Mrmerlin

NOTE to add on to Freds post,
the rear drive shaft cutout is not pinned by the bolt
IE held fast to the drive shaft and the input shaft .

SO,
that means the drive shaft can move in either direction inside the input shaft of the trans axle,
the shaft can move about 8 MM.

The idea with the rear pinch bolt inspection is to do 3 things.


1. Verify the bolt is not damaged from the drive shaft splines wearing into it.
The shank of the bolt can get cut from the shaft splines due to a migrating shaft.
Or that the bolt is not rusted or otherwise damaged from running in a loose condition

2. Verify the drive shaft cutout is properly centered in the input shaft,
once the shaft is centered and the REAR bolt is installed,
then the FRONT pinch bolt is tightened after prying the flywheel to the rear of the car.

3. Verify the rear pinch bolt is properly tightened to 66 ft/lbs

daveo90s4

JET951

The below post is one I put on Rennlist back in 2015

The torque tube " rear coupling" in-hex bolt that looses its tension over time , the bolt does not unwind it stretches with acceleration events over time & because the Auto trans version, the rear coupling inspection hole is well covered with heat shields ( out of sight & out of mind ) and so over time no one checks whats going on in there ( meaning is the single index bolt still tight ? )

For the last few decades we simply ( on major services ) just check & if needed reset the position of the T Tube shaft & always fit new index bolts ( F & R ) , grease the threads of these bolts ( do not loctite them ) because if you loctite them you will go back to check their tension / torque say 20,000 kms later only to find they are tight , but its the threads that are tight( because of the loctite) not the bolt head to coupling

And of course we always reset the TT shaft( centralise) if the said 928 TT shaft has migrated forward & they always migrate forward if the rear coupling has lost its tension because of stretch & with only one bolt , they stretch , where as the earlier 3 speed auto trans version they had two bolts on a sliding sleeve , this was an excellent design ( much stronger ) & if one looks at the last evolutionary step Porsche made ( the 968 ) the rear coupling had 4 index bolts ( YES 4 of them ) = perfection , this was the fix the 928 needed & would have received if the model continued

Just why the rear coupling index bolt stretches ( looses its tension ) much more than the front of TT coupling is a slight mystery , but I couldn't care less , it just does & its simple to maintain over decades as we have been doing

OH , in case you are wondering , do we see engine thrust bearing wear ?

Answer = NO

Why = because of us checking the above on a regular basis ( once a year or so ) = very simple , but more importantly we use high oil film strength engine oils & for our climate a min viscosity of 20w-50 & lucky for us most 20w-50 oils( last century viscosity for a last century engine ) are rich in ZDDP & engine /crankshaft thrust bearings get very little or no oil pressure at the thrust bearing surfaces ( between the two metal surfaces the bearing is = oil film strength & not the metal surfaces grinding against each other ) and you are going to struggle to get enough oil film strength with a 5w-40 this century engine oil unless you find a 5w-40 with more ZDDP than usual

I would only use a 5w-40 engine oil in a last century 928 or 944 or air cooled 911 engine if the ambient air temp did not rise above + 5 deg cel , meaning the average air temp is around minus 10 deg cel or below & it must be rich in ZDDP for the high loaded flat tappet overhead cams( 944 & 928 ) & thrust bearing

But what the hell would I know , I have only been working on Porsche cars since 1977 & thats only 38 years

PS } I am getting tired of warning people of these simple things , I should stop because we make a **** load of money fixing them when people keep doing stupid **** to these cars, like using low oil film strength oils & not checking the rear coupling on a regular basis

__________________

JET951

The below post is from the same thread back in 2015 , I was answering a question from Mike 77 }

Hi Mike 77 , to answer you're question about Major service & flex plate ,in the last few decades here is what we normally do }

First of all lets take an example of a 928S4 we have never seen before , in fact lets start with a "new to us" 928S4 that was booked in here two weeks ago for a pre purchase inspection & this car was from interstate & we have never laid eyes on this car ever

Year type = 1990
Version = 928S4 Auto
Kms = 167,434 Kms

In for pre purchase inspection

Among the normal checks , we ( like we normally do ) removed the engine pipes from their exhaust manifolds ( just 6 bolts & nuts ) , lower the exhaust a little & removed the bell housing lower cover , inspected & noticed the flex plate was pushed in a lot

