mike77

A year or so ago on my S4 I realeased the flexplate tension checked the play and torqued it back up again and thought job done. Recently saw mention of the rear pinch bolt in a post. I didn't check this. Lots of detailed info around about the flex plate but I can't find a decent post or guide for the rear pinch bolt. Anyone got a good link?

I'm also now wondering if perhaps just doing the flex plate could worsen forward migration? Surely the pressure on the flexplate is caused by the driveshaft migrating forwards. Wouldn't releasing the flexplate and reclamping just clamp the driveshaft further forward than it was before allowing the plate to flex again from a further forward position?

UpFixenDerPorsche

Got it in one Mike. (Quickly donning volcano suit in readiness for mega volcanic eruption. Will be worse than an "Oil" eruption).

Here in Australia all flexplate problems have been traced to loosening of the rear clamp bolt, allowing what you have described to occur.

When I was releasing my front bolt I was asking the same question as you have asked viz "what will stop this recurring?"

First reaction was to tighten the front clamp really really tight, and if it moved again, fit a bigger stronger clamp. I did the first one, but always felt uneasy from an engineer's perspective in that no underlying cause had been identified.

Then a post from Buchanan Porsche (928 expert here) highlighted the role of the rear pinch bolt, so I immediately checked mine, and found it was loose, damaged mid-way on the shank, and bent.

So I replaced it, torgued up with Gorilla Loctite, and all has been good since. However, since we still don't know the reasons behind it's loosening, a yearly flexplate check stays on the list.

Many ppl here will say that once they released the flexplate AND fitted an extra clamp of some sort, their flexplate has not moved again, or maybe very slightly.

OMHO this is because the rear clamp/bent bolt has jammed the shaft as far forward as it can go, meaning the front flexplate can no longer move forward, ever, thus proving the effectiveness of the extra gorilla clamp, except for the 'faulty' reasoning (understandable, since no one was looking at the rear clamp bolt).

Anyway, to my mind at least, there remained the unanswered question of the loosening of the rear clamp bolt (and I hate unanswered engineering/tech questions).

I come back to torsional oscillation of the central shaft, which in the case of an auto, can be a nest of snakes.

At one end of the shaft is the engine (delivering pulses torque), and at the other is basically a flywheel ie the torque converter.

As revs rise and fall, this combination will go through resonant and semi-resonant (harmonic) frequencies, with each of these frequencies inducing a peak torsional stress in the central shaft.

This is all normal grist-for-the-mill stuff with power transmission engineers, including those at Porsche (hopefully).

Not only are there torsional oscillations to manage, but also 'whip' oscillations viz those caused when a shaft ceases to run 'straight' but develops a bend, or an off-centre eccentricity, that causes it to 'whip' at some rpms.

This is where the benefit of intermediate torque tube bearings (and spacing) come into play. Essentially, the shorter the shaft span, the higher is the engine rpm needed to develop whip.

Conversely, should a bearing begin to 'walk' along the TT because of deterioration of it's rubber mounts (age, inadequate design etc), one section of the 'supported' shaft will lengthen, allowing whip to occur at lower rpms. This side-to-side rotating whip will further stress the existing rubber bearing mounts, allowing more whip, and in turn more bearing 'walk' inside the TT.

Next step: when a shaft 'whips' it momentarily shortens in length, imposing sliding stresses on the shaft clamp bolts on each of the central shaft.

In my experience (detailed earlier) it was the rear clamp bolt that allowed movement. Others have a differing view, but irrespective of which end, the fact is that 'a movement of something' resulted in serious 'flex' of the flexplate.

I'm going to stick out my neck and say that Porsche had it all basically right at the start, but they did not count on Brand Nutters like Us to still be interested 15-25 years into the future.

So, of all the fixes developed around this issue, FWIW I think the super bearing is the best long term fix, since these bearings, in particular their mounts, will stave off TT bearing migration (the cause of increased whipping/ bearing mount deterioration etc).

Cheers.

(Hey Constantine - well talk about some commission later LOL)

Mrmerlin

Up Fixin, thanks for the dissertation on the drive line.

My suggestions, When checking the drive shaft pinch bolts,
both ends of the shaft must be checked.

You will usually find the rear pinch bolt is loose,
so removal and inspection of the bolt ,
and the re centering of the shaft is usually necessary.

NOTE to check to see if the rear bolt is loose simply use a torque wrench set to 66 ft/lbs
you will find that it wont be close to clicking the wrench before it comes loose.

If the bolt had damage to its shank it should be replaced with a factory bolt ,
NOTE its a hardened bolt 12.9

I suggest to torque the bolt to 66 ft/lbs as also described in the TSB offered by Porsche,
also a dab of blue loctite to the threads.

For the Auto cars the rear heat shield must be dropped,
and this may also include dropping the rear half of the exhaust to get to the bolt.
For the 5 speed cars the port is easy to access

mike77

Ok didn't realise that this was a contentious issue such as oil can be. I use 5w40 btw

The steps I read were

1) release flexplate tension
2) rotate engine to remove tension from the drive train
3) torque flexplate bolt.

