928#00076

I have an early '78 that I need some guidance on a clutch replacement. I follow the 928 Forum regularly and can't seem to find a thread that addresses my issue. Twice now I have had the clutch out and back in and now out a third time.The clutch worked generally okay before I took it apart but it seemed worn and so I felt like it was time for a replacement. The first time out was to replace the following parts: pressure plate, clutch disks, throwout bearing and housing, pilot bearing, clutch fork nylon bushing. A previous owner had already replaced the clutch fork and ball with the later model. This was done in about 1983 from the date code on the clutch disks I removed. I was very careful to adjust the stops on the intermediate plate as specified in the WSM so I don't think that is the problem. Once it was all back together, I was able to drive the car about 100 miles without incident and then the clutch became noisy and erratic. Fortunately I was pretty close to home and so I just drove it a couple miles and put it back up on the jack stands.

I removed the clutch a second time, could not see that anything was really wrong but figured something was out of balance so I removed the flywheel and took the flywheel, intermediate plate and new pressure plate to a machine shop for resurfacing (flywheel and intermediate plate) and balancing. Not the type of clutch setup the machinist had seen before but they are very experienced and were able to do what I wanted. After reinstalling the clutch the second time, again I drove about 100 miles and the same thing happened. So I have the clutch out a third time and it is laying in parts on the floor of the garage. One thing I found this time was that the thrust washer holding the throwout bearing in place was frozen against the lock ring at a slight angle. A light tap with a hammer freed it up but It seems this may be a problem. I had noticed this the first time but it was not as bad. My question is what is causing this to happen? Here is a picture as it came out of the car and you can even see a tiny sliver of metal that has come off either the lock ring, the thrust washer or the bearing housing. Am I using the wrong parts? I purchased everything from Roger at 928srus so hopefully all good?

Here are a few thoughts:

The torque tube is a little noisy and could be causing some vibration. I am contemplating dropping all of that down to replace the bearings but would prefer to do it this winter.

I did not replace the intermediate shaft. It has some wear but does not look like enough to cause a problem but who knows. I know the shaft was seated properly in the pilot bearing when I reassembled. Picture of shaft is attached.

I did not replace the shaft guide tube. It looks good but again who knows. Picture in the car attached.

This forum is a great resource and I intend to keep this car but it sure can be frustrating at times to work on!

928#00076

Thanks to Mark Kibbort for sending a PM explaining he had the same issue:

"... thought that snap ring could just be pushed back in place... no, it came out and the TB became off center and caused the vibration at 4k onward. you need a new snap ring.. THAT is one of the most valuable and critical parts on the 928 .. tell the list I said so!!!!
---End Quote---
Thanks for the reply Mark! I will order a new snap ring. Is there something special I need to do to make sure it is seated in the groove of the TOB?
---End Quote---
No, a new ring will seat itself perfectly. it will be even all the away around in its seating, unless there is damage to the groove, which is very unlikely. I think I might have the same issue with mine... yet again, but its been a few years of racing. maybe it got dislodged for some reason... don't know ..
mention that we spoke on the list and compare the similarities. (multiple clutch removals, re-seating the snap ring, and still vibration until snap ring replaced) and then see what people say.

Anyone else have similar issues with this? Still not sure what causing the vibration.

GregBBRD

The round (cross section) T/O bearing retaining "snap ring" is pretty simple...get it into the groove.

The "early" rear clutch disc had a short central hub (splined part), which was changed to a "taller" hub.
This required a shorter T/O bearing collar, so the clutch disc did not rub on the guide tube.

Pretty easy to see where your guide tube has been hitting the later clutch disc....there's "shiny" marks on the end of it.

Additionally, the short shaft was made with longer splines area, for the increased clutch hub height. I can't tell if you have the early or late shaft, from your picture.

BTW...
I make replacement short shafts out of 300M, for half the price of a new factory shaft.

Also BTW:
Since you sourced your clutch discs from Roger, it might be worth noting that if he sold you his "rebuilt" discs, the actual clutch lining is thicker than stock. This total thickness difference will move the rear disc towards the T/O bearing guide tube and may require shortening the late model guide tube (already shorter than the original "early" guide tube) an additional amount, so the rear disc does not contact the guile tube.

928#00076

Thanks Greg. The clutch disks I ordered from Roger are identical so this may be a problem. They are not like the exact factory replacement disks which are each a little different. So you are recommending a new shorter guide tube and a replacement shaft. Here is an additional picture of the shaft - splines measure about 50mm but the picture distorts it. Please send pricing on both.

hacker-pschorr

What kind of lube are you putting on the shaft / splines? There's a specific kind of grease that should be used on there.


Many moons ago (2004 to be exact) the clutch in my 81 suddenly started to hang up randomly.

Upon disassembly this is what I found. Note the first pic there should be a "tooth" sticking out that keeps it located on the guide slot in the bearing sleeve:

GregBBRD

If you could take a picture of the two discs that Roger sent you, it would be easier to tell what you have/need.

There were three "hub" lengths, from Porsche. The original hubs (in your '78) were 20mm in length. The replacement hubs are 20mm for the inner disc and 29mm for the outer disc. Porsche made some rear discs (for a short period of time) that were 25mm tall.

