Black Sea RD

Hi Fred,

Checking the TT bearing units for travel is a pretty simple procedure and we believe a yearly check will help spot the start of them traveling or other problems with the drive line.

These checks should be done in conjunction with the front flexplate checks for automatics as well as clutch plate checks for the 5 speeds. Checking the TT bearing units will add a whopping 5-10 minutes more to being under the car.

One thing we have noticed is that many people do not do at least a yearly visual check of their engine bay, drive line, brakes, transmission, etc. This is a good practice to get in the habit of doing since spotting something odd can help catch something before it becomes a bigger problem.

And by the way, we don't recommend using any loctite at the front flexplate coupler. Just adds more problems for the mechanic/DIYer when working on the drive line. Take care of the migration the right way and never worry again about the drive shaft migration/TBF issue.

Cheers,

Dave928S

Sounds like very good advice to me ... if it ain't broke usually it's cheaper and safer to fix it before it does. It's a sad fact that so much repair and maintenance on not only cars, but lots of other equipment and property, is reactive. I see it all the time ... and wonder why simple repairs and preventative maintenance weren't undertaken. The belief is that it saves money but it's a fact that planned preventative maintenance costs less in the long run.

FredR

Hi Constantine,

Trust you are keeping well.

I recommend that owners of automatics sort their driveline out well and truly. The stock bearings do eventually wear and the rubber bushes they sit in do not last forever [esepcially in hot climates] and your kit is a real enhancement. Once that is done together with the torque converter bearings there should be no need to do any more for another 5 years/100k km minimum- after that more regular inspections become the order of the day.

If one is in there then by all means take a look/check the bearing location- it is easy and quick enough. The key is pre-emptive maintenance/upgrade and not a reactive approach especially for the clamping. Whether one uses loctite, the superclamp or the pklamp, something is required and the stock setup is simply not an option. The mind set that "the clamp has not failed in 20 years" is not acceptable.

I am happy with my driveline and have no vibrations I can detect and the flexplate coupling is well sorted with the loctite although I must admit having cross checked it twice at 6 months and 1 year since setting it, I have moved into "5 year mode" mentally. For sure if I as much as sense a vibration in the driveline I will pull the lot and move to your bearings and probably the super clamp if I survive workwise.

Regards and best wishes

Fred

Black Sea RD

Hi Dave,

Yes and agreed! If this was done more often there would be many more 928s on the road!

Seems as if you speak from professional experience!

Cheers,

Dave928S

Hi Constantine ... I've got a fairly wide experience base in Engineering, Electronics and Construction and I built and raced cars here in Australia for a decade in the 70's & 80's. I now work as a Consultant in the construction industry assessing, advising on and troubleshooting buildings, equipment and construction work. I've always admired good engineering and workmanship (and I tend to be a bit of a perfectionist) and cars have always been a passion. I've seen plenty of dreadful work in all those fields and every now and then some that's brilliantly executed .. thus my comments. As I've been posting here a bit lately, I should do an introduction thread with some photos ... as soon as I can ... and I can elaborate.

Jerry Feather

Hi Troy:

I think it is amazing and wonderful how so much useful information seems to come out of the woodwork, so to say, when someone like you posts an interesting problem about their 928. I really appreciate it in reading these threads.

My first impression when I read your first post about this was that the only way for this kind of flex plate failure to occur was to have the flex plate flexing in such a way that was not intended for it in the first place. The fact that there are no other threads about your kind of failure seems to confirm that. Since the flex plate is likely flexing in all the automatic cars as intended, but without failure, that suggests that in your car it is flexing differently.

I have dealt with fatigue cracks in metal frequently with my airplane and it is always vibration induced. In your case I can mentally picture the crack(s) you have been having, and without a picture, and I see the crack or broken-off corner of the plate about an inch to an inch and a half inward from one of the three corners.

I think that the way this kind of failure occurs is for the flex plate to be flexing in and out with each revolution of the drive train. This is going to happen ONLY because the crank shaft and the drive shaft are not rotating on EXACTLY the same axis. I was going to suggest that when I first read your post, but Nicole beat me to it--her post number 3.

