ZF Differential Rebuild Parts - Experience?

[Tonger]

I am looking to rebuild a coarse spline ZF LSD from a 915/02 and have a question about rebuild parts since Guard is no longer offering the rebuild plates and clutches. The original ZF clutches had a reputation for being rather short-lived - which is why so many folks would rebuild with Guard parts.

**Does anyone have experience to share with regard to the longevity and quality of the rebuild parts being offered by Porsche Classic?

**Has anyone tried the aftermarket diff parts from RacingDiffs.Com?

https://racingdiffs.com/products/porsche-zf-lsd-clutch-service-pack?variant=33422107279495

I am just looking to put the best quality parts back in the ZF with hopefully longer lasting parts than the original.

Thanks very much for any advice y'all can share!

[gled49]

Ive always used Porsche parts to rebuild ZF Diffs with 12 plates. 2mm pressure cones from SC, 928 inner lamina and 917 outer lamina that come in 1.9, 2.0, 2.1mm thickness. 928 & 917 are part number prefix. Plate stacking is outlined in service manuals. I’ve had good luck and have never had to rebuild any done this way. But there are different suppliers and parts that are in the market place. When I started doing these there wasn’t an aftermarket, so Porsche was my source.

[Matt Monson]

Originally Posted by gled49

Ive always used Porsche parts to rebuild ZF Diffs with 12 plates. 2mm pressure cones from SC, 928 inner lamina and 917 outer lamina that come in 1.9, 2.0, 2.1mm thickness. 928 & 917 are part number prefix. Plate stacking is outlined in service manuals. I’ve had good luck and have never had to rebuild any done this way. But there are different suppliers and parts that are in the market place. When I started doing these there wasn’t an aftermarket, so Porsche was my source.

The offerings from Porsche Classic are different now. You can only get 2.0mm under the new versions.

An early one is going to have wave washers instead of Belleville washers. I would recommend updating them as part of the service. I can provide a couple different thicknesses.

I’ve never seen the original ZF parts have a short lifespan. I just don’t think they are as good as our parts when it comes to racing. I would use them and not worry.

I’ve never held a set of racing-diff clutches in my hand, but the conversations I’ve seen on FB and IG suggest they arent very durable. Stick with ZF imo.

Regards,

Matt Monson
Guard Transmission llc.

I rarely check pms and don’t really come here to sell. Email me at gtgears@yahoo.com if you want some bell. Washers.

[gled49]

Just for drill I check with my dealer, and all numbers were available.
928 332 555 00 pressure cone, Belleville washer
928 332 551 01 inner lamina, with radial groves
917 332 552 10/11/12 outer plates
These parts only apply to early 12 plate diffs. Not for 904, G50 or fixed 40% Carrera diffs

[kevingross]

Originally Posted by gled49

Just for drill I check with my dealer, and all numbers were available.
928 332 555 00 pressure cone, Belleville washer
928 332 551 01 inner lamina, with radial groves
917 332 552 10/11/12 outer plates
These parts only apply to early 12 plate diffs. Not for 904, G50 or fixed 40% Carrera diffs

Glen, are the 2.1 mm outer plates once again available from Porsche? Last time I asked, they were back order no ETA.

I agree with Matt's experience and advice. I rebuild ZF LSDs all the time and have had good results following the factory's instructions and using their parts.

[gled49]

They said all 3 917 part numbers were available. The parts might not be right but the numbers were. The inner lamina are 87$ now. I think I use to do complete rebuilds for that.

[Tonger]

Thank you very much for your valuable feedback and experience - I will proceed with Porsche Classic plates/clutches and will check with MM about the Bellville washers.

Much appreciated!

[dpegan]

This post has been very helpful as I am also rebuilding two ZF LSDs at this time. As suggested I order new parts from Porsche for the rebuild.

One LSD is for a 901 going into a 914-6 track car and one is for a fine spline 915 that has been sitting on the shelf. I have a questions after reviewing several of the posts about Porsche LSDs on this forum.

Is there a preferred order of the friction discs and plates? The first and second pictures with the yellow cloth is the 901 LSD and shows the friction discs being stacked against the thrust ring. The 3rd and 4th Picture are the 915 LSD the with friction discs stacked together with one plated between them and the thrust ring. Any one have thoughts on the proper stacking order of the plates?