I'm wondering if step 1 should be release rear pinch bolt. Then when the engine is rotated the drive shaft has a chance to migrate backwards? Or is there another way to move the drive shaft back? I assume since I did have some bowing on the flexplate mine is now further forward than it should be.

I'll check out this Constantine clamp

mike77

had all this off two weeks ago for new rear brake lines and an rmb install

JET951

As John C has mentioned , its mainly the rear coupling index bolt that looses its tension over time ( the bolt does not loosen 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

Mrmerlin

My contention with using loctite is to keep the bolt in place ,
If the bolt should stretch then back out,
it will cause much more damage than just stretching and staying in place.

NOTE I did find one bolt out of the rear clamp,
and the shaft had spun inside the input damaging both parts

Since this bolt should be checked regularly,
IE Remove inspect and refit or replace.
using blue loctite isnt going to mask other issues..

NOTE when checking the bolt for torque I suggest to loosen it first to see how tight it was to begin with.
simply retorquing it isnt a valid check for this bolt.

NOTE you will also want to inspect the driveshaft migration and this can only be done with the bolt removed

mike77

I won't argue that 5w40 is the best choice as I'm thinking of moving to vr1 5w50 because on the hottest days I've seen oil pressure go to 1.8 and also has more zddp. That said 5w40 is in my 928 hand book along with 5w50 and both are referred to by porsche as superior oils. I also believe that 5w40 was the factory fill. But as I've said I'm probably going to change. If i didn't live in Scotland with freezing winters I would probably go with tbe flow and put 20 in.

Oil aside though. Some interesting replies. It looks like I definitely need to check out that rear pinch bolt. When you guys do a major service do you even mess around with the flex plate or just sort the rear out? Seems some of us may have been looking in the wrong place

Mrmerlin

you have to check both ends of the drive shaft ,
only doing one side will render the inspection a fail

UpFixenDerPorsche

Now now Brucey Let's not not spoil the party.

We agree completely on the rear pinch bolt issue, so let's not muddy the waters with other stuff.

:-)

JC aka UpFixen. LOL

FredR

Mike,

My take on this is that there are two potential issues in play and they need to be considered in isolation of each other.

Issue 1: The rearward assembly needs to be assembled correctly with the bolt at the correct tension located in the correct position. The only issue here is how often should it be checked and the procedure for checking it. If assembled correctly and working as designed it does not give problems.

Issue 2: The front flexplate clamp- when assembled and installed correctly can and often does slip. No one knows for sure why this happens on some [many] examples but it does and the results are often catastrophic. On the 5 failed motors I have seen the flexplate clamp had migrated along the shaft- it was not the shaft that migrated thus I do not subscribe to the notion that the flexplate clamp does not slip- it does and the real mystery is what proportion do?

As for the front flexplate I understand your clamp moved when you released the tension. If it did, how often do you check to ensure it has not moved again- did you paint some index marks to show the originally installed position and thus reference the position if the clamp has moved ?

If you have a front flexplate that is susceptible to this type of movement then you would be well advised to take secondary measures to ensure it does not go on to a full fledged case of TBF. Indeed I believe that any late [S4/GTS] automatic owner should take such measures- others may disagree. 16 years ago my clamp was migrating every two weeks, Porsche could not solve the problem- I did and the method I used [courtesy of this list] I still use today and my crank end float has not changed [I intend to check this tomorrow if I can take the heat].

Rgds

Fred

mike77

Hi Fred, sorry but your link isn't working. Is it the locktite method by any chance?

I didn't paint it but should have and I've not checked it since last year. Think this will be on the cards soon

As for re-centering the drive shaft at the rear. Is this a diy job once you are past the heat shield? Ie no special tools etc.

FredR

Hi Mike,

Indeed I use the Loctite approach. I did it once on my late S4 and it lasted 5 years until my big hit- the crash caused the shaft to give at the taper slightly but the Loctited flex plate clamp did not let go in that trauma. With the transaxle unit and shaft out I had no problems removing the clamp and that unit is still in service today in my current setup- again Loctited 8 or whatever years ago. If and when I have cause to remove the unit I will probably go for Constantine's unit given it is the type of thing I might have designed had I needed to but as you are probably aware, there are several other good options available that you can and should research opinions on.

I take some solace knowing that if my flexplate clamp were to move my motor exhibits a very distinguishable tell tale vibration at 3050 rpm. If the Loctite fails I am working on the presumption I will be able to spot this immediately but....?

As I am aware the rear coupling is a straightforward matter once you get access to it but as I have not attempted this activity personally you should follow what the likes of Stan [Mr Merlin] or the boys in upside down land have to say. Again, I am working on the basis that I will feel something if it lets go and the effective shaft length increases.

I personally think the paint mark should be seen as an absolute necessity- once there you only need to take a quick look to see if there is any movement. Indeed it is about time I had a look at mine!

The sad fact is we keep hearing about these failures happening- there was one about a month ago on the list. Once it happens the reasons why are completely academic.

If you need details of how to do the Loctite method just let me know. Weigh up the options and read what others have to say on the matter in general- for sure plenty has been written about it.

Rgds

Fred

JET951

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

Vilhuer

Nice long explanation about totally different problem. Which needs to be addressed but which do not remove another problem at front clamp.