on to any load considered in the design brought about by slippage of either or both of the flexplates. Once the flexplates have reached their travel limit the thrust is enormous and no oil is going to withstand that kind of loading when the thrust bearing is converted to a disc brake. The engine oil can and does protect the thrust bearing when exposed to the loads it was designed to take and that loading is not very much indeed which is why thrust bearings hardly wear when loaded normally.

With regard to removing the rear bolt you stated without any conditionality that when you remove them they can kick out of alignment. As advised earlier when I remove my rear bolt [with the front bolt still torqued] I have never seen any movement that needed to be reset and I would not expect to. I do agree with you in that if there was any kind of light loading the shaft stiction in the housing would resist that but if the compressive force was of the kind that causes serious damage I am pretty sure it would displace the job lot.

The point I have raised that has not as yet been addresed is whether you chaps [or anyone else for that matter] have ever seen a 928 automatic experience TBF as a result of slippage at the rear coupling- I have never seen this locally or on Rennlist and I believe it is mechanically impossible for such to happen even if the rear coupling does slip a bit. For this to happen you would have to lose a seriously large chunk of the bolt or the shaft.

With respect to examples that have been suitably reinforced at the front flexplate I do not see any relevance to your comment but maybe i am missing something? What i have said is that once the front flexplate has been secured no one has ever reported TBF- not once and that is all one really needs to know.

I can now understand why your management approach seemingly works and that is because wittingly or unwittingly you are stressing the bolts at or close to yield- maybe that is all it ever needed in the first place ironic as it may seem.

JET951

......

FredR

Hi Sean,

Something here does not quite add up. When talking about TBF I am talking about the phenomena that destroy's the engine on 928 automatic transaxle units. It starts with slippage of the front clamp, this in turn pushes the flexplates to their spring limit which I suspect is little more than a couple of mm., Once the flexplate is at its limit the shaft still pulls out and then puts immense pressure on the thrust bearing that acts more like a disc brake. The shaft seems to pull out of the clamp by about 10mm or so. The pressure eventually causes the thrust bearing to spin in the crankcase, the white metal melts and the crankcase webbing cracks - the owner generally does not have a clue as to what is going on until one morning he tries to start the egnine and finds it is seized solid after the white metal has cooled and set - game, set and match for the engine unless one has access to very talented [read very expensive] repair skills that are usually not practical. The white metal melts at or around 300C and at that those temperatures lube oil has no chance of performing any meaningful lubrication given its performance falls off a cliff once the temperature exceeds about 150C.

In the case of the GTS engine I pulled apart the rear most thrust bearing flange was about half gone, the rear most crankshaft thrust face was ground down about 0.5mm, the crank end float was 1.3mm and the crankcase was the new thrust bearing. The engine was full of swarf, and presumably the entire lube system would have been full of crap. In the heads the tappets would be toast because of the crud and I suspect the rods and piston coating will have fared no better- this is what we are talking about.

​​​​​​​I have no idea if you have seen such but for sure it is not pretty..

JET951

We under stand what you're saying Fred, and I do agree that the pressure pushing forward does add to the possibility of wear on the TB but we still put this as a secondary item and I will explain why.

Over so many years we have seen a lot of 928's in the shop, and we have seen them in all sorts of conditions. What im showing you with the pictures below was a 928 GTS that we saw for the first time only in the last 2 years, before that we have never seen or worked on it. It does have a full service record.
This is probably one of the more extreme examples we have seen and you can see by the pictures that the flex plate had a lot of force applied to it over a long period of time, in fact the flex plate is cracked shows of how far it has been distorted. But after checking the thrust bearing clearance it was still within factory installation spec of 0.18.

This is not the only example we have seen like this, but one thing they all have in common it that they were run on correct viscosity oil which were of high film strength. Not a 5w40 oil etc.

