928man

I know there have been some threads on thrust bearing failure but is relieving the thrust plate tension a necessary maintenance item that is routine for an 86.5 automatic 32V ? Also is the coupling torque setting by the factory of 59.5 lb-ft best or 65 lb-ft ? Is it also best to use blue locktite with a new bolt? This is the first automatic I've ever owned. Lastly driving the car 650 miles home from where I bought it in 100 degree heat, not far from home I made a stop and with the transmission in Park there was a loud humming noise comi ng from the transaxle area, after an hour parked & the car restarted the noise was gone, haven't heard it since. The car has 67,000 miles on it. Some guy told me it might be the differential, any help appreciated.

Mrmerlin

If this is your first 928 auto i would check both the front and rear bolts if your not sure order 2 new bolts and get a bottle of blue loctite before you begi.
Make sure to first loosen the front bolt then measure the front to back play in the flywheel then remove the rear bolt noting how much it takes to loosen it, most of the rear bolts are usually quite loose.
You can center the driveshaft with the front clamp , tighten the pinch bolt then tap the shaftclamp either way till the rear slot lines up.

you may have to loosen the front bolt and let the clamp slide a bit to move the clamp on the shaft.
Once the rear is in position then tighten the rear bolt then pry the flywheel back then install the new bolt in the front clamp , torque to 66 ft/lbs

FredR

There have been many debates about what causes the problem but everything points to inability of the front clamp to grip sufficiently. Over torquing the bolts does nothing to help alleviate the problem [it does not do any harm either]. The rear bolt should be torqued correctly and if it is, I have never heard or seen any evidence of it causing a problem. But the front clamp is a different case altogether- basically it is a disgrace- an archaic and seemingly inadequate design totally out of context for a Porsche design. Failure is not guaranteed but far too many engines have been lost ot this problem. Porsche will never admit to this but the evidence is sadly way too compelling.

Similarly loctiting the bolts will do nothing for you other than making bolt removal difficult. Earl Gilstrom's loctite solution requires the splines to be loctited on the front clamp with loctite 290 [I used 270 because we could not find 290 locally]. Some say it is difficult to undo after it is loctited- I did not find that to be the case and the splines can unbolted anyway to disengage it.

The prime indicator of a problem having manifested itself is crankshaft end float. Normally this wil be 0.2mm and the wear limit is 0.4mm. If you have more than 0.4mm chances are your engine is toast. Your engine will be vulnerable ot this problem if the flex plate is under tension. This can be checked by marking the interface between the shaft and the clamp with some white paint and then release the front clamp bolt - vulnerable cars often show compression of about 3mm [or more]. The problem can manifest itself without any warning or symptoms and generallhy there are no abnormal sounds to warn you aobut it. The only thing I noticed on my S4 motor was a vibration at 3050 rpm some 10 years ago. fortunately, due ot this list, I was able to diagnose the symptoms and I believe my motor was the 2nd one in the world to have the loctite solution applied.

I purchased my current GTS with a wrecked enigne due to TBF and analysed all the symptoms. fortunately I had the S4 motor to put into the GTS and it is still going strong today 10 years later with loctite holding it together.

There are three mechanical solutions to this problem Constantine's clamp [my favourite-backed up by lab tests], The Pklamp [an imaginative piece of lateral thinking] and a neat modification to the clamp design by some Dutch engineers [See Theo Jenniskens website]. I do believe they will all work.

Anyone who runs one of these cars without one of these solutions applied is a nutcase- welcome to the list.

Fred R

Tails

+1 on Mrmerlin's post and I would recommend that the front flexplate migration is checked at each yearly service and I would also suggest that when you reclamp up the front flex plate clamp you apply some white paint to the aft end of the clamp and the spline of the drive shaft to see whether there has been any movement.

Three methods of securly clamping front flex plate, lockite, Pklamp from 928sRus and the Super Clamp for Constantine. My recommendations are in the order as written with the best method being the Super
Clamp (IMO).

I would suggest that the noise you heard was the fuel pump, they get up quite a buzz on a hot day and a long drive.

Tails 1990 928S4 Auto

928man

Hi : And many thanks for all the good ideas! It really does sound like a poorly designed clamp as you mentioned, this problem shouldn't really exist considering what these cars cost new !