Is what I think I see (in your picture) correct? Are there metal rub marks on the inside of your guide tube?

The "replacement" guide tube is 49mm total length. The original guide tube is 50mm. They can be shortened (although an internal champher is tough, without a lathe.)

The "splined" surface on the original clutch shaft is 51mm. The replacement is 56mm. However, if Roger sent you two "short" discs (this would be a "reverse supercession" back to the original design) the rear disc may/may not "fall off" the splined area.

My 300M shafts are $375.00. They have an bit "extra" room at the rear, where the bolt passes through, to make sure there is enough travel to be able to pull the snout out of the pilot bearing (super important.) They also have a modified snout, from the factory design, to be better supported in the pilot bearing.

928#00076

Thanks for the feedback. The TOB that came out of my car looks similar to yours in that it has the notch in the side and the thrust washer has a tab that fits into it. Picture below. The TOB I was sent, which I think is the only one you can get, has neither the tab nor the slot. Picture also below.

Picture of Special Porsche Grease as sent by Roger also attached. I used copper anti-seize the first time but then he said I should smear on some of this so I did the second time.

928#00076

Greg - pictures of all discs are attached and here are the measurements:

First two pics - What I was sent: hubs measure 20.5 mm deep on both. Thickness is about 8.5 mm
next three pics - What came out: one hub measures 20.5 mm, other about 20.2 mm- virtually the same. Thickness is about 7 mm











My guide tube is 49 mm so it must have been replaced by PO. Picture included below.

GregBBRD

Yup.

The new T/O bearing is made in England, not by Sachs in Germany.

They changed some of the old design. Definitely more difficult to install and remove.

Put some of that high pressure grease on the fingers of the pressure plate, where the fingers and the washer (under the circlip) touch. This will help keep the washer from "machining" the fingers away, when it spins (it will.)

Mrmerlin

what color is that grease is it sticky?

When setting the stub shaft you should do a mock up without the clutch,
but use the guide tube and the stub shaft.
What you want to mark is the shaft where it comes out of the rear edge of the guide tube after you have seated the stub shaft into the pilot bearing.
then moved it to the rear about 3 to 4 MM.
this will seat the shaft into the meat of the bearing surface and prevent the shaft from bottoming out on the pilot bearing.

Then mark it . When fitting the clutch assembly all thats needed is to line up your marks to the guide tube and then tighten the pinch bolts.

NOTE the rear pinch bolt at the trans axle should be tightened before you tighten the 2 front pinch bolts.
The picture with the rusty disc hubs show a lack of lubricant,
I use a tooth brush to smear the splines of the shaft and the hub splines with the sticky brown Porsche grease

928#00076

Thank you for your input! I would say the grease is brown and sticky. When I am ready to reinstall the stub shaft I will use your procedure. During previous reassembly I was pushing the shaft in as far is it would go and then estimating 3-5 mm to pull it back. Next time I will mark it so there is no guesswork.

During the last disassembly, I found the rear pinch bolt at the transaxle was not fully torqued. This could have allowed some slight movement of the torque tube shaft and I am guessing that any longitudinal movement of the shafts would not be good for the TOB! I have since tightened that pinch bolt to spec but I am going to pull it again and put a little anti-seize on it.

The rusty hubs are on the clutch discs that came out of the car. I doubt that they have been looked at since 1984 and hence a lack of lubrication, if there ever was any. I made sure the splines and new disc hubs were "smeared in" before reassembly.

GregBBRD

A new Porsche disc measures 8.0mm, so Roger's discs are .5mm thicker.

With the additional thickness of Roger's discs, the rear disc is going to be moved back .5mm for the front disc and .25mm for the rear disc. I doubt that the square collar of the rear disc will touch the collar....and I don't see what I thought I saw in your earlier picture...looks untouched, other than rubbing during installation.

I don't see anything obvious that would be causing your noise.

I guess I should have asked this earlier.....was it noisy when released or engaged?

928#00076

The noise happened when the clutch was operated, putting pressure on the TOB. Pretty quiet once engaged and driving.

GregBBRD

OK. When the clutch is released, obviously the T/O bearing is under increased load, the pressure place is pulled up, the intermediate plate is allowed to "lift" off of the front disc (if there is enough travel), the clutch discs are not spinning, the pilot bearing is spinning. The torque tube shaft and short shaft are not turning (assuming the clutch is completely released.) Do you recall if reverse gear ground when the noise was occurring? (This would tell me if the clutch was completely released or still dragging, slightly.)

Can you describe what "noisy" is? I know it is a tough thing to do, but anything would help. Scraping noise? Rattle noise? Thumping noise? Screeching noise? Rubbing noise?

Can you elaborate on "erratic"?

GJB928

Pretty timely thread for me. Put my car up on the lift last night. I had an issue with my '78 5 speed clutch last year. The guide tube was definitely part of the problem. See thread here, https://rennlist.com/forums/928-foru...-question.html

I replaced the guide tube but after about 2-300 miles I started to hear a "rumbling" noise. This was most noticeable when cold and the clutch pedal not depressed while driving. I'm now wondering if the different length of the guide tube could be related?

Any thoughts?