Now if it were completely true what you have stated above, and we have thought about this before as you have, then all of these examples that we have seen that have the flex plate pushed so far out should in principal have thrust bearings that are worn or destroyed.
Regards
Sean

FredR

Hi Sean,

An excellent photo that demonstrates exactly what I have been communicating and I will add it to my limited collection.

So what do we infer from that photo and your post?- well quite a bit actually. Your conclusions, although understandable are [in my opinion] quite possibly flawed and I will explain that observation later on.

My first observation is that given the pinch bolt head is 16mm in diameter the exposed length of splines would appear to be 8mm. For an example in perfect condition that dimension is typically about 4mm or 5mm as per the two examples I pulled from my files- one is on a 28mm shaft [typical S4/GTS spec] the other on the 25mm shaft [typically post MY85 up to early S4's] as shown below.
.


MY85+

S4/GTS

Both of the above by comparison to the bolt head are set at 4mm and 5mm respectively as best I can infer.

Based on the photo you submitted the splines exposed amount to 8mm thus the shaft extension will be in the region of 3mm to 4mm. To most listers those numbers probably do not mean anything but to someone like myself who has studied the problem that degree of extension although it has clearly trashed the flexplate, is just not enough to cause TBF assuming my assessment of the motor I pulled apart is correct wherein the flexplate on the failed motor appeared to intimate something close to 10mm of shaft pullout. I saw something similar on two other TBF'd motors over here. Unfortunately I do not have any photos of the motor as we inspected it prior to being pulled apart but when I saw your photo my first impression was that the flexplate did not look as mangled as the flexplate I saw. If anyone reading this thread has a photo of the flexplate installed in a TBF'd motor please feel free to post such. Thus your perception that using 20w50 lube prevented TBF is flawed.

Now take a look at your photo once more- you can see clearly that the flexplate has started to bind on the flywheel rim and on at least one of the spars it has cracked- in part this to some extent would have lightened the load on the thrust bearing because that member is no longer solid- my "guess" is that the owner by then might have audibly realised something was amiss and came running to your shop to asses what the problem was given he was probably clueless about TBF- is that reasonably close to what happened? A question of more interest might be why did he come to you when he had a problem and not before?- did the main agents send him in your direction by any chance?

My conclusions-
1. This was the front flexplate not the rear so this establishes [not too surprisingly] that this problem on the front flex plate can and does occur in Australia albeit not too often due to your diligence.
2. As per my previous post there is no way any engine lube oil can or should be be expected to protect aginst this kind of event- it is not a design contingency it is equipment failure. For sure a lube oil with greater bearing capacity will stand attrition longer.
3. There will be a root cause for this issue that [as I am concerned] has yet to be fully identified but I suspect that I am now close to converging on such.
4. Suffice it to say that if we can prevent this kind of event happening TBF will not occur. This is what I achieved 23 years ago with the Loctite approach, I understand that auxiliary clamps like the PKLamp example in the attached photos work and as I stated earlier my dear friend Constantine has never come across a failure on any example kitted out with his superb clamp- a design that Porsche should have deployed after they somewhat amazingly dropped their original 25mm shaft design with the nose to fit shims and a locking washer and thus prevent this sorry sequence of events in the first place. That Porsche kitted the original shaft with such a design at the front flexplate but not at the rear flexplate in and of itself tells its own story.

With respect to the film strength of lube oil deployed in our engines I believe there is also a story to tell in that regard. The notion that lube oils with higher visosity have higher film strength was perfectly true 40 odd years ago but not today. In those days the additives did not contribute much if anything to film strength but today film strength is entirely down to additive technology. You are not old enough to remember but Bruce invariably will- in the 70's just about every engine oil used in water cooled motor vehicles was a 20W50 dino oil with brands like Castrol GTX and Duckhams leading the charge in the UK.. Such oils typically had a film strength of 70k psi . Synthetic lubes then became vogue and whereas they showed a small improvement in film strength their raison d'etre was more to do with consistent molecular chain lengths that facilitated longer life expectancy. The real dramatic change came with what I refer to as the modern era of "Witch **** lubes"..which as I recall took off big time in the late 80's.with the advent of modern additive chemistry. Today cutting edge lubes are hitting film strengths in the region of 140k psi with viscosities like 5w30. As I am aware the 20W50 dino lubes do not see anywhere near this kind of load bearing capacity- best I have come cross to date is Castrol GTX 20W50 that hits about 95k psi.