Then I have read the other information and found that of all of it, what Karl had to offer in post number 31 gives the best insight into what your problem is. So far as I can tell, he is the only one who has experienced anything like what you are dealing with, even though he was having his experience with the clutch plate rather than the flex plate. The problem is exactly the same as his. Until I read his post I could not visualize or conceive of how the drive shaft coud be rotating on a different axis, and I discounted it being a result of a factory defect.

I can drill a hole around a corner so to say, with a very long thin drill bit by flexing it into a curve and having the whole thing rotate while bent in the curve. That is more difficult as the bit gets thicker or shorter. Since the drive shaft of the 928 is relatively stiff I don't think it will work well rotating while curved. There is not enough "stuff" there to hold it in a curve while rotating. Therefore, if the Torque Tube is not in exact allignment with the crank shaft the flex plate must flex in and out at each corner with each rotation. It is going to fail just kike Carl's clutch disc. Since they are not rotating on the same axis they need to have a U-joint in between or something has to bend with each rotation. The flex plate is the only thing available to bend.

I think you need to find where the misalignment is in your drive train. The Tube is the most likely culprit. If it is bent either replace it completely or find how to true it up like Karl did with his.

I don't challenge all of the other suggestions except to say that most of them describe problems that are relatively common, but none of which have apparently resulted in your kind of faliure.

I hope this small contribution helps out a little.

Jerry

P.S. Another thought that I just had is that the noise you are hearing after the flex plate cracks pretty far or the corner has broke off, is because it is now easier for the flex plate to flex at the other two corners, but the cracked corner is now rubbing the broken or cracked surfaces against each other giving you the terrible noise you are hearing. That tends to confirm for me that the crank and drive shafts are not aligned.

Jerry Feather

Just to clarify my description above about the flex plate flexing in and out with each rotation--what I mean is that it is flexed in on one side while it is out on the other with each rotation and each side/corner changes in and out each rotation. That is opposed the the whole plate flexing in and then out all corners together at the same time. Is that confusing or what?

Jerry

Jerry Feather

Further, in regard to my Post Script, I think that if the corner is in fact broken off, or when it gets broken off, and the noise gets intense, then the flex plate has become a "wobble-plate" rather than a flex plate. It will still be flexing a very little bit but more of the movement will be simply a wobble in and out .

In fact, if I am correct about this theory of mine, if it is broken off, you could simply take out the broken piece and grind the broken surface off a few thousandths so it would not touch the body of the plate and put it back in for balance and drive the car like that. I would tighten the other four plate bolts to be sure that the plate stays centered, and I think you could drive the car some distance until you could get home or get a proper repair.

I hesitate to suggest this because I gave this kind of advise before and later found that the recipient of the advise--to get it home and fix it properly--was still using the stop-gap repair since "it is working just fine." That didn't have to do with and 928, but an airplane landing gear.

Jerry

Tails

Troy,

I gave my 2 cents worth on the basis that the drive shaft and crankshaft were in alignment and the possibility of a forward bearing migrating, so now I will give you my $1.00 worth for considedration and it is a little long.

The various order of induced vibration from an 8 cylinder engine, IIRC, are 4,8,16,32 etc and are rpm dependent.

The best way to describe induced harmonic vibration is by the method of twirling a button on a piece of string while holding the ends or the string with your hands. If you pull the string tight the button will increase it speed of rotation as you have reduced the radius of rotation of the button and if you decrease the tension in the string by moving you hands closer together the speed of rotation will decrease as the radius or rotation of the button decreases, same as when the rpm increase or decreases. The harmonic vibrations follow a sine wave curve, same as the button as it rotates, the amplitude is equal the radius of rotation and your hands are the nodes where the sine wave crosses.

Now if we transpose this image to your drive shaft, we have a sine wave that is connected via the flex plate at the engine fly wheel end with the aft bearing in the engine is a node, the bearings in the torque tube are also nodes and the bearing at the torque converter is the last node.

If all bearing in the torque tube are equally spaced and you draw a sine wave curve through each bearing as a node you will end up with a sine wave with equal amplitude, like a string on a gitar when pluked, however you have node crossing points in your torque tube where the amplitude is 180 degree of of phase and then it crosses over at the next TT bearing point and the phase difference is again 180 degrees. As the rpm of your engine increases then the amplitude of the curves will very their amplitude from the induced vibration caused by the cylinders firing, hence the term harmonics and the use of the harmonic balancer as it is capable of tuning out the amplitudes of the sine wave by varying the length of the overall shaft, crankshaft and drive shaft.

