Jerry Feather

Actually there is no vibration associaterd with this failure. The flex plate is simply flexing smoothly and continuously with each revolution with one side bent in and one side bent out. There is nothing to feel inside the car.

Jerry Feather

Jerry Feather

Hi Troy. Thanks for the picture. Actually the break is a little closer to the bolt holes that I envisioned. What you are going to find is that the torque tube is bent at or very near the front mounting flange. Karl in post 31 describe what this was like for him; and now James in post 60 is telling essentially the same thing. If the TT was bent near the rear that would affect to rear flex plate, but since it is your front plate the TT has to be bent near the front. Karl also pointed out that he bought a used TT and found it bent also and I think James reiterated this concern. Good luck with the rest of the rapair.

Black Sea RD

Hi Jerry,

Although I would tend to agree if the drive shaft was better isolated, there isn't much in the way of drive shaft dampening in the bearing units that would help negate the oscillations of the drive shaft as it goes round and round.

Many 928s have a vibration of one sort or another and most drivers tend to tune them out after awhile. It would have been interesting to drive Troy's 928 while it had this problem to see what if anything would have been felt by someone else other than the regular driver.

I did drive a 95 GTS with less than 50K miles on the clock. That was the smoothest 928 I have ever driven. Felt like silk!

I guess we have to see what Troy finds out with the "squareness" of his TT.

Cheers,

Jerry Feather

Troy, I would not mess with the flywheel. If it were somehow mounted crooked or off in some way it woud not cause the flex plate to flex like your is. They would both be held in the same proximity to each other all the way around each revolution. That misalignment would result in the failure of something else.

What you need to do is remove the torque tube. Then mount it in some kind of V-blocks made out of plywood or one-by or two-by lumber with a T stand sort of thing. Fix the blocks so they are firm on your bench or the floor if you have to. Then place the drive shaft ends each in the V blocks and then rotate the Torque Tube around the shaft. I think you will be able to see the front end of the tube wobble where it is bent. You can see a few thousandths with your eve if you can spin the tube fast enough, but if not then use the dial indicator approach to the face of the front mounting flange as you turn it slowly. You will find that it is not spinning or turning true.

Get another TT and have them gaurantee it and then check it the same way before you put it into service.

Jerry Feather

Landseer

On inboard skiboats and cabin cruisers the alignment of the long propshaft to coupling at the engine is a critical setup.

You might find some measurement techniques in a search on that subject that could be made with the tube still in the car.

Bill Ball

I didn't read through all the voluminous helpful information here, but scanned much of it. I'm not sure if this was already covered, but I'll ask anyway.

You mentioned you replaced the flexplate. Did you replace the clamp and flexplate or just the flexplate? Did you notice that one flexplate arm has a scallop cut out of it? That is a balancing scallop for that flexplate/clamp combination. The flexplate is sold bolted to the clamp, not separately. I think you may be able replace just the plate as long as you keep the scalloped arm in the same orientation to the clamp, but my vague recollection from the one I have done is that the replacement flexplate had a different sized scallop, so we replaced the plate/clamp combo. It's much harder to do unless you completely remove the TT, as while the flexplate easily clears the TT shaft end, the clamp will not unless the TT is removed or you do some tricky stuff (we did) to get the TT shaft end below the flywheel.

Dan87951

I'm curious if your torque tube is bent? If its bent I could see it putting extreme amounts of pressure on the flex plate from it bending and vibrating with each revolution. Since this is a job you don't wanna do twice I would get a reman torque tube from 928 intl. Mark will take care of you!

Tails

One of the good aspects of the flexplate is that it does not allow any bending stress to be applied to the crankshaft and therefore does not cause any crankshaft deflection in the aft crankshaft "throw".

However if the flyweel is not in vertical alignment to the axial centre of the crankshaft and drive shaft, it will cause a wabble to the flexplate, if the drive shaft and the TT are in axial alignment.

To ensure that you are not inducing any form out of alignment I would suggest that you do check out the vertical alignment of the flywheel by a dial indicator, thereby definately precluding it from having any influence on the wabbling of the flexplate. The problem maybe caused by an a number of out-of-alignments that give a cumulative affect to the flexplate.