The simple test procedures indicate what pressure it takes to break the oil film and thus inherently considers the impact that any ZDDP content has on the test specimen in terms of increasing its resistance to surface scuffing. The problem is that today engine designers do not consider 20w50 to be a commercially viable oil given today's engines run nicely on witch **** lubes - thus 20w50 is now a marginal market and profit margins do not exist for high end lubes thus what we get in those viscosities is pretty much the same as it always was.

My main concern with running modern lubes in our motors is what such would do to oil distribution within the motor. My "best guess" is that if we could fit a lube oil pump with greater capacity we could run such modern lubes no problem. We might even be able to run them "as is" but I would not recommend that unless and until I had engineering based proof of concept. Thus if cam wear was seen running witch **** lubes and especially first generation varieties of such, was such caused by reduced ZDDP, reduced film strength or reduced oil flow to the cams? Oil companies did not allow these lubes on to the market without knowing they could equal or exceed traditionally used film strength values

JET951

Your assumptions are incorrect unfortunately, If you can see on the picture above that I posted it has a black permanent marker line drawn drawn on the splines, we were told by the previous owner of the workshop it previously attended (now shut down, hence why I think the owner now came to us) that he marked those splines 3 major services ago or found about 60,000kms. At the time of us checking this the rear bolt had ridden up on the rear splines and damaged them in what I would say would have been 4mm on top of the spline so my guess without measuring would probably be 8mm in total movement at the rear bolt.

Im not saying that any of the aftermarket front clamps don't do anything, what im saying is that no-one is addressing the rear clamp that moves and we have seen it as im sure others probably have as well.

Unbelievably there was no unusual noise from this, It was simply just a new customer and we always check this first up.

To say that this is not enough tension on the thrust bearing to cause wear or destruction a bit unbelievable in my eyes. This its probably the worst one we have seen. Even though one of the fingers, or spars as I think you call it has cracked, the flex plate is now forced directly up to the flywheel. The contact is still there and it's still the same amount of force being applied continuously.

FredR

Sean,

Appreciate if you would be kind enough to list the assumptions you "perceive" I have made so that I can prepare an appropriate response to your interesting post.

From my perspective i have made one minor assumption and I reported that in my pervious post.

Constantine

Hi Bruce,

Interesting thread and appreciate your information. Seems the 928 drive lines in both automatic and 5-speed versions have some issues which can be difficult to understand completely.

From the work I do and have done in this area, I can't completely dismiss the front flex plate clamp pullout of the drive shaft as the main cause of TBF incidents in automatic equipped 928s.

Fred's findings, as far as drive shaft movement at the front clamp, is the same as I have found in my study and work in this area. I also confirmed the driveshaft did not move forward or "grow" in length as was surmised by others when the 928 engine TBF phenomenon was first discussed over 20 years ago on Rennlist. At least in the examples I reviewed and from my conversations with many others over the years.

For those 928 auto owners following along, a simple way to figure out what is causing the front flex plate pressure in their 928 auto would be to put two marks, one at the union of the front clamp and drive shaft and another on the drive shaft indexed by a straight edge just before it goes into the front of the TT. These marks should be done after loosening and tightening the pinch bolts front and rear, and after confirming there is no forward pressure on the front flex plate.

If forward pressure presents itself afterwards on the front flex plate, and before loosening the front or rear pinch bolts, check to see if there is a gap between the front clamp and drive shaft mark which would mean the clamp walked itself forward on the drive shaft. The second mark can then be checked using the same straight edge to see if it moved forward or is the same. If the same, that would mean the drive shaft hasn't moved.