If the front torque tube bearing migrates aft along the torque tube then the sine wave amplitude will decrease as one bearing moves closer to the other bearing and where the distance between the bearing and the flexplate increases the amplitude will increase and the forward flexplate will set up is wobbly vibration as previously described due to the increased amplitude of induce vibration acting upon it.

Now if the front torque tube bearing has not moved then this is a different story as the drive shaft should be detuned and be operating in axial alignment as designed.

Now Jerry's and Nicole's missalignment comes into play, so all needs to be checked to verify whether it is out of alignment and what is the cause.

We know that the flexplate and flywheel come in a matched balance set (this is probably to ensure that no harmonic out of balance forces are induced that cannot be detuned by the harmonic balancer) it requires the drive shaft axial alignment to be checked to the axial alignment of the crankshaft.

When the torque tube is buttoned up to the the bell housing and the gear box is should all be in axial alignment and pin located to be in alignment after any repair or disassembly.

If the TT bearing are in good order and conditon then the drive shaft, and crankshaft should be in the same axial alignement, however to check for misalignment is somewhat difficult in situ. Bill Ball is the master of working in the confined spaces in this area, view his avatar.

The missalignment could be that the torque tube is out of alignment with the bell housing, the drive shaft could be bent at the forward end or the flywheel has not be installed correctly and is out of vertical alignment with the crankshaft.

Ball Ball, Constantine and others have been in this area numerous time before, however I have not done this with a 928, but I have done it with long line shafts , so I would suggest that the forward flexplate should be disconnected from the flywheel and and the drive shaft checked for misalignment with the crankshaft by measuring the alignment of the flywheel to the flexplate at the outside fins of the flexplate.

I would also check the drive shaft by rotation with a dial indicator on the outside diameter of the drive shaft.

I would also check the vertical alignment of the flywheel to the crankshaft by rotating the engine with a dial indicator on a vertical preifery face of the flywheel.

If you have out of alignment of the axis of the crankshaft to the axis of the drive shaft and the drive shaft is not bent, then this would indicate that the alignment pins are either missing at the forward end or if they are not then the TT is out of alignment to the bell housing and the engine block.

If you have out of alignment of the flywheel vertically then this will have to be rectified and it will posssibly have to be removed, inspected, repaired and replaced.

When you have checked all of the above hopefully you will have located the fault and then the path should be clear for the required repair, however, if all is in alignment then we have another problem.

Keep up posted.

Tails 1990 928S4 Auto

Dave928S

Troy
A further check to those I made in posts 42 & 43, recommending dial gauge and other checks of flywheel, flexplate and driveshaft, to determine where the problem is .... check that your block to bellhousing and bellhousing to TT bolts are tight.

troy928

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Jerry, Tails and and everyone else that has contributed to this thread...
THANK YOU! Jerry you must have telepathic capabilities because you nailed it, here is an image of the flex plate.

Sorry I have not been able to post sooner but Mothers day cut down on my "playing with your Porsche time" as my wife would say. Finally got the damn loctie broken free...MAPP gas instead of propane finally got it.

Tomorrow I will measure the crank end play, and give you all details of shaft and bearing movement, but I think Nicole and others were right, the shaft is misaligned to the flex plate coupler from my inspection. I will keep you all posted with more details and images.

Cheers to all and thanks.
Troy

troy928

Dave,
I have done all the wrenching to my car except one thing, installing motor mounts.
After my experience with this shop, I made up my mind never again will I have someone else work on my car! (long story and this is when my problems began)
So you may be on to something...I will check that too tomorrow.

Cheers,Troy

Black Sea RD

Yikes!!

And this is your third one?

This sort of vibration should have been felt by you in the cabin area.

How does the drive shaft feel if you grab it and try to move it around?

Keep your findings coming.

Regards,

Nicole

Were the motor mounts done before the flex plates started to break?

James Bailey

I would suspect a bent torque tube housing. I have seen may over the years. If the mounting flanges at the trans and at the bellhousing are not true. Then the flex plate is forced to flex more and much more often than intended like once per revolution. It can also cause the rear plate which bolts to the torque convertor to fail if the trans input bearings are not 100%. Worth mentioning that used torque tubes tend to come out of crashed cars....