If the torque tube is bent, this will put a bending moment force onto the drive shaft, causing the drive shaft to bend in that area with fulcrum points at the TT bearings. This bending moment will impart a twirling effect to the ends of the drive shaft that are fixed points to flexplates causing them to wabble. Depending on where the bend is, that is if it closer to the forward flexplate it will impart a greater twirling effect and the amplitude of the twirling affect will be dependent on the amout of bending of the drive shaft.

The trouble shooting to find the fault could well be an extensive piece of work, however, some checks can be undertaken without a major strip down as previously indicated.

Tails 1990 928S4 Auto

Black Sea RD

Obviously they are not the only ones that can take care of TT problems.

We have a few good TT cores at the ready and with new better engineered internals too!

Good luck with your search Troy!

Jerry Feather

No, if the flywheel is not in verticle alignment with the axis of the crank****, the flex plate WILL NOT WOBBLE!!!! It will simply rotate in that "cocked" position all the way around keeping the same position in respect to the flywheel all the way around. However, if the crankshaft axis and the drive shaft axis is the same and then the fly wheel is somehow canted or cocked a little bit, and then the shaft is coupled to the flex plate bolted to the flywheel, that will put the foreward end of the drive shaft slightly off center causing it to orbit the axis in a slight circle causing it to appear to wobble. That type of rotation will likely transfer the wobble effect to the foreward torque tube bearing and that is likely to be where you will find some failure, not a break or crack in the flex plate.

Jerry Feather

OOps, I meant to say "crankSHAFT!"

Tails

Jerry,

Without labouring a point, I agree that the flexplate at the set bolt locations at the perifery of the wings will not wobble as it is attached to the flywheel, however the flexplate can wobble, but the extent of the wobble will depend on the the strength of the drive shaft and the location of the forward TT bearing. If the engine block, the crankshaft, the drive shaft, the torque tube, the torque converter, the TC bell housing and the gear box are all in axial alignment and the flywheel is canted in the vertical plane, if you take a measurement from the centre of attachment of the flywheel to the crankshaft to the end of splined coupling of the flexplate that is clamped to the drive shaft, you could calculate the out-of-alignment of the coupling to the centre line axis of the drive shaft if the flywheel is canted and this measurement will be a leaver to bend the flexplate at the centre coupling to the plate.

This out-of-alignment will cause the centre flange of the flexplate to wobble around the vertical axis if the drive shaft runs in line with the centre axis. This wobble will be absorbed by the wings of the flexplate, however there will be bending affect in the flexplate up to the attachment to the fly wheel.

At this juncture a picture would be worth 10,000 words.

We had better watch out now, as I can feel the pop corn coming out.

The point I was trying to make to Troy was that there could be a collection bending forces coming into play that could accumulate to cause the fracture and it would be advisable to discount the easy ones first otherwise there could be many $s and time spent in repair by renewal.

Tails 1990 928S4 Auto

Jerry Feather

Yes, Tails, I can tell what you are trying to say, but each time you say it you are wrong. When the flywheel is canted and the flex plate is bolted to it it too is canted. Then when you bolt the crankSHAFT into the flexplate clamp it may bend the flex plate a little to accomodate the misalignment. However, when this assembly rotates to the other side of the arc they will still be in the same relative position to each other, They will not flex the other way as you suggest. Therefore NO WOBBLE!

I know that Troy appreciates all of the help we can muster for him, but I think it is a potential waste of his time and effort to be messing with the flywheel when that is not causing his flexplate failure.

Black Sea RD

Hi Jerry,

Guess you meant drive shaft?

Troy sure has gotten his money's worth from this thread!

Really would like to hear what he finds as he digs into the problem a bit further.

Keep us posted Troy!

Jerry Feather

Yes, There is probably enough verbiage in this whole thread to write a script for a new video on the Turbo Encapsulator.

Driveshaft! Driveshaft! That's the word!!!

Jerry