tda1

Yesterday I did a flex plate inspection. There looked to be about 1-2mm of bow. Now my question is this. do you loosen the allen bolt holding the drive shaft to alleviate the stress on the plate? Why do the write ups have you loosen the plate and observe the movement? Is here preload or is this relieving tension?

I am looking at having my torque tube rebuilt by Constantine. Once that is done I plan on doing a VCB supercharger (stage1) any thoughts on whether a 25mm or 28mm shaft would be better? I currently have a 25mm. I'm concerned about sheering of course...thanks

Avar928

Loosen the allen bolt to relieve tension. I believe some write ups say mark the splines and observe movement to record as a measure for how much load was there and as a reference for the future to make sure it doesn't move at all or not as much.

Mrmerlin

Bill Ball has a thread

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

NOTE make sure that the rear pinch bolt is retorqued before tightening the front pinch bolt.

You will verify the looseness when you go to loosen the bolt it usually spins free with a lot less force.

Remove the rear bolt then add a drop of blue loctite to the threads then torque to 66 ft/lbs

UpFixenDerPorsche

Flex plate problems come from movement of the collar on THE REAR SPLINES

Drove myself batty trying to understand how the front collar could move on the shaft (it doesn't) until I looked at he other end and saw the rear collar pushed back onto the gearbox shaft splines with slightly bent pinch bolt.

New pinch bolt + Loctite and .

I now check that end when I change the oil.

HTH

JET951

That's exactly correct , its the T Tube rear coupling inhex bolt that stretches the most & the T Tube shaft migrates forward , that's why you will find the front coupling inhex bolt is always much better torqued up as compared to the rear coupling inhex bolt ( providing they were both torqued up at the same time previously & yes the rear coupling inhex bolt ( in Auto trans version ) is harder to access( heat shields etc etc ) , hence why its seldom checked

Remember , much less force is required to "flex" the engine flex plate than what you would expect to be

FredR

Whereas both ends have to be installed correctly I can only conclude from your comments in upside down land that things work back to front down there [pardon my well intended humour].

The modifications to correct this problem all relate to the front flex plate connection- guess why?

Needless to say if the rear one is not done correctly it will slip but I am pretty sure that most of us on this list who have been dealing with this problem for a long time will tell you it is the front connection that let's go. Dealt with this in 1999 with loctite 260 courtesy of Earl- no problems since [touch wood]

regards

Fred

Black Sea RD

From all the research we've done on thrust bearing failure (TBF) of 928 engines, we also agree with Fred R.'s comments.

Although both front and rear connections should be checked to ensure they are correctly secured, the real problem stems from drive shaft pullout at the front flex plate coupler. This is due to a front clamping system that Porsche had initially designed with more parts to secure the drive shaft within a very certain range in the front coupler. Later Porsche stopped using the other parts and only used the one pinch bolt to secure the drive shaft in the front flex plate coupler. Many have found this is not enough to secure the drive shaft in place.

Going back to the OP's other question, he is asking about using a 25mm or 28mm drive shaft for his TT rebuild since he plans on super charging his 928. We told him that 25mm drive shafts don't shear as often as the 28mm drive shafts and the 25mm drive shafts would be the better choice. This was before he knew what drive shaft he had in his 928.

We also discussed that he should query this Forum for other's opinion since there are owners who have a 28mm drive shaft in their super charged 928s with no problems. Since he has a 25mm drive shaft we believe he is good to go with the caveat that we have even seen 25mm drive shaft shear since they are all getting old.

That's why we have stood up our new 928 automatic 25mm drive shafts from a stronger material so owners could have an option when rebuilding their TTs.

All this was discussed with the OP and your comments to him would be helpful.

Cheers,

JET951

Hi Fred , have you looked at your'e rear coupling ,because if you have not then you may be in for a suprise .