If the two marks show different results, then it will give the owner and their tech further information and possible problems to inspect, like the rear pinch bolt problem as you have found.

I also advise clients to replace the drive line pinch bolts during a TT R&R in all the Porsche models we service. We always recommend using Porsche sourced pinch bolts to assure the correct strength of the bolts since aftermarket ones might not hold the needed torque value over time.

Cheers.

JET951

Hi Fred,
Your assumptions as I read them from post 378:

Paragraph 5, You are assuming that, due to your studies of the problem, that the force upon the flexplate shown above in my post is just not enough to cause TBF.

To me this should be well enough as it's even broken the flexplate and is even continuing to apply even more force as it's bottoming out on the flywheel. In my opinion/ assumption would be if the the pressure on the flexplate is enough to wear/ Destroy the TB then it surely would have in this case. What I do say though is that I do agree that it does add to the possibility of wear due to the constant pressure.

Paragraph 6: That because the flexplate had bonded to the flywheel the load would be lightened on the thrust bearing

I would completely disagree, the force is still present as its resting right up against the flywheel. In fact I would go even further as say you can clearly see the distortion in the flex plate between the central hub and where it is hard up against the flywheel. As the spring in essence is now shorter it also takes more force to distort like it has.

Paragraph 7: your assuming that this was due to the front flex plate slipping.

Even though I did say in my post 38, that this was marked by the previous mechanic at around 60,000kms ago with black permanent marker that you can see clearly on the splines. Showing what appears to be in my opinion as no slippage has occurred at the front for the past 60,000kms. But the rear bolt rad ridden up the rear splines.

Paragraph 7: You assume that there is no way that engine lube can and would be expected to protect against this kind of event.

I would counter this by saying a much heavier force is present upon the thrust bearing in manual transmission cars when you activate the clutch. Not only that but also we have only experienced TBF in 928's that have been running significantly lower weighted oil. Again this is my assumption.

Edit to add one more, Last paragraph: You assume that oil companies do not allow these lubes on the market without knowing they could equal or exceed traditionally used film strength values.

There has been many cases where a new oil has been released to market that has significantly underperformed on the previous generation of lubricant. This goes for fuels as well. Whether it be oil manifacturered trying to meet strict new emission targets or just plain simply not knowing the importance of ingredients such as some manufacturers did with reducing ZDDP to ridiculous amounts or substituting it with Boron. Now the said oil companies don't pay for the damage to your engine and barely loose any reputation due to them being so big and aggressive with any negative feedback. Think of all our customers engines that we have inadvertently kept safe from wear due to the use of the correct oil, and yet other independent specialists around us having trouble for so long with both water cooled and air cooled cars with wear to camshaft lobes, excessive valve guide wear, rocker, thrust bearing etc

Regards
Sean

FredR

Quote: Your assumptions are incorrect unfortunately,

Response- many thanks for clarifying your position- I will respond in detail via a separate post.and demonstrate how my perceived assumptions are not assumptions at all with one partial exception. With regard to the photo you submitted I could not make sense of what I saw which is why I stated that the shaft had only pulled out 4mm or whatever and that such could not possibly cause TBF. I think the increment of rear slippage on top of this may explain the state of the flexplate but I am not at all surprised that the thrust bearing is not at all worn yet alone trashed for reasons I will explain later.

Quote: what im saying is that no-one is addressing the rear clamp that moves and we have seen it as im sure others probably have as well.
Response: I would say that you have addressed this issue by lubricating the bolt and then torquing to spec. We [on this list] have addressed the problem to some extent by fitting new bolts and torquing to 66 ft bs in the dry condition. Not as effective as what you have been doing perhaps but I have had no probems to date.

Quote: Unbelievably there was no unusual noise from this,
Response: That is surprising given there is a total failure of one spar.

