John Struthers

Anyone remember the thread or recently checked their Auto's for the walking crank disease?
I was looking for that descriptive thread from about a year and a half ago and can't find it .
Snow had a good one from several years ago but it went along with the rest of those in-depth post and threads during the Great Pissing Contest.

I finally got the software download from Kodak and want to do a digital step by step which would be a lot easier under the guidance of a BTDT coach.
Also need to know if any of the BTDT crew have come up with a allen wrench/socketed drive tool that won't break while trying to release pressure on that clamp bolt. Preferably a tool that doesn't demand a pound of flesh and a quart of blood as a user fee
Thread lockers? Type and #
Has anyone disassembled a motor with the main journals significantly cut by the 928 crank milling machine? If so, did it push the bearing(s)?
If it pushed the bearing(s); does it look like the bearing 'STAYED' where they had been pushed -to the front- ?
OT
Dennis, I see you are awake
Anthony,
Hide the sHARk, More weather heading your way
TIA

John Speake

John
There is a very good descrition of the procedure on Tony's site
http://members.rennlist.com/v1uhoh/index.htm

It is recommended you always use a new pinch bolt, and tighten it 10% more than manual figure

Dennis Wilson

John,

Thust bearing failures due to migration have only been reported on the 4 speed autos. Thought yours was a three speed. In 84 the factory manual was modified to check for thrust bearing pressure (pg 39-141). While you are in there it wouldn't hurt to check it. The tool for my 78 was either allen or triplesquare (having a senior moment). I do remember buying a three piece triplesquare set at the local Autozone. Of course the manual doesn't list or show the tool. The key to using these with minimal sacrificial blood letting is to make sure the bolt is clean so the tool meshes completely. A little PB Blaster wouldn't hurt either. It may actually help disinfect the wound when the blood letting occurs. ):

Dennis

Lizard928

I have a quick question for yah, on my 86.5 4 speed auto its Vin is later than 1000, there is a hole on the bottom side of the bellhousing which has a plastic plug there and is right in line w/ the pinch bolt, now my thrust plate is in perfect condition and there has been walking on the bolt, but is the hole there from factory or did a shop add that for easy ability to loosen and retighten the pinch bolt?

WallyP

This is a "final" report on the Flexplate hub Loctite test. ---It works.


Background: The engine crankshaft thrust bearing has worn out on some
automatic transmission cars. There is much speculation on the cause of TBF
(thrust bearing failure) There are many possibilities. One of the
possibilities is the migration of the front flexplate hub, causing constant
pressure on the thrust bearing. "Stuttgart Mechanics," whoever they are,
suggested increasing the torque on the hub pinch bolt by 10% (torque to 110%
of recommended torque). We found that this did not stop the migration.


Test: I tried using penetrating Loctite (290) on the hub splines. This has
been tried on at least 10 cars and has been successful. I have not heard of
any hub movement after using Loctite.


Results: Loctite seems to be able to stop the hub movement. The pressure
against the TB is significant, you can't check for wear until you release the
pressure. The TB wear on the 4 cars that I checked was within factory
specifications, so presumably the pressure was not causing a problem.


One byproduct of eliminating the pressure on the crankshaft is: It may reduce
or eliminate a vibration that happens at ~3050 RPM. Even if it doesn't help
the TBF problem, maybe it will stop potential problems from the vibration?
Could this also cause the TT bearings to fail?


Recommendations: Porsche designed the hub to not move. It is moving, and
putting pressure against the TB. The "Loctite Fix" is cheap insurance. If you
have an automatic, why not do it? While you are down there check the rear
pinch bolt, I hear that they can loosen up and cause the driveshaft splines
to strip. Maybe the hub movement causes the rear bolt to work loose? One car
had worn threads on the rear pinch bolt, so maybe it is a good idea to
replace both pinch bolts. P/N 931.412.240.00 and install with Loctite 242.


Please do NOT think that this is "THE FIX" for TBF. This is one of the
possibilities. One other possibility that has not been brought up on the list
is torque converter "balooning". Apparently other manufacturers have seen TBF
caused by failed torque converters. It would be interesting to know if
Mercedes ever has this problem. I have heard that it happens on Chevys or is
that Chebbys? I have heard the advice that if you replace the torque tube,
then while you are in there, replace the torque converter bearings. Why? Do
they go bad? Is this insurance against "balooning"?


Constantine's sliding coupling could reduce the possibility of TBF much more.
If "balooning" is a problem, can the coupling slide enough to compensate? If
your torque tube needs replacement, then that would be a good time to install
the sliding coupling. Just a little more insurance.


Loctite Procedure:


1) Drop the exhaust at the manifolds. (this may not be necessary on some cars)
2) Remove the bell housing bottom cover.
3) If you want to measure the amount of movement of the flexplate hub, put a
dial indicator on the forward side of the hub. Or you can measure the amount
drive shaft spline visible on the rear side of the hub, before and after
loosening the hub clamp screw.
4) Loosen the hub clamp screw and watch for hub movement.


