ZF Limited slip install - DIY job?

[katigbr]

I recently purchased a ZF LSD, a new R&P and some new gears to install in my early 915. I orginally thought of doing it myself. In reading the Porsche Shop manual the process they describe to adjust the pinion shaft and ring gear uses a lot of specialized measurement equipment that I don't have access to. I'm capable of doing a decent disassembly and reassembly job but am intimitated by the precise measurement and shimming required. Is installing an LSD with new R&P outside the capability of a home mechanic without the specialized measurement equipment?

[Early_S_Man]

Possibly not, if you carefully measure the shim pack in the transaxle currently, then determine any difference between the etched numbers on old and new ring & pinion sets, and adjust the shim pack thickness appropriately ... and measure the backlash by the 'Plasti-Gauge Method.'

Are you planning on renewing the clutch disks and shims inside the ZF as in the following diagram?




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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[katigbr]

Early_S_Man,
Thanks for helping me out. That sounds like it will work. What is the worst case scenario of being outside of the optimum contact pattern? Noise? Accelerated wear? Breakage risk? I will be running over 250hp with this early mag-cased 915 (billet bearing retainer and side cover being installed).

Regarding Plasti-Guage, am I correct in assuming that the objective would be to ensure that I've got a "full" contact pattern across the surface of the ring gear tooth equivalent to more or less the pinion gear length. Is this correct? This would allow me to verify that the backlash is set. Do I need to worry about the ring gear being centered in the differential case, i.e. ring and pinion in contact using max possible gear tooth depth?

I'm not cracking open the ZF as it is in ready to drop in condition. Do I need to do anything to the internals prior to installing?

Thanks again.

[Determined]

Bump to join, I'm considering doing the same.

[0396]

sub.. interested too.

[billybek]

10 year old thread.
Always nice to see some of Warren's posts pop up again though.

[Walt Fricke]

Guys

There are three things which need to be set in the differential area:

Pinion depth

Preload

Ring gear side to side location.

The trickiest one of thes is pinion depth - how far toward the axis of the ring gear should the end of the pinion be. Ring and pinion gears are matched sets, with data etched on each so they don't get mixed up, and to help you set their depth. Pinion depth is set by shims inside the middle housing of the transmission. The good news here is that you are not changing your ring or pinion gears. So you don't have to worry about that one. It won't change from what it is supposed to be no matter how you futz around with the differential.

Preload means how much the total width of the differential assembly pushes the carrier bearings outward. How much they try to bulge the side plates of the transmission out. There is a set specification for this, which you need to know. That spec is based on which brand of bearings you are using, so it does not depend on the type of differential. It is adjusted by the shims which sit between the bearings and the differential body: more total shim thickness, more preload.

The side to side part locates the ring gear in the proper right/left plane to the pinion gear, and sets that aspect of the depth of the mesh of the teeth. The factory procedure has you setting that by measuring the lash between the gears. This is actually pretty simple to measure, though it gets tedious. You adjust this by moving shims from the right side to the left, or left to right, until you are within spec.

The reason why you might not want to just take out the old diff and plop in the ZF is that the important measurements of the two units may not be exactly the same. If the bearing seating surfaces are exactly the same distance apart, you wouldn't have to fuss with preload - you could use the same total shim pack as you had been using. But how to measure that down to a couple of thousandths? You will need more than a 6" caliper.

Similarly, if the ring gear mounting flange surface is located in exactly the same place relative to the bearing seating surfaces, again you can just swap what is there. Also tricky to measure.

However, the job can be done, and I think is normally done by trial and error rather than measuring parts.

It is a real pain to install the bearings, measue something, and then take the bearings off again to adjust the shim pack, and repeat the process. The bearings are a press fit, and are hard to get off. Ordinary gear pullers don't really work, and it is easy to damage the bearing carrier part. The best method is to use a set of old bearings which have been honed out so they are a slip fit for all this cut and try. And shops usually have a bunch of different thickness shims on hand, which speeds up doing the swapping back and forth. You will start out with only what you had, which may not be enough, or it may be too much, but if you remove the thinnest one that may be too little, or it may not divide down to what you need.

The preload is supposed to be set at a certain number of inch/ounces. Nobody has that kind of torque wrench in his tool box, and they turn out to be quite expensive as well. There is an alternative: you measure how large the gap is around the differential cover before you cinch it down with all those nuts and washers. Not as accurate, though.

Take a look at the tutorial on transmission work - it has a whole discussion about how to go about all this.

[Walt Fricke]

The above based on reusing the R&P, and just installing a different diff.

[TDIDog]

I bought a car that some do it your selfer did the R&P. Cost me a new R&P & big bucks to get it done properly. I wouldn't want to be the guy you sell that car to!

[herman maire]

Does anybody have a source for new clutch disks and shims ?

[Determined]

bump on the source for shims.

[gestalt1]

Mcmaster-Carr has shims that will work, jut measue the old ones and order an assortment of thicnesses to work with.

[herman maire]

I checked the mcmaster site but they just have so much stuff not really specific to porsche.

How would you order the correct belleville washer? Doesn't it have to be applying the correct pressure?

Is there any other sources for ZF clutches for porsche differentials

[KTL]

Pelican can supply you with the friction plates. All you need are the Porsche part numbers from the parts diagrams

I actually had Guard Transmission supply me with the necessary friction plates. Reason being is I asked Matt to see if he could reconfigure the breakaway torque rating to a more typical 40 or 50 percent symmetric diff. Didn't have the spare parts on hand to cook up a Frankenstein diff, so as long as he had it, I asked him to do simple rebuild of my ZF 20/100 G50 LSD with the correct friction discs. Had it all nicely reconditioned by a pro and nobody wanted it when it came time to sell....... :P

[Walt Fricke]

Herman - I think Gestalt was referring to the shims which adjust the ring gear, not the pieces you would use to adjust the working of the limited slip.