The rear T Tube coupling ( with its one inhex bolt 931 421 240 00) is on average twice as likely to loosen off its torque value ( bolt stretch ), this in turn causes wear of the male splined shaft inside the female splined shaft ( torque convertor drive shaft ) , this shows up ( when looking up into the tiny round inspection hole rear of T Tube ) as red ( rust ) powder , once you see this you have wear of the splines ( male & female ) = not good at all

Now the worst bit is once this occurs , the female splined shaft ( this is the one with the three cuts in it ) will crack / break if left too long in this mode , meaning not torqued up = movement of splines , but in the mean time it wears away at the male splines of the TT shaft

The other thing is before we get to this dramatic stage the T Tube shaft migrates forward bit by bit & this is where we see the front flex plate deflection( engine end flex plate ) , whats even more interesting is that with the heavy defection cases , one will see teeth marks on the side of the rear T Tube coupling inhex bolt flank ( non threaded side of bolt 931 421 240 00), these teeth marks is where the flank of the inhex bolt has been dragged up & into the male splines of the T tube shaft , we have a selection of these heavily damaged bolts at work , if I get a chance tomorrow I will post up a picture of one

All one has to do is just check the rear TT coupling on a regular basis , after first replacing the bolts with new ones & NO locktite on threads , just a smear of grease & torque up the front & rear coupling bolts

PS } we have been checking the rear coupling inhex bolt of the 928S 4 speed auto > through to the 1995 GTS every major service , we have been doing this for over twenty years ( 20 + years ), then with the manual trans 928 ( ALL Year types ) the same applies , every major service check & Torque & if in doubt , replace bolts , check alignment ( migration of male shaft & reset if needed )& we have completely eradicated the problem by this simple checking / tightening etc on a regular basis , even on 928S4's with well over 400,000 Kms , no flex plate defections , nothing because the problem is just not there any more , we only see issues with these cars if they have not been checked on a regular basis & because so few people know , we end up with a **** load of work that was avoidable & yes we work on a lot of 928 Porsche's ( own one my self + other Porsche's )

Subnote 2 } If you apply Loctite to these bolts , all that happens is the next time you check them ( tightness )they will appear tight because of the Loctite , but in fact the inhex bolts have stretched ( rear mainly ) or put another way the rear twice as much stretch as the front coupling inhex bolt, just why this is the case is not really my worry , all I know is its completely controllable , but it must be maintained because of a built in weakness ( yes the female hollow shaft with the three large machined slits)

The next common thing with these ( 928S 4 speed auto > 1986 & the 928S4 GTS 4 speed auto & ALL Manual trans versions ) if the final breaking of the female rear coupling , yes the one that was weakened by Porsche by putting three large machined slits in it , but this final insult only happens with loosness in this area which in turn allows movement & breakage , its completely avoidable if checked ( tighten the single coupling inhex bolt ) on a regular basis ( not hard ) & saves a **** load of money, not for me but the customer / owner

Its been that bad with this problem over the decades with new to us customers with 928 4 speed auto trans versions & All Manual Trans versions that have not been checked / maintained in this area that we keep in stock new 960 116 042 02 1987 to 1995 4 speed auto, torque convertor drive shaft in stock ( its a hollow shaft with the three machined slits & the rear flex plate as one assembly )

Whats interesting is that }

A ) The 928 3 speed auto ( 1978 - 1983 ) had a male ( solid ) drive shaft that drove the torque convertor , meaning there were two solid shafts being connected by a ( female ) sliding / clamping sleeve ( 922 421 051 02 ) this is the same clamping sleeve that is on the 944 / 944S / 944S2 / 951 manual trans input shaft ( solid ) to solid T Tube shaft & naturally this clamping sleeve has two inhex bolts ( 931 421 240 00 ) , yes that is 2 bolts not one in this area , this is the reason we see no movement in this area on the 3 speed auto trans version or the 944 series for that matter , its an excellent design , the same can not be said for the hollow / split version used on all 928 manual trans versions & all 928 4 speed aut trans versions , it can be managed but its a weak point

Interestingly , Porsche up graded strengthened this clamping sleeve design used on the 944 series & 3 speed 928 auto trans even more with the introduction of the Porsche 968 6 speed manual , for it recieved a 4 bolt clamping sleeve ( connection between T tube shaft & Trans input shaft ), this is the perfect design , extremely strong & never moves

Now Fred how do I know the above I hear you ask ?

