How to Rebuild a 911 Factory Fine Spline ZF LSD

[Determined]

I've done a search in the forum and I can't find any details about how to tear down and rebuild a factory 911 ZF LSD fine Spline from a Carrera (84-86).

Can anyone let me know how to do it and what is needed? Thanks for your time and support.

[BertoSpid]

Hi, i am very interested by this subjet !
i have mine (G50 RS / 964) to taer dowm and rebuilt too

[Driven97]

Full disclosure: I've never even seen a factory LSD let alone rebuild one.

But from what I've seen the mechanism is pretty simple. You have alternating plates (11&12) where half key into the diff housing, the other half hey into the output gears. In the center is a block (4) where the pins (6) sit. The block has ramps on it. When torque is applied, the pin acts on the ramp to force the block apart. This squeezes all the discs together, which locks the diff. The more torque applied, the more it locks.



The magic is in the ramps. Getting those right (or wrong) can greatly change how the car reacts to certain situations. Here's the deceleration ramp on my (aftermarket) LSD so you can see how the pin acts to wedge the plates:



Anyways, when a diff wears it's usually the plates. They rub against each other, so that's why they wear. Looks like they're ~$40 each for OEM plates. Maybe Matt at Guard has a more modern technology plate that can go in there?

Also when those plates wear, clearances increase, and you lose preload, which is how locked the diff is without torque applied.

[Driven97]

Oh, one more thing, I didn't see it in any of the guides on this site, but you can pull the diff with the engine / trans still in the car. I did it.

[BertoSpid]

Thank's a lot, Matt !

i think pull off the diff with the engine in my car, and your site will be very helpfull

[Driven97]

Fair warning - "don't do as I do."

[KTL]

You need to have a target "stack height" to get your diff innards the right dimension. So you need to know what your stack has to be and then select replacement discs of the correct dimension for it all to fit together.

Matt's right that the basic behavior of the diff starts with the ramp angle on the pressure ring pieces (#4 in the parts diagram). But the diff is also affected by how much preload you build into it, which can be adjusted by the disc thickness chosen, which affects how much compression is being placed on the belleville spring washers (#15 in this diagram, which are strangely absent from Matt's diagram!)



Also note that modern OEM Porsche discs are prone to wearing out very quickly. Very quickly. Aftermarket discs like those from Guard are of a much more durable material (plasma sprayed?) and last much much longer

[Matt Monson]

Hello,

Thought I would pop in and let people know that we have discontinued service of the ZF LSDs. Not only is Porsche servicing them these days at about the 1/3 of the price of our parts, the real reason is bigger than that.

We are a motorsports company. The newest ZF LSDs are 20 years old, and the oldest 50! In racing we time out parts regularly. Even our most bulletproof of billet LSDs realistically should be completely retired after about a decade, and less under some applications. What we have seen in the last couple of years is catastrophic failures, of bodies and drive gears, on ZF LSDs that are being used in Club Racing. This poses a significant concern to us with respect to the safety of the drivers, as well as our liability in an increasingly litigous society. My liability insurance doubled last year!

While I understand that most of you guys are building hot rods or doing restorations that will never see the stresses of a race car, it's an all or nothing game. I can't say that I'll sell to street tuners but not racers. And even if I did, how would I control where the parts went? As such, Guard Transmission no longer sells parts to service or recondition ZF LSDs and will only sell our all new units, and service them. And since we've been doing LSDs for close to 2 decades, there are even certain GT units I will not rebuild and will advise that the client retire it. I hope this makes sense to all of you. I'm not going to debate the decision if anyone posts with comments in protest to our decision. My decision is made, it was a decision I considered for a long time, and with the ready availabilty of ZF original parts through Porsche, it's a decision that doesn't leave you guys out in the cold. That last part was a big part of me being able to rest easy that I made the right choice.

Regards,

Matt Monson
Guard Transmission LLC

[Matt Monson]

PS.
Determined, you are in good hands here. The guys will help you build your unit.

Kevin,
The reason the first diagram lacks bell washers is because the early ZF diffs up until the 915 used what they called a wave washer instead. It was the same size and shape as a steel plate, except it had been bent to have ripples in it. Really problematic design because they always cracked and broke at the bend, leaving 0 preload. We have historically replaced them with bell. washer when doing any of the old 904/901/914 units.

Regards,

Matt

[chris_seven]

The ZF of this size is very popular in the UK as it is used in a number of Escorts.

We have been making Friction Plates, Flat Plates and Thrust plates for some time now.

Friction plates are Moly Coated and thrust pates use the same material as the original parts.



We have just drawn ramp drivers, uprated Crosspins in 300M VAR Steel and will be making pinion gears, side gears and end plates.

We are have also just introduced our new 901 Plate Diff which has a Chromoly Case, moly coated plates two sets of ramps machined into the ramp drivers and 6 plates per side.

The side gears work with Lobro Output Flanges.

WE have the first one installed in a 2 litre Special Stage Rally Car which has a reasonably powerful Twin Plug Motor and is very short geared using a 6:29 Ring and Pinion. It is geared for 110mph at 8000rpm in 5th.

I hope this will be a demanding environment for the unit and that it will stand up to the demands imposed.

[KTL]

Sorry to hear Guard doesn't service ZF diffs any more. However, I completely understand why.

Still, we appreciate the feedback you always gladly share. Thanks for the clarification on why no cup washers in Matt's diagram.

[Matt Monson]

Originally Posted by KTL

Still, we appreciate the feedback you always gladly share. Thanks for the clarification on why no cup washers in Matt's diagram.

You're welcome. I come here because it's my community, not because I am trying to promote my company. I'll always share if I know the answer.

[Geary]

Originally Posted by KTL

Also note that modern OEM Porsche discs are prone to wearing out very quickly. Very quickly. Aftermarket discs like those from Guard are of a much more durable material (plasma sprayed?) and last much much longer

Yes .. brass, carbon, etched, and coated discs all end up the same performance-wise to plain steel ones (as found in early ZFs like the 904) rather quickly. Porsche Motorsports discs are much better, while Guard discs last for a VERY long time (although the plasma bombarding step during manufacture is quite expensive).

[BertoSpid]

Thank you very much Guys for all the informations

i think do this job before summer, and, if i do, i will post some pictures...

(It's a G50-10)