Landseer

I've checked our 86 on 5 different occasions in the last two years and it repeatedly has had a mm or so of movement, which I release. Its had the bolt replaced once and has been torqued each time to about 66 lbf. I need to do something more with it, especially as my wife begins to thrash it more. Two years ago we found the car with more 3 mm flexplate movement at 73kmi. Did not use loctite on bolt or splines. Have checked torque before releasing and its 66 or so but still has had shaft migration.

The 85, which I've driven harder this year, has been released twice with 2mm play and is now clamped with a Porkclamp as a supplement. Last summer when we first checked this car it had a very significant bow in the flexplate at 71Kmi

The 84 got a Constantine when it got its 89 transmission. Its circlip and shimkit was missing from the TT when the car was found with about 140Kmi.


All the cars are well-within specification on endplay. Thank goodness we learned about this issue and have at least released pressure.

Hayk928

That's not fully true - I spoke to two engineers at Porsche Classic - they are aware about that thing (btw. why is there a hole for maintenance in the bell housing?).

Funny enough, they both linked on rennlist and told me about Constantine's Superclamp, Roger's PKLAMP and even the Loctite solution. Don't think, they are not following you in your improvements...

Best

Marc

FredR

I did not say they are not aware of it I said they will not admit to it- that would cost them big time! I believe they have settled claims against those who made enough noise/legal threats etc but then gag the recipient. A chap in Germany asked this list for help- we gave him al he asked for and in turn we asked for feedback- guess what?


Fred R

Landseer

They follow, but didn't lead.

FredR

You have three automatics, you experience migration on all three, you clamp one and run the other two without additional clamping or loctiting the splines? Anyone home?

In case you have not understood how dire and un heralded this is- when it happens, the first you will know about it is when you are in sh*t street and that will be after you arrive there.

Best wishes

Landseer

I have a constantine clamp on the 84.
A Porklamp on the 85.
And I've been meaning to get at the 86 that my wife drives for a while to augment it, but have released the tension routinely. She has been driving it lightly until recently.

The issue is real for the early 32V cars, for sure, not just boosted S4 cars.

4 of the 5 cars in the driveway were rescues in every sense of the word, saved and now doing service as daily drivers.

The 85 was parked by the OO in 1999 with a lot of flexplate bow, maybe 5 mm. Was next started by us in 2009. It measured on the low side of specification.

You should wear a hat over there, it gets warm.

FredR

The issue seems worse for the 32v cars, the earlier 16V ones seem much less problemmatical but whether they are impervious to TBF remains to be seen - I have seen about 6 32 valve models fail over here- consideering the total fleet size of about 30 cars during that period [maybe less due to personal exports, wrecks etc] that is a significant proportion.

Do something about the remaining one pronto- it would be a shame to see the motor trashed especially for someone so committed.

Yesterday afternoon I stripped the window drive mechanism on both doors- I think 50% of me had vapourised by the time I finished- not pleasant at all and no a/c or garage- just shaded area. There was on wind and the humidity soared- urrrrrgh.

Regards


Fred R

Hayk928

Don't know if I got you correct, Fred. If you're concerned that I didn't update about my TBF car, I'll have to say sorry, but there are currently no news; The car's been parked dry and from my buddy in England I got a '86 manual with minor TLC for cheap to enjoy yet and restore step by step. In other words: I'll rebuild the S4 engine hopefully at a later point in my life.

My apologies as well for being not always online due to my other hobby - curing an intestinal carcinoma.

Cheers

Marc

mickster

Take into consideration that this was designed in the 70's using technology from the 1930's that was mostly abandoned by 1950 (see Tempest below).

Outside of muscle cars, which used a front mounted transmission and more traditional Hotchkiss driveshaft, and the odd Pontiact Tempest, torque tubes rarely carried as much torque as the 928 32 valve engines produced.

Today with computer modeling and all that has been learned, it would be far easier to design a better clamp (see Constantine's clamp).

Also remember these cars were designed for high speed driving on the Autobahns where torque would be lower at high speeds.

There is no question the clamp was under-engineered (rare for Porsche-well usually addressed after a few years) but not for negligence, just lack of real-world experience.

I've pondered if a motor sitting typically an inch lower on crushed motor mounts actually puts more load and stress on the torque tube, flex plate and clamp.'

Food for thought.

Michael

Black Sea RD

Hi Michael,

Not really.

The motor, TT and transmission are all bolted together as one unit. Lowering either end just angles the whole assembly, nothing within the assembly is changed in orientation to the other.

Kind regards,