Answer = I have been working on Porsche cars since 1977 = 37 + years & have owned my own independent Porsche cars repairs & service workshop since early 1994

PS } last but not least , 928 / 944 engine thrust bearing wear / destruction , this gives me a laugh from time to time over the decades , I looked on with horror when humams starting using low oil film strength engine oils ( Low Viscosity so called synthetic engine oils )in last century Porsches , yes 928 ( all ) , 944 / 968 ( all ) are last century Porsche's & wow did thrust bearing wear take off in the last 20 or so years , but with high oil film strength engine oils we see no thrust bearing wear at all , even when these poor cars come in to our workshop for the first time & we check the engine flex plate & its pushed in hard , but the crank axial play ( thrust bearing play ) is within new spec , meaning not even close to max permissable wear limit , but this is only when these poor cars were on either a decent 15w-50 or better still a good 20w-50 , however the same 928S4 with a 5w-40 low oil film strength oil , the thrust bearings are toast

Black Sea RD

Hi Bruce/Sean,

From the many torque tubes we have rebuilt over the years, most of the rear spline damage we have seen is of 5-speed drive shafts when the coupler you describe above, that is also used in 944 5-speeds, was not secured to correct torque values and allowed the drive shaft to slap around in the coupler and beat the splines to a knife edge. We have not seen this type of damage with automatic drive shafts.

Most of the damage we have seen with automatic drive shafts is when the rear pinch bolt was shoved through the rear connector between the splines and drive shaft and not the grooved area of the drive shaft.

We have also released tension at the front flex plate and have witnessed the front flex plate move back onto the drive shaft. We then checked the rear coupler and did not find any movement/problems with the drive shaft or pinch bolt. This has also been witnessed by many other owners/mechanics over many years.

One other thing that could play into abnormal rear drive shaft wear is when drive shaft pullout occurs on 928 automatics, the whole drive line is put under tension which forces the drive shaft rearward and can cause some of the damage you notice at the rear coupler, to include the premature wearing of the torque converter bearings and the engine's thrust bearing.

As far as TBF, it is interesting to note Porsche put a maintenance bulletin out on TBF saying that TBF was due to improper maintenance procedures, nothing about oil film strength. This is quite telling since this means 928s equipped with automatics were suffering TBF even back then.

Cheers,

tda1

I apologize for my ignorance and confusion but I'm getting further lost here. I understand the concept of the front and rear of the drive shaft getting out of wack (migrating forward or rear). What I don't see in any of the writeups/pictorials is anything that shows the rear being adjusted. I'm not sure what to look for. Everything I see seems to show the front, the flexplate, and the flywheel. Where does the rear drive shaft connect? What will I need to remove to access that? do I need to do anything with it?
I feel the need to make sure the torque tube is the real issues before I invest 3K in something that isn't my real issue.
Am I totally missing something here? apologies...I'm a newb

tda1

this is my drive shaft...25mm.

FredR

Bruce,

Thanks for sharing your experiences -always appreciated. As I stated in my note the rear connections have to be maintained correctly. What you did not say was how many 32V motors you have seen destroyed by TBF and why they failed. Over here I have seen 7 motors destroyed by TBF and whereas that is not a lot in the overall scheme of things it represents about 15% of the fleet and that is what I know of, there could be more that have gone that way that I do not know of as some examples left the country and I do not necessarily see every 928 that is in distress albeit I know most of them. Heavens knows how many I have read about on this list but interestingly I only seem to remember one failure that was put down to slippage at the rear and then I am not sure whether it was total engine failure or more limited local damage to the shaft- doubtless others will know .

One of the motors I have seen fail was the original motor in my current GTS and that failed because of slippage at the front splines. My original S4 bust the main drive shaft behind the front splines just after purchasing it with 70k km on the clock at the time. I was given all kinds of nonsense as to why it failed- non of them making any sense mechanically. I reckon it failed because the front flex plate clamp slipped setting up a vibration/misalignment leading to fatigue failure. I then replaced the torque tube with a new one and within two weeks an annoying vibration came to light at exactly 3050 rpm. I then came across this list and read about the problem with TBF- horrified me. We checked the clamp, marked the shaft with paint, released the clamp bolt and the shaft popped back 3mm. Retightened it, vibration dissappeared- two weeks later returned. Then Earl came up with the Loctite 260 solution on the splines- worked a treat- no problems since -still use it today.

I still fear a failure but work on the premise that if something slips- front or rear- I will know about it. I have never used a Loctite formula on the pinch bolts and do not recommend it.

As I am concerned TBF is an automatic model problem, it is caused by excessive axial thrust and I have a well buggered thrust bearing and gouged out crank that proves it. The thrust brearing is not designed to take a heavy axial load.

Whereas I agree with you 100% on using a 20w50 oil in our motors, I am a little puzzled by your perception that synthetics have low film strength. That contradicts most of what I have read about them which suggests they perform better at higher temperatures and keep their lubrication properties much longer because they have much more consistent moleular chain lengths than dino oil. Manufacturers recommend low viscosity synth oils because they have sufficent film strength at lower viscosities. Most of the more knowledgable on this list believe they are not suitable for our cars because they have low zinc content that results in premature wear of the cams making such not suitable for motors with flat tappets.

My favourite engine oil is Red line 20w50 but I cannot get it here these days. That has plenty of zinc in it and it is fully synthetic. I think I can safely say no dino oil has anywhere near the same film strength as this stuff but kindly correct me if you know something different.

Regards

Fred

daveo90s4

To the OP:

The rear bolt is up above the rear heatshields that themselves are above the exhaust. With the heat shields removed you'll see a rubber bung. Pull that out and you will see the rear clamp. Rotate engine (I use ratchet set in the tightening, not loosening, setting to turn the front crank bolt to line up the rear bolt head with the bung hole. With the front flex bolt already loosened, to remove any existing longitudinal forces within the central shaft, then remove the rear bolt and replace with a brand new never been used before bolt. While doing this be careful the collar does not slip off the outer tube and hide further up inside the torque tube. Fun fishing it out if it does. With the front clamp still loose, torque the rear bolt up ( of course making sure that the bolt is running in the machined groove in the shaft and that the shaft is not way too far forward). Once the rear is completely done, then you can turn your attention to the same at the front.

daveo90s4

To the OP no. 2

Before torquing the brand new never before used front clamp bolt you might want to check the wear on the crank thrust bearings. This involves gently levering the flywheel fore and aft with a pry bar and having someone with a micrometer measure how much fore and aft movement there is.

Do a 'search' on rennlist about thrust bearing failure and / or flex plates and / or torque tubes / central shafts for lots of discussion on thus topic

Cheers all

JET951

Here is a photo of the rear T tube index coupling bolt that has the (splines) damage to its flank from the splines on the T Tube shaft , as you can see , the thread is perfect , no damage from being forced through by a human who didn't get the hole aligned ( which does happen as well, thats another matter ) but not in this case , this is the classic case of migration of the T Tube shaft due to the simple fact that the Rear T T coupling had a much lower clamping force as compared to the front of TT coupling which had greater clamping force or put another way the rear TT coupling was not that tight because of bolt stretch & the front TT coupling was tight enough to hold ( hence the Flex Plate Deflection )

Because remember ( simple physics ) that if the front coupling was the problem , it would not hold the flex plate in the deflected state , it would just slip back into place

But the rear coupling when loose ( most common ) then the extra grip of the TT shaft being held " just enough" by the black nylon covered bearing" Inner" sleeve / insulators are just enough friction to hold the flex plate in the deflected state ( this is very simple stuff )

If you look at the end of the T Tube splined shaft( trans end) , you will see where there is a half moon machined cut ( entire circumference ) that the splines that emigrate out in both directions are chamfered / angled so this is where the index bolt is dragged sideways up onto this chamfered / angled point

The next thing you will notice is that at no point in time has the bolt turned , its been stationary the whole time while being mauled by the splines , the bolt tension has been reduced by stretch only over time

Thats why for decades now , this is not an issue , it is easily maintained , because we have no issues with ones that have been maintained ( problem solved twenty years ago ), that 21/22 + years ago was showing up as an issue , meaning we were seeing 928s 4 speed Auto > rear coupling force reduced as compared to the front of TT coupling due to something then was a mystery , but it took little time to figure out .

To answer Fred's question with engine oil , what I was trying to say is simple observations day after day , week after week , year after year , over decades is the simple fact that we only ever see }
A) Thrust bearing wear when the engine oil is "Low on Oil Film Strength "
B) Cam Lobe , Cam


Regards
Bruce Buchanan
Buchanan Automotive