Quote: To say that this is not enough tension on the thrust bearing to cause wear or destruction a bit unbelievable in my eyes. This its probably the worst one we have seen.
Response: If that is the worst one you have seen then chances are you are not going to see TBF- ever [thank goodness]! Considering that you do what you do and as per Bruce he used to do the same in the main dealership, if other main dealer franchises also did the same then TBF is not going to be anywhere near as endemic as in other parts of the world and especially so considering the ones that are destined to fail have probably done so aleady.

There are two things you need to consider viz -
1. Considering the thrust bearing area and the film strength of the oil, theory says it will take some 70 tonnes of axial thrust to break down the oil film. Considering that under normal circumstances the axial thrust load is going to be no more than say 50kgs tops it should be no surprise that what you saw did not fail or even wear. Of course in practise the failure loading, although highish, will in fact be nothing like this number as the oil in the thrust bearing face will over heat, the film strength will collapse above 150C and everything goes **** up after that.
2. What you saw on that GTS is not the final stage before failure but was likely the stage before. As you can see the fractured flexplate spar is now sitting on the rim of the flywheel but it is still in a position to flex albeit at a slightly higher spring rate. As the shaft pullout continues to expand at some point the flexplate will collapse even more into the flywheel dish until such time as it goes solid against the crankshaft hub or the bolts that fix the flywheel to the crankshaft. Once this has happened any further pullout increments will cause the axial thrust to go ballistic and then it is game, set and match for that motor.

JET951

Hi Constantine,
I will say off the bat that your product is a great addition to the front clamp and does reduce the amount that the front can slip. But I respectfully disagree with your and Fred's findings on the rear. Ever since our discussions in the thread started by Cosmo Kramer My interesting Pklamp experience in 2010 https://rennlist.com/forums/928-foru...erience-5.html we have been doing exactly that test with marking the front coupling along with the shaft in alignment with the bell housing. Even though we are a small shop here in Sydney we look after an incredible large portion of the 928 community cars as we are one of the 928 specialists in Australia, In the past 12 years since that thread we have checked over 250 928's, some multiple times due to some customers may be daily drivers and some less as they are collector cars. We get to see a very large sample pool first hand

What we have seen during this time is movement of at the rear clamp as well as the front, We even have some of the Pklamps, the Ritech clamp and your Super clamps on customer cars and it certainly looks to be keeping the front clamp secured to the shaft. (we have seen small amounts of slip with some of the cars). But some have zero movement and yet the flex plate has been pushed in. How would it be possible to have any deflection of the front flex plate is the front clamp is working absolutely perfectly with an additional aftermarket clamp? the rear has to slip.

What we are seeing is the marks on the shaft no longer align with the bell housing and represent a migration of the shaft in a forward direction.

I will add that we see this mainly on customer cars where the customer is a more aggressive driver and likes to give their car a good rev every now and again compared with some of our customers that may just drive the car with little acceleration events and below 3000rpm.

Regards
Bruce

FredR

Sean,

As promised my version of events:
Quote: Paragraph 5, You are assuming that, due to your studies of the problem, that the force upon the flexplate shown above in my post is just not enough to cause TBF.
To me this [the deflection?] should be well enough [to cause TBF?] as it's even broken the flexplate and is even continuing to apply even more force as it's bottoming out on the flywheel.

Response 1: When the engine in question has not suffered TBF and as per your advice has not even shown any signs of wear this can only be an observation of fact. The flexplate is designed to transmit the engine power and has a lot of excess capacity in that respect. The ability of the flexplate to absorb axial displacement is very limited - it can easily cope with the shaft length fluctuations needed which I suspect are very small as in 1mm or so.

For purposes of discussion assuming the relationship between axial displacement and reactive load is a cubic function, if 50kgF displaces 1mm then to displace 10mm the load would be 5 tonnes assuming the flexplate is functioning normally- now try to visualise what happens when the flexplate is no longer flexing and is locked solid against the flywheel- the load reaction will increase rapidly and disproportionately.



