951and944S

That diff has been worked on before.

In your 1st picture, post #14, the view you have through the rectangular windows at the bottom of the external splined (4 prongs) show rotational wear that is impossible to occur since that plate is at one with the diff housing.
The only way this wear pattern can occur is if the plate was previously flipped, facing an internally splined (clutch) disc, which is normal since the discs "slip" while clamped due to the pressure of the diaphragm conical pressure plate.
Unless the picture is deceiving and that is the cup spring I am seeing but if it is, it should just look like gray metal, like the one in your last pic next to the broken washer.
There were two cupped spring plates right...?
One on each end....?

Can you snap a shot of the steel plates (4 prong) and the clutch discs....?

There is some damage to the diff housing, probably from a previous failure which warranted the fix attempt when they flipped the steels.

Lets see them.

The amount of the measurement (total depth of housing minus stack of new discs with thrust rings) that clamps the plates via compression of the cup spring (diaphragm conical plate) is not very much, (1.2mm from memory - could be wrong on that).

I'd also like to know for sure there is no noise when you run the engine with clutch released.

Not convinced your clutch disc drive isn't damaged.

The diff is easy to restore to like new condition, as long as the plates with the outer tangs don't hang in the housing due to damage to the tang slots.

Pretty much stuck with original parts for sourcing and for what the are, the price per plate is pretty high.

I will walk you through the diff refurb.

T

951and944S

Kevin, what do you make out of the damage to the ID of the housing itself at the 1 o'clock spot, to the right of the window...?

Previous failure...?

T

Alex89

I did read a bit further in the service manual and found the section talking about this. Mine does have a yellow dot. Interesting to know it wasn't necessarily an assembly mistake.

Obviously not what I was hoping to hear but I appreciate the feedback. I got your message earlier and will get back to you.

This is what I was trying to point out with that photo but I realize I wasn't clear. The piece with the wear pattern is the conical spring. I can't see any way that this wear pattern could happen. It also appears on the inside of the housing, visible in the photo I took looking straight down (circled in red). The spring plate should be sandwiched between the housing and the plate with the 4 tabs. I don't see a situation where it would rotate to cause this wear. There is no wear on the other side of the diff housing.




And the spring washer itself (those spots are just marks in the oil, not dents)


I think what you are seeing is just a reflection on the inside of the housing. I didn't notice any visible damage to the housing or to any of the plates, except for the inexplicable wear pattern on the one spring plate and the broken washer. Here are a a couple pictures I took earlier of the inner friction plate and the outer plate off one side (the other side looked very similar). I can take more later this week if you like.





I've had the heat on in the garage so was avoiding opening the door to run the car. But the heat's off now so I made you a quick video

Thanks for the help. The tangs seem fine so I think refurbing the diff should be pretty straightforward if I decide to do it myself. Might not make sense if I'm getting someone else to rebuild the rest of the transmission. I have to do some more research and see.

951and944S

Ok, yeah, that's what I was asking.
The picture was deceiving in that it looked like there was an outer splined plate against the diff housing with no cup spring on that side.
People will "flip" those plates and install so the side never contacted by the clutch disc faces opposite when installing new discs.
Fine if they measure in spec and are not warped.
You need to mic the thickness of the clutch discs (inner splined) to tell which of the thickness options you have, you can bring this diff up to Turbo S spec with the thicker, more robust clutches if you have access to a machine shop that can reface the pressure ring (thick part with 4 tangs) to precisely set your clearance.

When you have to replace the ring and pinion or the case, you need to accurately measure the diff carrier center line to the face of the installed pinion gear.

When neither of these items requires replacement, you can replace the bearings without the complication.
Especially in a case like yours, only surface pitting of the races due to metal contamination from the oil running through them, you can carefully press these off and accurately measure the sum of the bearing + race with a depth gauge on a metal table or surface. Then you do the same with your new replacement bearing.
When you see that the new and old bearing is identical in this fashion, you can safely restore the transmission to it's original settings.

You will need a hydraulic press, a bearing splitter and a thin jaw puller at a minimum.
Most of the puller and pusher tools I have are self fabricated from differing steel tubing diameters, etc.

