Rebuilding a ZF Limited slip

[Tom Oxner]

I have a ZF limited slip (fine spline) that I ran in my 72 911. I replaced it with a TBD for autocrossing. The ZF did fine at first. It launched really hard with both tires spinning. Over time it got to the point where only one wheel would spin at launch. I assumed theat the clutches were worn and removed it. This is the later model LSD with only one clutch per side. When I disassembled it, the clutches did not seem to be worn very badly. Any advice on critical measurements on the component parts as well as where to get replacement parts. Thanks,

Tom

[Grady Clay]

Tom,

Reassemble it just the way it was and measure the torque necessary to have each clutch slipping. This is done with an (inexpensive) indicating-type torque wrench and not a click-type.

You can easily calibrate the torque wrench by clamping the ½” drive in a vice and hanging known weights from the handle.

I suspect you will find the torque out of spec.

This is why the 1-disc/side LSD is barely suitable for anything other than generating heat.

Best,
Grady

[Jon B]

The ZF diffs with single plate/disc per side usually require 7-25 lb-ft torque to break the clutches, when tested as shown below (better with ring gear and bolts fastened). The outer plates were available in 3 sizes: 1.9mm, 2.0mm and 2.1mm. The housing flange can also be shaved, but it shortens the diff and requires a new preload/backlash adjustment.

[spuggy]

Sell the late ZF so-called "snow-only" diff, use a 40/65 LSD set up for maximum clamp (~70 ft/lbs of torque) and marvel at how much more planted and controllable the car feels on acceleration and deceleration...

[Geary]

Originally Posted by Tom Oxner

This is the later model LSD with only one clutch per side. Any advice on ... where to get replacement parts.

When Porsche began making discs and plates in half-thickness (in order to squeeze twice as many active discs into their GT3 LSDs), Guard Transmission did the same with both late style and early style discs & plates. Matt @ GT can easily rebuild your later ZF with 4 active friction discs.

[Tom Oxner]

Thanks for the advice. I had already done the "torque wrench test" and found that the amount of torque required was less than that specified in the factory manual. I will give GT a call to see what they have. Thanks for the help,

Tom

[Determined]

Can I have Matt's number as well. Thanks, AO

[0396]

sub.. interesting