With the front hub clamp screw loose you can check for TB wear and also
tighten or replace the rear clamp screw. Note: Tightening the rear clamp
screw with the front clamp screw tight and with preload may be why the rear
clamp screw threads are damaged.


Replace the rear clamp screw (same P/N as front, P/N 931.421.240.00). Remove
the rubber plug in the rear end of the TT. Rotate engine until the bolt is
visible. See shop manual page 37- 4 #14. use Loctite 242, torque to 65 ftlb.


Check crankshaft end play:


Install dial indicator on flywheel and carefully pry the flywheel back and
forth. You may be able to check this measurement with a dial caliper or
vernier caliper. The spec for later engines is .0024" to .0076". Wear limit
is .016". Metric .110 to .312. wear limit .4.


5) Before installing the front clamp screw, pour Loctite 290 penetrating
formula in the slots in the hub.


6) Install new front clamp screw P/N 231.421.240.00 using Loctite 242 and
torque to 65 ftlb. It would be logical to pry the crankshaft back before
tightening the bolt, but I don't think that it matters. The flexplate
provides for plenty of "float".


7) Paint the shaft splines white on the rear side of the hub. Then whenever
you have the car up in the air you can look through the hole on the bottom of
the bell housing and see if it has moved. An inexpensive fiber optic extender
on a penlight can give you better visibility. They are available at
hardware/home center stores ~$5.00.


8) Reinstall bellhousing bottom cover and exhaust.


Loctite says that it takes 24 hours to cure. If you can't wait 24 hours
before driving the car, I would wait at least 1 hour and then drive gently
for the first 24 hours. I know that is a lot to ask, but do it.


It's probably a good idea to periodically check crankshaft end play. If the
hub has not moved, you can check end play without loosening the clamp bolt.
The thrust plate flexibility easily allows back and forth movement of the
crankshaft.


Earl Gillstrom '88 5Speed


This is an earlier post by Cap'n Earl on the RennList 928 email list. Hope that it helps.

Erik - Denmark

Wally and Earl,
Good description as usual when you two expert 'lead the pen'
I do not have the TBF problem, but I do have the vibration around 3000 RPM
------
You said: One byproduct of eliminating the pressure on the crankshaft is: It may reduce or eliminate a vibration that happens at ~3050 RPM.
------
Can you explain this 3000 RPM vibration phenomena?
------
You also said: One other possibility that has not been brought up on the list
is torque converter "ballooning". Apparently other manufacturers have seen TBF
caused by failed torque converters. It would be interesting to know if
Mercedes ever has this problem. I have heard that it happens on Chevys or is
that Chebbys? I have heard the advice that if you replace the torque tube,
then while you are in there, replace the torque converter bearings. Why? Do
they go bad? Is this insurance against "balooning"?
-------
Looking at the exploded drawing, (I never BTDT) I think, if these converter bearings are OK (without axial play) they will prevent forwards movement of the shaft (because the bearings are locked with expandable locking rings), so only if this bearing are worn out, I accept the ballooning theory!
I also understood this problem is not so often on older models, so it mush have something to do with over torque of the trans shaft - On newer models we even see that the trans shaft is breaking due to torque- What happen before the shaft is breaking (winding and unwinding), and can that be the reason for the forward movement?
I would very much like to hear others opinion about this never ending ballooning discussion - I mean that too bad if we cannot find the real reason for this problem - One of the way could be to look at the torque/dimension for the first and latest model - I think the shaft dimension is the same and the torque increased 35%!

PS: I check regularly my pressure plate (I have the last model with distance washers - X measure - who gives a negative pressure on the crank shaft) and also I checked the axial play/trust bearing when I last year overhauled my engine

Steve Cattaneo

Here is the write-up on the other possibility











Steve Cattaneo
Addict
Rennlist Member
Registered: May 2001
Location: new york
Posts: 594


928 engines, with automatic transmissions; have a high rate of crankshaft thrust bearing failure. There is a condition known, in the transmission industry, as torque converter ballooning. The torque converter physically increases its size and exerts an outward pressure. Over revving the engine and excessive torque converter pressure are contributing factors.

The torque converter has two main functions. It acts as a fluid coupling connecting the engine power, through transmission fluid, to the transmissions planetary gear set. It also multiplies the torque of the engine and transmits this torque to the rear driving wheels, propelling the car at a torque multiplication of 2:1 in a Porsche.

The transmission oil pump charges (pressurizes) the torque converter with transmission fluid, when the engine is running. As the TC spins, at engine speed, there are forces in the torque converter exerting pressure on the crankshaft thrust bearing through the torque tube drive shaft.

These pressures are:

1. Rotary fluid flow generated by the rotating fluid.
2. Vortex fluid flow also generated by the rotating fluid in the torque converter.

These forces create a centrifugal force in the torque converter, which push on the torque tube drive shaft in turn pushing the crankshaft forward cutting off lubrication to the thrust bearing.

Excessive torque converter pressures can multiply centrifugal forces. The main causes of excessive pressures are:

1. Restriction in the cooling circuit.
2. A plugged transmission cooler in the radiator.
3. A crimped cooler line. (Torque converter fluid is the feed oil that goes to the radiator to be cooled.)
4. Highline pressure (working pressure).
5. A bad modulator.
6. A stuck pressure regulator valve.
7. A disconnected vacuum line to the modulator.

These can all cause excessive torque converter pressure. The engine rebuilding association blames the torque converter for thrust bearing failure in most cars. The transmission industry claims the failure is due to a flaw in the crankshaft design. Both agree that modifying the thrust bearing or the crankshaft can be beneficial in some engines.

If you remove your engine or transmission make absolutely sure that you adjust the pinch clamps on the torque tube drive shaft (according to the workshop manuals). Mercedes Benz issued a tech bulletin on torque converter ballooning. They want you to lay down the torque converter on its primary pump flange (hub) and measure from the work surface to the top of the flywheel mounting ears. If it’s more then 121.5mm, the torque converter has ballooned and needs to be replaced. Everyone knows, and if you don’t know, 98% of the internal components of our transmissions are Mercedes Benz parts. Porsche made the cases for both the 3 and 4 speed.

If you’re rebuilding your engine, have the crankshaft or thrust bearing modified for an increase in lubrication. If your one of those who disconnected the vacuum modulators vacuum line, you should reconnect it. If you want a firmer shift install a shift kit or adjust the modulator by one turn only.

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
__________________
Steve C
928S 83
The Great White928
85 S2 EURO
02 MB AMG 350HP SC
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08-27-2002 02:35 AM

John Struthers

Thus spake the Guru's.
Thanks Dennis, John, Wally, Steve.
Either way or both ways it would seem that a solid effort in tension
release, increased torque values, Loctite application, rear clamp retourqe, proper -not to wild there, fellas- pressure regulation, and a T/C balloning inspection or best combination there of is a good idea.
For those of us in the ranks of the lazy shifter shifter camp - aka " Shiftless Bastards" I see no reason not to schedule this as a 'ought to' inspection every year or two.
Thanks again guy's.
BTW The reason I brought this up is a very low whirring sound has developed and is barely audible thru the 'Hump' at idle-car stopped- in any forward or reverse gear. No sound in neutral or park. Not detectable above idle.
Lost my personal favorites maintenance files to the win 32 worms.
Back up to disk people, it's the only sure way to save your STUFF.

Earl Gillstrom

Steve,

Excellent write up, but I have a few questions.

Do 928 engines have a HIGH rate of thrust bearing failure? I have heard of a few, but is the 928 failure rate higher than normal? Higher than MB?

I have thought that torque converter ballooning is the major reason for TBF, but the reasons that you point out make a lot of sense. If I had an auto I would do your checks immediately.

You don't mention torque converter bearings, yet many suggest that you replace them while you are in there for other repairs. Should they be a maintenance item?

The only thing that I still wonder about is: why does the front flexplate hub migrate all the way forward on the shaft to put continuous high pressure against the thrust bearing?

The "Loctite Fix" that we have been doing seems to relieve that force. Does this reduce the wear on the thrust bearing?

Every 928 that I have checked (8 or 10) has had the tension (some have been jammed hard against the flywheel) but they have all had good thrust bearing clearances, (.006" to .008").

Should we be doing the "Loctite Fix"?

I am glad I have a 5 speed and the only thing to worry about is that extra flat spring attached to the drain plug magnet!

Steve Cattaneo

Earl, I commend you on your excellent write up as will.


I have heard of five 928 thrust bearing failures. I am sure there are a lot more as will as engines with excessive crank shaft end play. I have never heard or read of any MB TB failure. In most engines including 928s there is only one thrust bearing (center bearing) installed into the block. The thrust surface area is only ½’ not a lot of surface area when there is a load on it. Good engine builders add oil groves on the thrust surface. In 1990 Porsche issued a technical bulletin for the installation of the torque tube.

Central Tube - Installation
9222porsche01
May 5, 1992
Model
928
Group
3
Number
9203
Part Identifier
3903
Subject:
Installing Central Tube
ATTENTION:
Service Manager/Service Technician
Models Affected:
All Model Year 928's with automatic transmissions
Concern:
Installation procedure of central tube.








Repair Information:
After the central tube is bolted to the forward housing, first tighten the six (6) dhve plate attaching bolts (A in Figure 1) to 32-39 N-m (23-28 ft.lbs.). Then tighten the clamping screw (B in Figure 1) to 75-85 N-m (54-62 ft.lbs.). Following this procedure will ensure necessary ninning clearance for the crankshaft thrust bearings.
Any damage to the engine which has been caused by improper central tube installation is NOT a warranty matter.





If the transmission or torque tube is remove, change the converter bearings, no only do they support the toque converter like a pilot bearing they also support the torque tube drive shaft. Bad torque converter bearing will cause a front seal leak.


I don’t know why the front hub migrates forward; any constant load on the crank will prematurely wear the thrush bearing. The loctite fix will most certainly help. I would also install a new bolt, torque a little higher than spec. checking the crank end play as a maintenance item is also a good idea

Manual transmission also place thrust loads on the crank especially if you use a hi performance clutch or if there is no clutch pedal free play.