Snowball the 81 white 928

noticed a nice thread on nichols site about this inspection procedure. is there anything else out there for review or can someone chime in with abbreviated procedures here?

thanks. going to do this inspection asap.

an apple a day keeps the doctor away:

loctite, sealer and cash, keeps your 928 from turning to trash!

Greg86andahalf

http://members.rennlist.com/v1uhoh/pinch.htm
http://www.landsharkoz.com/techtips/tttbf.htm

http://www.928oc.org/tip/tip598.htm

Snowball the 81 white 928

so, the gist of this is that the driveshaft is growing? when does it stop, after it protrudes like a spear from the front?

Steve Cattaneo

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I’ve been hearing 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.

Steve C
The Great White



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Posts: 239 | From: new york | Registered: May 2001 | IP: Logged

WallyP
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User # 763

posted 02-24-2002 23:10
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Capt'n Earl has done a great deal of very good work in this area on the 928. Here are some earlier posts from him on this subject.
Wally


This is an update to the use of Loctite on the driveshaft/thrustplate hub on
automatic cars to reduce hub migration that pushes on the crankshaft and may
cause engine crankshaft thrust bearing failure.

On test car "D" we did the Loctite procedure but, we did NOT let the Loctite
cure for 24 hours due to geographic constraints. After 600 spirited miles the
hub has not migrated. I would still recommend the 24 hour cure if at all
possible.

I have attached my previous Emails with procedure for your information.

Earl Gillstrom '88 5Speed


I have some possible good news for automatic owners that are worried about
crankshaft thrust bearing failure. One (now two) of the cars that I have been
monitoring for drive shaft movement in the front hub of the thrust plate has
stopped moving. We did an experiment on Gary Knox's '88 automatic (car "C" in
my previous email) that seems to have stopped the movement of the shaft. Here
is what we did:

Removed the pinch (clamp) bolt in the front driveshaft hub. This relieved the
thrust plate tension and allowed the shaft to attain the proper position.

Poured Loctite 290 (penetrating formula) into the three slots in the hub. The
intent was to Loctite the shaft to the hub without disassembly. Disassembly
requires removal of the torque tube. You should "prime" parts before
assembly, but it is pretty time consuming to R&R the TT.

Installed a new bolt P/N 931.421.240.00. using Loctite 242 before assembly,
torqued to 65 ft.lb. Porsche had recommended to Fred Rourke's mechanics,
replacing the bolt and tightening to 110% of recommended torque. The standard
torque is 59 ft.lb. The Loctite was our idea. I doubt that the Loctite on the
bolt was necessary, but it can't hurt.

Let the car sit for 24 hours. Loctite says that a full cure takes 24 hours.

The car now has about 2000 miles on it since we did this, including two days
of drivers education with NO hub movement. You can say that this is not a
long term test, which is true. But we checked this car previously and found
that the hub had moved .016 in about 2,000 miles and one DE. The first check
we did, there was .105" movement (40,000 miles). So it is very encouraging.
This is not an expensive procedure. About $6.00 for the bolt and $6.00 for
the Loctite. It takes about an hour or two depending on experience. Check the
crankshaft end play while you are in there.
We painted the shaft splines white where they enter the clamping hub. After
reassembly we could look through the hole in the bottom of the bell housing
cover and see the painted splines. We think that we can do a quick spot check
for shaft movement without any disassembly at every oil change by looking in
the hole. Maybe a small diameter fiber optic light would make the job easier
but the white paint helps a lot.

I have included my first email on this, as well as the procedure. Car "D" was
done on 11/11 using Loctite on the splines and was added to the original
email. Unfortunately we used red paint, so it may be harder to check hub
movement without disassembly.

