Limited Slip Friction Discs

304065 · 06-07-2006, 12:02 PM

For a 901 ZF limited slip, how often should the friction discs be changed? This is a full race application, 155 HP, about 30 hours of operation per year.

Is this a complex operation? Does it entail setting the backlash? Are the parts available or NLA?

Thanks in advance. Geary, I would particularly appreciate your expertise here.

Steve@Rennsport · 06-07-2006, 12:19 PM

Hi John:

I'm not Paul, but I've done a few of these over the years,...

FWIW,....I measure the breakaway torque of the LSD every year and track its wear & functionality. When its at the end of the tolerance range, I simply pull the thing out the side cover, remove the ring gear and take it apart to replace the friction discs.

Its pretty easy to do and one simply must keep track of the order the plates (fiber & steel plates) to maintain the same locking factor and replace the ring gear bolts each time.

Either Paul or the Dealer can get you plates,...

DG624 · 06-07-2006, 12:47 PM

Steve, is the 915 similar to the 901? Was the LSD standard or optional?

304065 · 06-07-2006, 05:10 PM

Ahh, no less a personality than the good Dr. Weiner to the rescue!

I suppose that in the factory shop manual there is a procedure for measuring the limit torque. I'll do some research and report back.

'11 was $9.04 each, 6 req'd
'00 $21.05 x2
'21, or '22 or '23 can't find in the Pelican system.

304065 · 07:45 AM

The 915 lsd has one common part, but its discs carry a 917 p/n (nobody get excited, 917's had 920 part numbers!

Steve@Rennsport · 06-07-2006, 05:49 PM

Hey John:

Looks like Porsche has discontinued those friction discs in all 3 thicknesses. Those numbers are not in their current pricing. I haven't bought any since my last order,......

Call Paul Guard.

The book has the breakaway specs for 40%, 50%, & 80% lockups. If you cannot find that, drop me a direct e-mail and I'll get those for you.

JasonAndreas · 06-07-2006, 10:49 PM

Quote:

Originally posted by john_cramer
'21, or '22 or '23 can't find in the Pelican system.

The '21 part number was superceded to 951-332-551-00.

Geary · 06-08-2006, 07:16 AM

Hi John

The price for friction discs has increased to (gasp) $59 apiece.

Paul

304065 · 06-13-2006, 05:57 AM

Thanks Paul. Where most guys freak out at the prospect of paying $59 each for the discs, I'm just happy they aren't NLA. It comes from restoring a '66.

So last evening I dug out the workshop manual and noted with interest the factory procedure for testing the lockup percentage. With the carrier in a vise and a couple of special tools affixed, you apply a torque wrench and note the figure in meter-kiloponds.

Here's my question: is there a "shade-tree" method for doing the same thing without disassembly of the transaxle? Can I jack up one rear wheel and apply my torque wrench to the wheel hub castle nut and note the breakaway torque? Will that be accurate enough to know a worn-out diff from a good one?

Thank you gentlemen in advance for any advice on this matter.

Geary · 06-13-2006, 01:32 PM

I imagine this would be a reasonably accurate test.

goldgunner · 06-13-2006, 02:17 PM

Question for you experts:

I recently had my 915 rebuilt by a local Porsche shop that has 'done hundreds' over the years.

I'm sure my 80SC has never had a transmission rebuild: had the usual first gear problem.

According to the shop the bearings were good - is this expected and usual on a 25 year old transmission?

The transmission turned out to be a factory LSD - but I'm looking at the itemized work ticket and no LSD components / plates appear to have been replaced. Is it probable these were worn and should have been replaced?

Thanks,
Jim

ChrisBennet · ChrisBennet's Garage

Quote:

Originally posted by john_cramer
The 915 lsd has one common part, but its discs carry a 917 p/n (nobody get excited, 917's had 920 part numbers!

I think you meant 917 parts carried a 912 part number.

There was an article in the May issue of Excellence magazine called "The Cheap Seats! A Bargain 912 Racer". The owner of the 912 got a hard to find factory racing oil cooler for it from Carl Thomson (noted Porsche race car restorer and owner of his own 917). "Carl joked that we 912 owners need to stick together."

-Chris

beamonk · 06-13-2006, 05:04 PM

I was wondering how to measure wear on a 915lsd. We took mine apart and my wrench said they were beautiful and to leave them alone. We haven't gone through the tranny yet, but I was wondering if we should mic them or get new ones to increase the locking % as this will be a driven to the track track car.

benjoys02 · 06-13-2006, 05:30 PM

hi everyone,
i have an '81 sc which i think has an LSD. i raised the rear of the car earlier today and manually rotated the driver side rear tire forward and the passenger side moved in the same direction with no hesitation. just to confirm that the car has LSD, is there a serial number that would indicate such?
thanks!

lally911 · 06-13-2006, 06:34 PM

The 12 stamped between the 915/73 and the 74F indicates factory LSD install. Location for this number is on the bottom of the tranny near the drain plug.

benjoys02 · 06-13-2006, 08:04 PM

thanks! i'll check my car again tomorrow.

Steve@Rennsport · 06-13-2006, 10:40 PM

Quote:

Originally posted by john_cramer
[B] Here's my question: is there a "shade-tree" method for doing the same thing without disassembly of the transaxle? Can I jack up one rear wheel and apply my torque wrench to the wheel hub castle nut and note the breakaway torque? Will that be accurate enough to know a worn-out diff from a good one?

Hi John:

You betcha there is,....

With the rear end of car up on jackstands, pull the CV's off the axle flanges on the transmission.

Make a pair of 2"x 6" aluminum (or steel) bars. Drill a hole in the middle and install a 10mm bolt & nut. Drill two holes near the edges that line up with two of the CV joint holes on opposite sides so you can bolt this up to each axle flange on each side of the differential housing. You'll need some short CV bolts,...

Now, while holding one side of the diff with a socket and breaker bar, attach your torque wrench to the opposite side and slowly pull up the wrench and observe the torque setting when it moves.

Works great!!!

dvkk · 06-13-2006, 11:43 PM

Quote:

Originally posted by john_cramer

Here's my question: is there a "shade-tree" method for doing the same thing without disassembly of the transaxle? Can I jack up one rear wheel and apply my torque wrench to the wheel hub castle nut and note the breakaway torque? Will that be accurate enough to know a worn-out diff from a good one?

I don't see why this wouldn't work. Have you got some comments, Steve? Paul?

304065 · 06-14-2006, 04:33 AM

Chris, truth be told, I originally wrote 902, because I was thinking 912, before realizing that I really DID mean 912. In any case, I went to Grady's excellent compilation and realized that the 917's box had a 920 part number. All in a day's work.

Geary · 06-14-2006, 08:38 AM

These friction discs are the most common disc in the industry, used in the LSDs of many marques. They have been sold under a variety of part numbers (although actual thickness may vary slightly), including part #917 332 522 11.