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 a condensed version of my previous Emails due to rennlist size constraints.

Earl Gillstrom '88 5 Speed

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.

Earl Gillstrom '88 5Speed

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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
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Constantine,

The major advantage of the "Loctite Fix" is that it is inexpensive and easy to do. The Loctite "Fixes" the worn flexplate hub so that you don't have to replace it. You have to pull either the engine or the torque tube to replace the flexplate/hub. If you are going to do that, then why not install your sliding hub?

Since your sliding hub is still in beta test, do the "Loctite fix" now and when the sliding hub is available then install it.

If hub migration is the only cause of TBF and the "Loctite Fix" corrects the problem, then the sliding hub may not be necessary. I don't think that hub migration is the only problem causing TBF.

My advise to automatic owners is: do the "Loctite fix" now. If you have the TT or engine out, then, install the sliding fix. If you like insurance, then, pull your TT or engine and install the sliding fix. If you don't install the sliding fix, then check your crankshaft end play often.

It is fun to "second guess" Porsche engineers. What if they would have used the 5 speed drive shaft, including the clutch stub shaft and then installed a clutch hub in the flexplate. Then they would have solved the problem and simplified the parts list as well. Maybe the automatic driveline was designed first? Maybe they underestimated the significance of a 35 pound "flywheel"
(torque converter) on the end of a 10 foot pole? Ain't hindsight great?

Earl
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Earl and all others doing the Loctite fix:

as you mentioned, on some cars (including mine) it is necessary to drop the exhaust in order to take off the flywheel cover.
Since I didn't want to do that, I managed to do everything through the little access hole while leaving the cover in place.

For illumination I used a simple brake-light bulb soldered to a wire and connected to the jump start terminal. That bulb fits nicely through the access hole and (using a wire) can be hung over the shaft, so that it stays in place.

The access hole is also large enough to get a socket wrench extension through, in order to loosen, replace and tighten the pinch bolt.

To apply the Loctite I used a special kit designed for infusions (the needle is connected through a small diam. plastic tube to the syringe) After fixing the needle to a wire (about 20 cm long) , I could apply the Loctite to the slots in the flex plate hub.

Even the paint marking worked through the little hole: this can be done by re-drilling the nozzle of a regular spray can to about 2 mm diam and insert a brass (or plastic tube) about 20 cm long . If you squeeze the end of the tube almost shut, it works nicely as spray extension.

All that was done within 15 minutes
Hope that helps

Kurt

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despite of the manual which says the bolt should be torqued to 60Nm, its mentioned in a service bulletin that 80Nm is appropriate.

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The Loctite 290 fix failed !!!!

The loctite fix I did a few months ago was checked yesterday. Regretfully the clamp migrated over the axle. The paint dot that I used as a marker clearly shows that the loctite fix is not enough. I will be looking for a proper solution as this loctite "fix" is not reliable for me.

John.