I may have some of the diff thrust washers in good condition if you can't source one.
The front pinion bearing was in scarce supply last time I needed one but if you get the trans code off top of bell housing, you can verify that one is available.

I'll keep an eye out on your progress but we are prepping a 968 for the NOLA race this weekend, then at the track from Friday-Sunday.
I'll be in Nashville for work the following week but will have some free time during hotel boredom to help you if I can.

T

951and944S

BTW - listen close to the bell housing (front) area with car running, or better yet, you get under there and have someone depress and release clutch multiple times while you are listening close.

There is a strange noise.

This can be transmitted through the shaft to the rear but it sounds like the metal drive potion of your disc is cracked or broken.

T

Alex89

I'll try listening like you suggest. I'm also planning to pull the torque tube to rebuild it so I'll give the clutch a good inspection while I'm in there. Honestly I don't think I'm up for rebuilding the transmission myself. If I had more time and was better equipped I might give it a shot, but I don't think it makes sense at this time. I'm looking into shops who can do a rebuild as well as considering a used transaxle I could swap in instead. Thanks for your help.

If anyone has a 951 transaxle for sale please send me a message.

KevinGross

I think you're referring to the inside surface of the case, hmmm, I had not noticed that as damage, thought it might be just a reflection artifact due to the camera, flash, or lighting. Perhaps if the case gets cleaned up and a photo of that area taken. If indeed there are marks in the inside wall, I'd attribute them to debris getting sucked into the LSD. For whatever reason, my experience has been that with parts failures inside these transmissions, the LSD has a weird ability to ingest debris in ways that score the clutch discs, pressure rings, outer plates, etc. Seen it a lot.

Well, there is nothing to stop the cup springs (Belleville washers) rotating. But given that they are situated (or should be!) between the case top / bottom end and an outer plate that is splined to the case (four big tabs), you gotta wonder what would turn them, what would put a turning force on the spring that would lead to the depicted scoring. The scoring on both the case bottom and the cup spring is not something I'd consider normal: pretty sure it indicates that some debris got in there. Or that the fluid was very degraded, not providing the basic level of lubrication expected.

I have to confess that I don't get to see "normal" as much as I'd like to. Many of the transmissions sent to me are being rebuilt by new-to-them racers / owners after a lifetime of passive abuse by someone else. So they are a mess. Bless all my clients who do take care of their transmissions! I did just walk into the shop and take a peak at the LSD going into a transmission for a new client, which LSD came out of a race 944 whose original transmission grenaded. As a point of comparison, he was lucky, no debris ingested, no scoring of any kind, and the case ends and cup springs are clean and smooth.

T, thanks for your great thoughts here. But... not sure this would be worth it in terms of cost. Bear in mind that machining 0.5 mm off each of the pressure rings would only be half the job: you'd also have to ensure that the new working surface is hardened suitably. I have not Rockwell tested these parts' surfaces, not certain of the original spec but would want to be sure we get it back. Probably cheaper just to buy new pressure rings. The Turbo S LSD ("AOR") was also blessed with moly coating on the two cross shafts where they ride in the spider gears, which was great given the wear we see to them. I've seen examples where the spider gear teeth were also moly coated, but this was not done to all the AOR LSDs as best I can tell.

For a street-only car, I really don't think the AOR upgrade is needed. Except maybe the input shaft with its hardened 1st and 2nd gear teeth. It's inexpensive and really worth it.

Gonna politely disagree. In theory, and given perfection in measurement and parts conditions, maybe. But in reality, in 22 years of rebuilding these puppies, I have never, ever put one back together with the same shims it came with. Hypothesize age, fatigue, etc. as the cause -- dunno for sure. And not as bad as a magnesium case type 915 transmission (911 '72 to '78) where I have had to put 2 mm net additional shimming to get the preload right. Yup, you're reading that right, 2 whole millimeters. For the 944 boxes, never so much, but it always, always moves around. Given how stressed the pinion gear is, especially in the eight-valve, normally-aspirated transmissions, resetting the preload and backlash is a must.

Yup, it got shifted to the Classic catalog, which is Porsche-speak for "don't hold your breath, mate" However, it is available at the moment as PCG.311.220.L. T, good luck with the race! Cheers,