Glenn M

My 88 has the circlip grove at the end of the shaft. So where do you move the crank, aft or forward?

Glenn

PorKen

Below is an interesting take on the problem. Instead of the TC expanding, the converter is pushed forward by pressure in the converter against the converter hub, especially when the cooler circuit is blocked/restricted. (Note that 928's have banjo fittings at the cooler connections, and >20' of cooler pipe.)

http://www.gmcws.org/techcenter/99-12tc27.html

Far as I can figure, you'd still have to have worn converter bearings to facilitate the movement.

Bill Ball

The factory procedure is to lever the flywheel/crank to the rear (aft).

If you had the circlip, retainer and shims, the procedure would be to loosen the TT clamp, unbolt the flexplate arms from the flywheel, lever the flywheel/crank to the rear and install shims until the flexplate is held 0.3 to 0.5mm from the flywheel surface, then bolt the flexplate arms to the flywheel and tighten the clamp bolt. See dimension 7 below. The details of the method in the WSM show more involved measurements, but it boils down to dimension 7.

Black Sea RD

Nice article and susbstantiates what I learned along the way of getting to our new flexplate clamp. The articles do focus on GM products and their engine/transmission interface is of course different from the 928.

Also interesting about the different finishes on the crank journals and the use of inferior bearings in engine rebuilds. Another reason to have someone like Greg B. do a 928 engine rebuild who is a known quantity.

Constantine

Bill Ball

Update:

Date: May 5, 2009
Mileage 201,970

Flexplate preload released: 2 mm
Crank endplay: 0.008 in (0.20 mm) Unchanged from June 26, 2007

heinrich

Thank you, Bill and Tony

heinrich

What about crankshaft surface machining then .... could that be the answer in some way? Interesting find, Ken.

heinrich

Actually I think we can almost be sure it isn't that, Ken ... because if the torque shaft were being forced forward by the torque converter, the clamp would theoretically move BACKWARD on the splines, not pull out forward. Which still returns us to the theory that the shaft twists under torque; shortens momentarily; pulls backward out of the clamp; and when twist force is released, relaxes and lengthens again .... time after time after time, resulting in a forward-migrating clamp and flex plate.

PorKen

I don't go for those fancy theories anymore. I think it's fastshaft wrap, as you say.

I checked my flexplate again, with the prototype PKlamp on there, yesterday. Still no movement (I scribed the shaft at install). This includes a bunch of dyno runs, and lots of jackrabbit starts.


The question that's knocking around my (empty) head now is: if the clamp no longer moves, should I preload the flexplate as if it had shims on it?

Bill Ball

The factory procedure produces a small amount (0.3 + 0.2mm) of resting preload, BUT that is with the crank pushed to the rear. So, the actual functioning preload is much smaller. In my case, with the crank forward, the preload would be only 0.1 + 0.2mm. Hardly worth fussing over.

Black Sea RD

Those fancy theories have never been a question for us about the cause of most of the drive shaft pullout problems seen by 928 owners. However, the other possible causes can't be entirely forgotten either.

As far as preloading, the answer is in the Porsche manuals. Porsche did not specify this preload any longer after they stopped using the circlip, shims and bearing at the front of the drive shaft. We always recommend the crankshaft is pulled back toward the transmission before tightening our Super Clamp, which is the very last step before putting the lower bell housing cover back on. This negates the possibility of someone tightening the clamp with forward pressure on the thrust bearing.

HTH,
Constantine

mj1pate

If I choose to install the circlip, shim(s) and bushing(?) on the leading edge of my drive shaft, and the flex plate isn't distorted, is it just a matter of installing the circlip and shims so that there is zero tolerance between the circlip-shim combination and the rear mating surface of the flex plate? I'm not sure that I understand how the bushing fits in with all this from the shop manual (pg 39-141) I haven't done this before and would like not to make a problem where one doesn't exist today.

Bill Ball

Michael:

Here is my interpretation on how to do it from earlier in this thread.

https://rennlist.com/forums/4875285-post93.html

Note it will take some trial and error my way, and you will need a small rather than beefy circlip plier to get up into the cramped area between the flywheel and the flexplate. I did not have a lot of trouble doing it on a 91 S4 with a rebuilt 25 mm TT.

mj1pate

Thanks Bill. I realize that I'll have to get back under there, to start figuring things out. From the picture, it looks like a flat feeler gauge has to be pushed down between the flex plate and the fly wheel?

Bill Ball

Yes.