Noticed the red painted ( factory paint ) front coupling inhex bolt was still quite tight

Removed the heat shields covering the rear of T Tube inspection round hole & turned the engine until the rear coupling inhex bolt head came into view ( could just make out a slight "red dust " covering the rear coupling & bolt head ) this red dust is rust from micro wear particles from the T Tube shaft male & female splines , this one , meaning the amount of rust powder was mild as compared to others we have seen , this is because of the rear coupling is not tight
Went to loosen the rear coupling inhex bolt & it was already loose "ish" , but this rear coupling bolt had not turned by itself , it was just stretched ( as bolts do ) & already knowing that the front coupling inhex bolt was no where as loose as this rear one ( all very normal ) , because no one knows to check these on a regular basis

Next step , removed both the front & rear inhex bolts & moved the engine flex plate back into position , this flex plate had been in this bent position for some time because it didn't want to straighten up probably because it had been in this bent position for years

Then we connected the dial gauge & measured the crankshaft / thrust bearing play & it measured 0.22mm , which is well within the wear limit( 0.40mm ) , but was / is in the more than the non wear specs ( 0.06 - 0.192mm ) , meaning the thrust bearing on this engine was/is worn a little , but this can be stabilised easily with high oil film strength engine oil , like we have been doing for decades

Subnote } we have had a few ( new to us ) 928S / S4's / GTS were the engine thrust bearing wear is into the .30 mm range & yes these cars were all on so called synthetic engine oils previously( low oil film strength ) and with varying degree of flex plate deflection & when we go to measure these very worn ones ( later monitoring ) the previous wear spec has completely stabilised when on a 20w-50 that is also OK in AW ZDDP

Subnote } the high the viscosity the higher the oil film strength , meaning going from say a 5w-40 to a 20w-50 & both these oils have identical AW ZDDP , the 20w-50 ( just because of viscosity ) has higher oil film strength ( all very simple stuff ) , but most 20w-50 last century oils a richer in ZDDP as opposed to this century 5w-40 or 5w-50 oils & thats why when we do a thrust bearing measurement of say a 928S4 that has been on a 15w-50 or better still a 20w-50 oil in the years before this check , the measurement is always in the non wear spec area ( 0.06 - 0.192mm )& that's only because of Oil Film Strength

This is the reason I mention oil film strength in the context of engine thrust bearings is because all thrust bearings in these engines must have excellent oil film strength ( its that simple)

And that is the reason we have see these cars with very deflected / pushed in flex plates but because like the 928S4 I mentioned above these 928's were just lucky that they were already on decent oil film strength oils ( like 15w-50 or 20w-50 ) and hence NO thrust bearing wear that was going to be an issue , or put another way , yes the flex plate was flexed , but the thrust bearing was protected by an extremely effective slippery oily bearing = The Engine Oil = who would of thought ?

Because remember , the engine thrust bearing MUST not make metal to metal contact , if it does the soft thrust bearing metal wears & then finally galls = disaster & its preventable

Now back to the pre purchase inspection }

The next thing we did ( after measuring the thrust bearing wear & writing it down ) we repositioned the T T quill shaft back to its central position , meaning the half moon cut out in the rear of the TT shaft aligns perfectly with the coupling , meaning the new bolts will go straight in , then we install new genuine inhex bolt ( with a little grease on the threads ) so we can go to the rear coupling inhex bolt on the next major service & then the next major & so on to check its torque , this later service check only takes ten minutes , because we only have to move the heat shields sideways a bit to give access to the inspection hole to check this very important Rear Coupling inhex bolt & of course a new inhex bolt for the front coupling

No Loctite on the threads of the inhex bolt/s , because this will give you the wrong torque / feel the next time you check its torque / tightness ( say 1 or 2 years later ) because the threads will be locked but if the bolt stretches ( and stretch they do ) & the underneath of the head of the bolt will not be holding the clamp with the force required

Its all very simple to maintain & we have no issues at all

Now why do we go to this trouble on a pre purchase inspection on a 928S4 that we have never seen before ?

Answer = We / I want to know do we have an excessively worn thrust bearing or do we have a thrust bearing with a little wear but within spec & we can control this & stabilise it ( like the car mentioned above )

All simple stuff & no need to ever wear engine thrust bearings at all & yes its a team effort between the engine oil & the unneeded extra deflection / slight constant push forward of the flex plate being in this position , meaning its a combination of two things

All straight forward stuff
__________________

JET951

From a Rennlist post I put up in 2014 }

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 .




Regards
Bruce Buchanan
Buchanan AutomotiveAttached Imageshttps://rennlist.com/forums/attachments/928-forum/801035d1390959219-1987-s4-auto-flex-plate-question-dsc_0016.jpg

Mrmerlin

Bruce thanks for posting this.
I wanted to share a few ideas with you and things I have found while mainlining these machines.

The reason I suggest to use loctite is so the bolt can not come out unless you want it too.

For rear bolt inspection just putting a torque wrench on it and making sure its tight is not how the bolt inspection is performed.

This is how I perform a pinch bolt inspection.

You should set the torque wrench to the 66 ft/lbs then remove the bolt,
if it has stretched you will know how easy it is to remove loctite or not.

If it has not stretched then you will also know how much force was required to break it loose as it will be very tight.

The bolt should be removed and the driveshaft should be checked to verify its still centered, or you could have a bolt that looks like your last picture.

Clean the threads of the bolt, apply a drop of blue loctite and torque to 66 ft/lbs.

FWIW I like to use the old bolt if its not damaged IE damaged threads or cuts on its shank or a bent bolt constitute damage.
So I replace with a new bolt usually put into the front clamp, and the old front bolt goes to the back since its work tempered.

Since the bolt has stretched some its gotten to its working position, thus its not going to be stretching much more.
for as many years as I have been doing this,
I have never found one of these bolts to have been snapped, or soft or over stretched
or any other issues with clamping or the bolt getting loose on any car I have serviced, and subsequently checked.
So I know this procedure works.

I had a car where the rear pinch bolt did come out,while the shaft was turning.
The bolt cracked the the rear bell housing snout and also damaged the input shaft clamp and the driveshaft,
the bolt was also damaged and bent.

NOTE the threads were clean, this made it easy for the bolt to come free and then get spun into the outer housing as it came out of the clamp.

This is why you do want that bolt to not have a choice to fall out, it comes out when you remove it. So a drop of loctite will keep it place.

JET951

Hi Mrmerlin , many thanks for you're post , I have read many of you're posts over a long time & they always make sense , you're slightly different approach to the maintenance to the Torque Tube front & rear coupling / inhex bolts & shaft positioning all makes sense , its just I have found that a light amount of grease on the inhex bolts threads gives me / us a great result & like you we replace these inhex bolts with new ( genuine ) ones & continue to do so on each 928 at regular major service intervals

Keep up the good work

Constantine

Couldn't agree more with the statement in bold above.

When we embarked on our efforts to find a solution many years ago about why automatic 928s were getting hit with TBF, we came to the conclusion that a free floating system like the 5-speed driveline would be the ultimate fix. We believed that clamping the automatic 928 driveline front and rear was the root of the problem.

We explored a few options on a free floating system but came to realize that making one the way it should be done would be costly and we did not believe many 928 owners at that time would want to pay for such a system. Hence we came up with the Super Clamp, our thoughts being if Porsche wanted to clamp the front drive shaft, lets make one that would be better for the task. But we always wanted to make the free floating system we dreamed of at the beginning.

At this time, 928 prices are going up and more owners might want to pay for such a system going forward. All that would be needed is a new automatic drive shaft with the front made to resemble the OE 5-speed drive shaft and a new flex plate coupler to mimic a clutch disc union to the short shaft. An OE 5-speed coupler, 5-speed short/clutch shaft and pilot bearing can be used. The only other possible modification would be an access hole at the front of the automatic torque tube to mimic the one found on the 5-speed torque tube to loosen/tighten the rear pinch bolt of the front coupler.

Although we can develop such a system, we have a few other "irons in the fire" as they say.

Maybe Greg Brown would be interested? This is based on the fact Greg Brown is already making drive shafts for both the 928 automatic and 5-speed versions, and can make the necessary modifications to the torque tube if needed.

Just my .02 cents worth.

GregBBRD

My thoughts are the same as my actions, on client's cars:

Install a supplementary clamp on the front flexplate as a temporary "fix". Note: None of the supplementary clamps I've installed have ever moved, but I watch them very closely on client's cars....I'm just not 100% convinced they will never allow the torque tube shaft to slip.

When the engine or transmission is removed, install a Constantine clamp...extremely robust, cleverly designed part. I'm 100% sure that this clamp will never allow the shaft to slip.

I do not "proactively" encourage people to change their 28mm shaft to a 25mm version, however, if I'm rebuilding a torque tube (always with Constantine's bearings) and it has a 28mm shaft, it is not going to go back in.

Constantine

Greg,

Thank you for your continued belief in our products. Means a lot.

Best regards.

FredR

Constantine,

Believe it or not it is almost 20 years to the day since you floated your then proposed clamp design to me after my shaft failure. I said then that a taper lock was the way Porsche should have designed that front clamp and you have proved that to be the case. The stock clamp looks like something one might find on Joe's Brough Superior SS100 kickstarter. As to why some of the stock clamps work and some do not I guess we will never really know the answer. One thought that came to mind was that in the days when they were new folks here used to drive the *** off these things and just maybe our hot climate and the heavy right foot syndrome form part of the equation. At that time I had no options and Porsche could not stop the clamp migration but the Loctite did for me and still does and as I intimated quite some time ago, as and when I have to remove the torque tube one of your clamps will go back in there assuming they are still available,

Had Porsche applied such design from the start I wonder just how many owners would not have had the anguish of experiencing TBF and if my theory holds, snapped 28mm drive shafts. To my knowledge about 20% of the S4 and GTS fleet experienced TBF and 10% had snapped 28mm shafts. Incidentally, all the TBF'd engines failed due to slippage of the front clamps with the clamp having migrated somewhere in the region of 10mm along the shaft and mangling the poor things. I have also seen examples of the rear clamp pinch bolt being knarled up and the 28mm shaft snapping at the rear end of the shaft but I have not seen a TBF'd motor with the front clamp in the correct position and the rear clamp having pulled through.

The history of the failure mechanism behind TBF also makes me smile somewhat when I recall folks proposing it was possibly caused by torque converters ballooning amongst other things. Once the front clamp has slipped and the effective length of the shaft has increased by the notional failure slippage of 10mm, the simple fact is that the rear face of the crank thrust bearing is under heavy compression all the time and this in effect blocks the oil feed to that thrust face as the feed to that central main bearing then discharges through the front face of the thrust bearing and at some stage in the length growth process the thrust bearing spins, the white metal melts and the first the owner knows about it is when he tries to start the motor the following morning or whenever and finds it is seized solid due to the white metal re-solidifying. Although I did not own it at the time, the original motor in my current GTS was in such a way. They managed to get the motor free by somewhat brutal means but the damage was done. No engine oil in existence could possibly protect the motor from such attrition. How do we know this?- white metal melts at around 240C, the best synthetics start to break down at temps over 150C and at 240C I suspect they would be well and truly thermally cracked!.

Majestic Moose

Always an interesting discussion and all I have to add is my anecdotal experience. This issue is largely ignored on 5 speed cars. Of course there is no flex plate to worry about but when swapping out my transaxle I found that although the rear pinch bolt was tight, the shaft had migrated out of the transaxle input sleeve by a considerable amount. Unlike the previously posted photo of the chewed up pinch bolt, mine was undamaged but instead wore into the drive shaft.





For reinstallation I cleaned all the splines and applied blue Loctite onto the splines and fitted a new bolt with blue Loctite as well. I agree with Stan that the only way to inspect for shaft migration at the rear pinch bolt is to remove it and look for correct alignment of the cutout for the bolt in the drive shaft. The bolt cannot move relative to the access hole in the torque tube because it is fixed in position by the cutout in the transaxle input sleeve so a visual inspection without removing the bolt will look perfect; but it is the shaft that moves.

Transaxle input sleeve:


Part of my maintenance plan will be to remove and dispose the rear pinch bolt, inspect the shaft alignment, and replace the bolt every third oil change or so. It is only a few dollars and a few minutes of work on 5 speeds.