Here is the first email sent on 9-24-00.
I have been assisting 3 friends with automatic transmissions in trying to
determine if crankshaft thrust bearing wear is excessive on their cars. For
simplicity, I have named the cars A,B and C. Here are the findings. "Hub
movement" means: with a dial indicator on the center of the thrust plate hub,
loosen the hub clamping screw and record movement of the hub. This shows the
preload on the crankshaft. Presumably, no preload is desirable.

Car Year Mileage Hub movement CS End play


"A" '89 85K .120" .040" .009"

"B" '89 114K .100" .002"

"C" '88 44K .105" .016 .007"

"D" '89 58K .134" .005" (added 12-05-00)

We also checked the difference in preload caused by a cold car and a hot car
using car "C". After making the initial check and finding .105" hub movement,
we tightened the hub clamping bolt with no preload. The car was then driven
~2000 miles including one DE. We then checked the car cold and found .016"
preload. Hub migration? We reclamped the hub and drove the car ~30 miles and
rechecked the preload hot and found .003" preload. It appears that the
shaft/torque tube do not change enough in length from cold to hot to affect
the preload. Or was the .003" change caused by migration? Car A was also
rechecked after 2,000 miles and the hub moved .040".

It also appears that the hub does migrate forward enough to put significant
pressure on the thrust bearing, although the wear on cars A and C seems
acceptable.

To try to stop migration on car "C", we used Loctite 290 penetrating formula
on the hub splines and 242 on the threads of the clamping bolt, although the
bolt does not appear to loosen. We will check periodically to see if this
stops migration.
( the results are above)
On cars "A and "B" we used 271 on the threads. All bolts were replaced and
torqued to 110% as per advise from Porsche.

One thing that we don't understand is: why does car "B" only have .002"
crankshaft end play? On car "A"and "C" the crankshaft "clunks" back and forth
when we pry them. Car "B" does not appear to move, but the dial indicator
shows .002" movement. The specification is .00236" (.06mm) minimum. It is
hard to believe that a car with over 114,000 miles has less than the minimum
axial play. (Update: 12-05-00 an oil analysis showed no problems.)

Is this a symptom of the dreaded thrust bearing failure? Is there another way
to check for thrust bearing failure without tearing the engine down?

Earl Gillstrom '88 5 speed (.007 CS end play) 99,000 miles

Here is the procedure:

Drop the exhaust at the manifolds.
Remove the bell housing bottom cover.
Install a dial indicator on the rear side of the forward Drive Shaft Hub.
Loosen The hub clamping screw and watch for hub movement.
On the three automatics that I have done this on, the hub moved back between
.100"and .120". This preload puts pressure on the rear face of the crankshaft
thrust bearing. If the hub does not move more than a few thousandths of an
inch when you loosen the pinch bolt then all is OK. (Update 12-05 -00,
Car""D" had .134" movement.)

After the pinch bolt is loose then put the dial indicator on the flywheel,
(not the flexplate). Pry the flywheel back and forth to check the crankshaft
end play. The spec for later engines is .0024" to .0076". Wear limit is .016".

If your end play is more than .016", then you are experiencing "Thrust
Bearing Failure".

If you are installing a TT, clean the shaft and hub splines with Loctite
primer before assembly. Tighten the forward hub pinch bolt last.

Pour Loctite 290 (penetrating formula) into the three slots in the hub. It
doesn't matter if hot or cold or the position of the crankshaft.

Install a new bolt P/N 931.421.240.00. using Loctite 242 before assembly,
torque to 65 ft.lb. The standard torque is 59 ft.lb. I doubt that the Loctite
on the bolt is necessary, but it can't hurt.

Paint the shaft splines white where it enters the hub for future verification.

Reassemble cover and exhaust.

Let the car sit for 24 hours. Loctite says that a full cure takes 24 hours.

Look through the hole in the bottom of the bell housing every oil change to
check for hub migration.

The pinch bolt is 10 mm with an 8 mm "Allen" head. Hardness is 12.9.
Part number is 931,421,240,00.

I will try to assemble a list of results if you try this experiment. Let me
know how you make out.

