904 LSD open heart surgery

[ajwans]

Gurus, can I get your advice on what exactly should be inside the 4061004004 LSD. The ZF booklet shows 3 of part 4, outer disc, before the first part 5, inner disc.

I just busted open my LSD and found that I have alternating inner and outer discs.



What's the stock configuration for one of these? If the ZF document is correct I should have an outer disc in place of the 5th from the left inner disc.



andy

[Jon B]

What's the stock configuration for one of these? If the ZF document is correct I should have an outer disc in place of the 5th from the left inner disc.

Andy, refer to page SR 28b in the factory workshop manual.
Your differential was arranged to have the maximum number of friction surfaces in use, which Porsche called 75% anti-slip effectiveness.

Jon B.
Vista, CA

[gled49]

As Jon says, that is the max setup you can have for that diff. The 904 diff doesn't have any preload in it, all the inner and outer lamina are flat. Later diff's used an undulated (wavy) outer lamina, or a belvil (pressure cone shaped) washers, in both the outer most positions in the diff to create static tension, both wheels turn the same direction when one is turned. Under load or de-acceleration the thrust pieces compress the plates providing power to both wheels. This is true in both types of diff, 904/911. If you don't want the max setup in yours combine inner and outer lamina; i.e., outer, outer, inner, outer, inner, inner. You will go from 12 friction surfaces to 8 total, all without buying any plates. With no preload I would run what you have. Have you driven this setup. Because ring gear doesn't have to be removed to change the setup, you can just pop the diff out while it is in the car and change it ezy-pizy. G

[Jon B]

What's the stock configuration for one of these? If the ZF document is correct I should have an outer disc in place of the 5th from the left inner disc.

Andy, I believe the illustration you posted is one that I posted a few years ago. It shows the early, original ZF arrangement of discs and plates.
If you go to page SR 23 in the factory manual, you'll see an illustration with the same arrangement for an early differential.

If you go to page SR 28a, you'll find a page with a different illustration and the notes "From manufacture date of 6 Mar 67" and "50% anti-slip effectiveness", and on the bottom "For use in particular sport events, the self-locking differential can be modified to yield anti-slip effectiveness of 75% (see page SR 28b)."
As I said before, your differential was arranged for a 75% anti-slip effectiveness, as shown in the illustration on page SR 28b. Yours is either a ZF manufactured after 6 Mar 67, or an earlier ZF updated to 67> specs.

Jon B.
Vista, CA

[ajwans]

The picture is actually from a little ZF book that contains almost the same information as the workshop manual but does not mention the later variants of the LSD.

Regarding part 3, "catch plate (steel)", mine have a 0.002" step worn into them by the ends of parts 7. Is this a problem? If so, resurface or replace?

If replace, where would one find parts for this beast?

My ZF book makes no mention of wear on this piece.

andy

[gled49]

I doubt it would be easy to get parts, the side gear #7 thrusts against the ring, and I don’t see a real problem using the parts. What kind of service is this diff going to be subjected too. G

[Jon B]

Regarding part 3, "catch plate (steel)", mine have a 0.002" step worn into them be the ends of parts 7. Is this a problem? If so, resurface or replace? If replace, where would one find parts for this beast?

As you said and as Gordon also explained, the steel disc #3 is a hardened buffer between the side gear #7 and bronze disc #2.
The splines of the side gear are wearing into the steel disc.

You have several choices...

You can run the part number 904.332.562.00 through Porsche or Freisinger, and see if new, genuine discs are available.
I'm not aware of anyone remaking that part, but you can do a search.
You can continue to use the discs as they are, and have a combined extra play of 0.004" in the gear mesh. It's not ideal, but for mild street use I don't think it will be a serious issue.
You can de-burr the worn sides and face them toward the bronze discs, which will eliminate the extra play but eventually wear a 0.002" step in the bronze discs.
You can grind the steel discs to eliminate the steps, face the undamaged sides toward the splines, and make new bronze discs that are 0.002" larger.
You can have new steel discs manufactured, but they'll need to be of similar hardness.

As Gordon asked, "What kind of service is this diff going to be subjected to?"
If this is for a street car, a ZF with slightly worn clutches will still be reasonably effective, and still reduce your time to grocery store by 1 to 2 seconds.
If this is for a race car, I'd suggest that you retire your ZF to the display case and purchase a newly manufactured LSD.



Jon B.
Vista, CA

[Elferhelfer]

Hello,
I don´t if I may help you, but here´s an specialist for LSD. He helped me ones for an early 901 gearbox with LSD. He´s based in Germany and may help for parts ans special LSD´s for 901/904/908/917... Unfortunately his page is only in German.

https://www.antriebstechnik-schmidt....ensperren.html

https://www.antriebstechnik-schmidt.de/bilder.html

[ajwans]

Jon, any chance I could bother you to measure the thickness of your steel thrust washer there? I don't trust the one I have as somebody put a 0.6mm shim washer behind it.

andy

[Jon B]

Jon, any chance I could bother you to measure the thickness of your steel thrust washer there? I don't trust the one I have as somebody put a 0.6mm shim washer behind it.

Andy, it would've been better to ask before I re-assembled the differential and put it away :-(
I'll have a measurement for you shortly...

Jon B.
Vista, CA