Quote: Paragraph 6: That because the flexplate had bonded to the flywheel the load would be lightened on the thrust bearing
I would completely disagree, the force is still present as its resting right up against the flywheel. In fact I would go even further as say you can clearly see the distortion in the flex plate between the central hub and where it is hard up against the flywheel. As the spring in essence is now shorter it also takes more force to distort like it has.

Response 2: I stated that when the spar cracked the superimposed load on the crank would have dropped- simple physics. My thinking was that the spar cracked whilst the flexplate was working overloaded axially and thus the load on the thrust bearing had to be lightened at the point of fracture. It seems that you are assuming that the flexplate cracked whilst the spring leaves were sat on the edge of the flywheel and maybe that was the case. Once the flexplate decks down/jams on that ledge of the flywheel the spring rate will increase but note there will still be a spring reaction.

Bottom line- the above makes no difference to the slippery slope the engine is on.



Quote: Paragraph 7: your assuming that this [the damage?] was due to the front flex plate slipping.
Response 3: No assumptions were made here- just statements of factual observation. I concluded there was a slippage of about 4mm or 5mm on the front clamp and you concurred with that. Fact- there is no way 4mm or 5mm of clamp slippage would cause TBF and I stated so. Indeed I do not think such slippage on its own would cause damage to the flexplate.

In your follow up post you then stated belatedly that there had also been an increment of movement at the rear clamp - that info makes sense regarding what is seen – that the rear clamp had moved circa 4mm on top of the front clamp slippage of a similar amount simply completes the picture.



Quote: Even though I did say in my post 38, that this was marked by the previous mechanic at around 60,000kms ago with black permanent marker that you can see clearly on the splines. Showing what appears to be in my opinion as no slippage has occurred at the front for the past 60,000kms. But the rear bolt rad ridden up the rear splines.

Response 4: Your syntax in the original post rendered the comment impossible to comprehend so I could not read anything into it. First hand experience locally and on this list tells me that major engine damage around the thrust bearing always involves slippage of the front flexplate, it is quite conceivable there could be an increment of rear clamp slippage as well but I have never seen TBF caused exclusively by rear clamp slippage and so it seems, neither have you chaps.

Not related but perhaps you can advise what total mileage your example had covered when you saw it.



Quote: Paragraph 7: You assume that there is no way that engine lube can and would be expected to protect against this kind of event.
I would counter this by saying a much heavier force is present upon the thrust bearing in manual transmission cars when you activate the clutch. Not only that but also we have only experienced TBF in 928's that have been running significantly lower weighted oil. Again this is my assumption.

Response 5: My position is based on engineering analysis- no assumptions that I can see. There is nothing wrong with what you say about normal thrust loads- the actual thrust load on an automatic transaxle unit is going to be considerably less than the thrust load generated by pushing the clutch in on a manual tranny or so I would think. The axial thrust load on the auto models should never vaguely trouble the thrust bearing and nor should it on manual models however let’s put the manual tranny version to one side as it is irrelevant to our dialogue.

I would think the axial thrust loads will be ridiculously small as in something like 20kg or so for the auto box if my estimation of the flexplate spring rate is correct - whatever the number it is so small it is irrelevant compared to the load capacity of the thrust bearing. Simple theory suggests it would need 74 tonnes of axial force before the oil film breaks down but I rather suspect the crank would be pushed out of the crankcase long before that level of force could be generated! As I illustrated in response No1 above we could easily be talking about 5 tonnes F maybe more. Either way I figured that excessive axial thrust leads to a greatly reduced oil flow from the rear face of the centre main bearing, the system overheats locally, the oil breaks down, the film strength collapses, the bearing is wiped followed by melting of the white metal and ultimately the thrust bearing spinning in its seat and cracking the crankcase webbing- game over for that engine.

My best guess is that by the time axial displacement of the kind seen in your photo [8mm] takes place the spring rate will be going through the roof hence the flexplate mechanical damage that is seen.

My take is that the axial displacement indicated on your photo did not reach its final [devastating] stage but that was pure luck. You state that because the flexplate is now in contact with the flywheel [slightly inboard of the flexplate mounting bolts pcd] that it is now in hard contact but as I am concerned that is not the case and it can still “flex”. The killer blow comes when the front of the flexplate boss comes into contact with the crank hub or the crank mounting bolts – that as I could tell is when a firm and final hard contact is made and that is when it is most likely game over.



Quote: Edit to add one more, Last paragraph: You assume that oil companies do not allow these lubes on the market without knowing they could equal or exceed traditionally used film strength values.
There has been many cases where a new oil has been released to market that has significantly underperformed on the previous generation of lubricant. This goes for fuels as well. Whether it be oil manifacturered trying to meet strict new emission targets or just plain simply not knowing the importance of ingredients such as some manufacturers did with reducing ZDDP to ridiculous amounts or substituting it with Boron. Now the said oil companies don't pay for the damage to your engine and barely loose any reputation due to them being so big and aggressive with any negative feedback. Think of all our customers engines that we have inadvertently kept safe from wear due to the use of the correct oil, and yet other independent specialists around us having trouble for so long with both water cooled and air cooled cars with wear to camshaft lobes, excessive valve guide wear, rocker, thrust bearing etc

Response 6: The oil companies do not set the performance standards - that is done by NGO’s like the SAE – the oil companies comply with their edicts whether by fair means or foul. Nothing assumed here.

Rest assured I know exactly how big oil companies work having over 40 years experience working with four of the biggest.

Are all oils the same? – far from it as I can tell. Ideally one needs to know the film strength of the oil one intends to use- that or stick with one you know works for you.

I have little interest in discussions about ZDDP- it has been used since WW2 and is decidedly old tech. I believe there is a lot of technical ignorance about the stuff and what it does or more to the point does not do [it has zero impact on film strength]- for our 928’s try to get a quality 20w50 with a reasonable amount of the stuff in it, a film strength of circa 100k psi [or better if such exists] and be happy with that.

Nowadays film strength is dictated by the quality of the additive package not viscosity. Modern engine designs revolve around 5w30 grade oils or so it seems to me and they thus get more focus as companies battle for share of what presumably is the biggest market segment these days.

Constantine

Hi Bruce,

Thank you for the kind words on the Super Clamp.

Your customers are indeed fortunate to have you and our son look after their 928s and to manage such loading in their drive lines to stop TBF from happening to them. I have talked to my share of shops who bill themselves as Porsche specialists and they were not at all knowleagable about the 928 platform, and about this very issue which can cause TBF.

As for your findings further pointing to the rear clamp as the main issue when you see front loading of the front flex plate, even after other clamps were installed, that is very perplexing. Clients I have worked with in the past have said their flex plate loading was stopped with our Super Clamp and after years of use. Same results have also been found by owners by using other devices, as Fred can attest to by using loctite to stop such migration in his 928 for many years now.

One thing that hasn't been discussed is the number of bearing units used in 928 auto TTs and how three would be much better at controlling the drive shaft deflecton under load. When we rebuild 928 TTs, no matter the year or transmisison type, they all leave with three 928 Super Bearings in them. I found that in 1987 Porsche stopped using three bearing units in their 928 auto TTs, as they had done in the 1985 through 1986.5 auto TTs after they increased the S3 engine output, and just went to using two. Since the S4 engine output was increased even further, the drive shaft was under more load. Then sometime in 1988 Porsche changed to the 28mm drive shafts in the 928 auto TTs and still only used two bearing units. With the research I have done in this area when developing our product line, it seemed to be an engineering blunder to leave such a long length of the drive shaft unsupported in the 928 auto drive line.

Could the use of more bearing units help in reducing the drive shaft migration by managing the deflection and thereby reducing the loads seen at the front and rear clamps?

The take away from this thread, especially by 928 automatic owners, is to check and manage such loading in their drivelines so TBF does not occur during their ownership.

Best regards,
Constantine