Earl Gillstrom '88 5 speed

Hope that this helps someone.


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posted 02-24-2002 23:41
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STEVE C
The Great White 928
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Snowball the 81 white 928

here's another one:

<a href="http://www.atra-gears.com/crankshaft/" target="_blank">http://www.atra-gears.com/crankshaft/</a>

Greg86andahalf

Snow,

The pinch collar wrongly allows the hub to slip forward. Imagine it slipping and re-gripping closer to the flywheel. The flexplate absorbs alot of this forward pressure or preloading, which is good. Since the flexplate is being pushed, it has a loaded force which "pushes" on the crankshaft, which is bad, hence the bearing failure. What you are doing in this repair is releiving the forward pressure of the TT on the flexplate.

When I loosened my pinch collar, the hub popped back a good amount. That is the flexplate unloading. Now, measure the amount of play in the crank by moving the flywheel fore and aft. The locktite procedure is an attempt to keep the hub from migrating again. If you put some index marks in the right spots, it is possible to inspect future migration by looking up into the housing through the hole with the rubber plug at the housing bottom.

I had my forward exhaust down for a O2 sensor replacement, allowing acess to the housing. The pinch collar procedure didn't take that much time. You can use a dial indicator or a dial caliper with a bit of invention as to where to take accurate measurements. Needless to say, take a few measurements, keep them for future comparison, and mark on the shaft, where the hub is after you completer the repair. In the future, your'e looking for foward slipping.

Greg

Donald

Is it correct that this problem results primarily from automatic transmissions?
TIA
Donald

Greg86andahalf

That is my understanding. There is some thought that it is also only in the later MY cars, but I think it is good PM to check it in any automatic MY car. My 86.5 had preloading but it did not effect the thrust bearing.

Snowball the 81 white 928

has anyone ever tried to add a second pinch bolt to the collar (for added clamping power) following removal of the torque tube, you know, the old "two hands are better than one theory", or are there balance and/or clearance issues involved? this is really a weird problem but from the link i posted above it does not seem isolated to porsches but affects chevy's and large diesels as well. Picture a jumprope at rest and in use. at rest it might be 10' long. when in use, as it is spun and it's arc widens in the center, it's length shortens. MY THEORY is that worn torque tube bearings allow the driveshaft's arc to widen in the center, thus pulling back from the collar assy in front. maybe more occurence on the automatics because only 2 bearings used instead of 3 on the manual trannys (a fix might be to use 3 bearings on TT rebuild on the automatics, anyone ever tried that?) this is simplistic and there appear to be many other theories, but the only other one that makes any sense is that of torque converter ballooning, but that would not explain the shaft being pulled REARWARD out of the collar instead of being pushed FORWARD into the collar. i cannot subscribe to the theory of the driveshft "growing in length" due to winding and unwinding. i think a 3rd TT bearing might wrap this problem up and put it to bed forever.

Snowball the 81 white 928

i just spoke with a very trusted mechanic and he too feels that this problem is related to the health of the TT bearings although he found it hard to believe that a solid steel driveshaft could have THAT much flex in it to cause it to pull away from the collar. i then argued that the driveshaft is about 6' long and that 2mm of movement is entirely conceivable due to flex. he also made the suggestion of drilling some type of pin into the collar and driveshaft (as has been suggested by wally on one of his earlier posts.)

what my mech did point out that was very interesting is that if you will view the picture in the link below, that the orientation of the pinch bolt in the pinch clamp should be PERPENDICULAR to the "slits" in the collar in order to maximize clamping force. as shown in the picture, clamping force is NOT maximized as this particular type of clamp does not maintain uniform clamping pressure "all around" as would, say, a screw-type hose clamp. this argument is blown to hell, though, if there are 2 additional "slits" that cannot be viewed due to the angle of the photo. I DON'T KNOW WHAT IS THERE AS I HAVE NOT BEEN IN FOR A LOOK-SEE.

as a temporary time-to-maintenance extender, i think that if one feels that their TT bearings may be suspect, don't demand top performance from the car (ie: ragging it out) as this, in combination with worn TT bearings, is what may be causing the flex that causes the driveshaft to pull AFT.

my mech also agreed that the earlier, lower hp/torque engines may not have enough oomph to flex the driveshaft relatively as much as the later engines. he also felt that it was interesting that the problem did not show itself as much on the manual tranny cars with 3 TT bearings.