Thank you,
dan

[gled49]

Locking value is determined by contact surfaces between inner and outer plates. The pressure cone is always stacked against an outer plate. I only use the new inner lamina with radial groves. I personally use 80% but reduce preload down to 10 or 15 pounds. Sometimes impossible to do. I also try not to use an inner plate against the thrust ring. So my 80% is cone, outer, inner, outer, inner, outer. 4 contact surfaces per half. This info is for ZF 12 plate coarse or fine spline. I’ve done this long enough to wonder how a pair of 2.5 cones were used in your 915. (Was the housing machined)

[kevingross]

Following up on gled's comments, with your four-clutch LSD you have the option of "40%" and "80%" locking, depending on the stacking order of the plates. Your photos both show "40%" set-ups, with four active inner plate-to-outer plate interfaces and the inner (clutch) discs next to each other. This is generally the best option for track, autocross, and street, and it's how I set up almost all my customers' ZF LSDs.

To get the greater locking, you alter the plate order to put eight inner-to-outer interfaces into action. Something like
pressure ring -> inner -> outer -> inner -> outer(s) -> cup
This "80%" lock is generally not a good idea unless you are a very skilled racer who can manage its effect on handling. There is a lot of mythology (my polite word) spoken esp by certain vendors promoting the idea that more lock is always better. It isn't.

To answer your question about order (I think), it doesn't particularly matter when you have more outer plates in the stack than needed for configuring the lock, as is the case with your LSDs in photos.

Hope this is helpful.

[dpegan]

Very Helpful!

Thanks for your advice. I don't know the history on these units I have so I am just trying to get them back to a stock setup and spec. Spent about $500-$600 on parts for each so far. We bought the 901 LSD sight unseen as it was in a 914 side shifter that had supposedly been rebuild. As we wanted to regear it for the track, we disassembled the transaxle and found a few problems like improper shimming. After seeing what was inside the transaxle it was decided to open up the LSD to make sure it was OK and you can see what we found in the pictures. The parts are only getting more expensive to repair/rebuild so as most know, better to pay a fare price for a reputable rebuild than a cheap price for a part that does not have the documentation.

Thanks again your all the insight.

[dpegan]

Forgot to ask. What book did the diagrams come from?

[gled49]

Volume I service manual, in the updated section covering 911 Mag trans. ALU case differential info doesn’t apply.

[dpegan]

thanks

[gled49]

Another note if u check preload, use the drive flange to drive flange method, not the early housing to drive flange described in the early service manuals.

[dpegan]

Another question. When setting up to measure the slip torque/preload using drive flange to drive flange method are the flanges bolts torqued down or are they left snug? I ask because when I am measuring the slip torque on the LSD it changes depending on flange bolt torque.

Thanks

[Matt Monson]

While the manual outlines the lower acceptable breakaway torque to determine when to rebuild, you can’t really accurately test a “green” freshly rebuilt LSD. They come down upwards of 20% during the first 500-1000 miles of break in. It’s not really a valid”tuning” metric during rebuild, even though many think it is.

[gled49]

Dpegan, turning torque shouldn’t change weather or not drive flanges are bolted tight. Coarse spline diffs had special longer drive flanges for LSD, you should confirm this.

[dpegan]

Interesting. I'll confirm the Flanges are correct. Does that mean that the slip torque can be 20% higher than max factory range?

I did get a hold of some early factory manuals. Interesting thing is that the slip torque/Preload is different in the manual for different years and percent lockup. It also looks like one method for the housing clamped in a vise vs the method using the flange clamped in the vise might give different results?

Here is what I have from some factory manuals;

1967 to 1969 part no. 904.332.053.00
50% locking effect
slip torque, 7.2 to 10.8lbs/ft

1969 forward no part no provided
50% or 40% locking effect
slip torque, 30 to 57 lbs/ft
75% or 80% locking effect
slip torque 43 to 85 lbs/ft

1976 Turbo/930 manual
40% locking effect
slip torque, 29 to 58 lbs/ft

1985 to 1988 Carrera manual
40% locking effect
slip torque, 7 to 20 lbs/ft

After assembling my two LSDs at 40% configuration with new parts, the 901 has a slip torque in the 45lbs/ft range and the 915 is at the max of 20 lbs/ft. I still need to verify that I have correct flanges for the 901 LSD as Gled recommended.

Thoughts?

[Matt Monson]

Open differential flanges measure 56mm to the shoulder from the end of the flange. LSD flanges are 6mm longer.