<a href="http://members.rennlist.com/v1uhoh/pinch.htm" target="_blank">http://members.rennlist.com/v1uhoh/pinch.htm</a>

have i stirred up the kettle enough for today?

Greg86andahalf

Snow,

Clearence for a second clamp is there. I read a interesting post where someone was asking the question : why clamp it? If the hub rode on the shaft, like a driveshaft yoke on a rear wheel drive car, the preloading would be eliminated. I suppose that different materials and splines would be used if one were to try and make it work that way.

So your'e thinking that the shaft pulls back, the collar regrips, then the whole thing rams forward? That explains how the pinch collar travels forward up the shaft. It's really the shaft travelling back. Interesting.

I know little about how the shaft attaches to the back at the trans, but there has been discussion of that attachment becoming loose as well. What if the rear attachment is a bit loose, allowing aft movement of the shaft, then it regrips with a forward surge, causing what is described above?
That could explain the pullback of the shaft at the front, allowing for migration. HMMMMM.

When I loosened the pinch collar in front, the hub popped back on the shaft. Maybe the shaft is suppsed to be further aft. The only reference I have is a sticker on the shaft which was scraped and bunched up by the hub, probably on original install. When the hub popped back, it returned to the spot where the sticker was. So, would that mean the shaft is in the correct position? Maybe a rear inspection is in order.

Snowball the 81 white 928

someone stated earlier that the rear pinch clamp fits into a groove in the driveshaft, so i don't suggest much movement there.

Mike Schmidt

One theory is that the drive shaft shortens when torque is applied by the engine, the same way that a towel shortens when you wring the water out of it. When the application of torque to the shaft is stopped, the shaft untwists and returns to it's original length. The shortening during the twisting pulls the end of the shaft out of the front clamping sleeve, and then when the shaft returns to it's original length it's exerting forward pressure. How much the shaft twists and shortens is anyones guess, but it would twist when torque is applied by the engine. There are some automatic cars out there that have three bearings in their rebuilt torque tubes.

Snowball the 81 white 928

yes, my $$$ is on that theory. here is one more question. suppose some schmuck were to strip the clamp with the clamping screw. THIS IS HYPOTHETICAL AT THIS TIME! is there enough room to replace the clamp without removing the entire torque tube (ugh) by unhooking the collar from the drive flange, unhooking the drive flange from the flywheel, removing the drive flange through the lower access area, then sliding the collar toward the flywheel and off of and clearing the driveshaft. would the collar slide forward enough to slide off of the driveshaft and allow the clamp to be passed between the driveshaft and the rear of the collar? big $$$ question, who has the answer?

John Struthers

Now this was a tasty post, indeed!
Pattycakes comes home tomorrow, I'll park her till I have a chance to peek on Saturday.
I tend to lurk in 2nd between 3200 and the high end of the 5000 rpm range. purpose of which is to be "on the cam" and at a legal road speed when a roll on event occurs. In particular when the unsuspecting checks me out, pulls along side and pushes in the clutch to give me a rev', he is usually out of kilter on the rpm/road speed when I INSTANTLY pull away. If he downshifts at to high a road speed to get his rpm back up he usually goes over the top and has spent several seconds shifting while I'm walking up that wide powerband of ours. So, it would appear as though my working pressures should be on the high side and consequent TC balloning probable.
Time to mend my ways.
Thanks to all on this series of post and replies.
John S 'still in the ford' <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />