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Fri Feb 20, 2015 12:25 pm

A number of years ago, I had internal components manufactured to modernize Porsche's early 904 LSD, made by ZF. A year or so ago, I learned that my upgrade kits also fit the VW IRS Type I LSD. So I thought I'd create a rebuild thread clearly showing this upgrade.

Both the 904 and VW IRS LSD were manufactured during the same era .. a time in which ZF was still experimenting with various internal components to determine optimum function and longevity. Both LSDs share the same approach to lock-up: multiple active plates having no friction material (plain steel), and no Belleville washers (thus no static preload on the plate stack). This is called a “Zero Preload LSD”. During the following decades, friction materials improved, and the importance of static preload came to light.

VW LSD components spread out:

This LSD is in far better shape than average, but the plates are in obvious need of replacement:

Because most 904 LSDs are used on the track for vintage road-racing, my 904 kit provides four friction discs along with four hardened and REM-polished plain plates. And, we of course added a pair of Belleville washers to the kit to introduce static preload. Two of the LSD's original plates are retained as spacers.

For street use, 40% lock-up is more desirable, so the smaller 40% kit utilizes only 2 friction discs, 2 hardened plates, along with the Belleville washers. Six of the LSD's original plates are retained as spacers. As this VW LSD is destined for street use, this is the kit we're installing. Just two of these modernized friction discs (along with the addition of Belleville washers), provide the LSD with more plate friction than the six OE plain steel discs.

The individual components are carefully inspected, and any imperfections addressed.

Nicks in LSD body:

Highs spots worn into the pressure ring grooves:

This LSD body had very nice grooves, but still required a bit of cleaning up:

The nubs of each pressure ring had their chamfer increased just a tad:

Measurement of the various internal components assures proper clearances and “free space” within the LSD. I'm going to rush over this section by just showing a few photos, as this procedure isn't really required for bolting in this kit:

The first step in reassembly is to install the small thrust washers with grease on their backside, insuring that they stay in place during reassembly:

At the point that our first side gear / output shaft is installed, the diff body will need to sit on something that accommodates the protruding shaft. So, I've readied some large sleeves for the body to sit on:

The first Belleville washer is installed with convex inward, concave outward (outer edge contacting diff body, inner edge contacting plain plate):

Then the inner three spacer plates:

The single hardened plate is then installed:

Because the ID of the Belleville washers are centered within the LSD by the splines of the output shafts, ..

.. the Belleville already installed must be centered below the various plates:

Then the first friction disc is installed:

The inner pressure ring is installed:

With the diff body now propped up on my sleeves, the first side gear / output shaft is installed. Although not shown, every part installed is liberally slathered with gear oil. The shaft-to-bore contact area gets an oil/moly mix slathered on:

The cross shaft / spider assembly is now dropped into place. If the cross shafts don't seem to properly seat in the pressure ring's notches, the output shaft's splines are hanging up on the ID of the Belleville washer, and you will have to back up a couple steps to again properly center the Belleville.

Now is a good time to squirt extra oil on the spiders and inner side gear.

The left-hand side gear / output shaft is now installed, along with the left-hand pressure ring:

The outer friction disc and hardened plate are installed ..

.. and then the 3 outer spacer plates:

The outer Belleville is the last internal part:

Before setting the cover plate in place, I'll slather some moly/oil mix onto the contact area:

Properly installing the diff cover is the most critical step. We're going to use a pair of long 6mm bolts in the two set screw holes for initial alignment, and four 10mm nuts & bolts with HD washers to pull the cover on perfectly square. Slow going with tiny twists of each consecutive nut minimizes the scuffing between cover and body. Now that we're using Belleville washers, the cover requires a bit of effort to install as the Bellevilles are compressed. A 27/64” transfer punch or drill bit serves to keep the ring gear bolt holes in cover and main body properly aligned. Failure to properly align the two may cause problems during later ring gear installation.

The two long 6mm bolts used for initial alignment are now replaced with the small set screws. These two dinky set screws are all that holds the cover down until the ring gear is installed. Since this cover is now preloaded by Bellevilles, it's important to use this cover bolt-down procedure in reverse whenever removing the cover, so that no threads are damaged.

The LSD is now ready for the ring gear to be reinstalled.

Fri Feb 20, 2015 1:07 pm

Thanks for posting.

Your discs are ones Bruce Tweedle used when he rebuilt my ZF. And it works great on the street.

Mon Feb 23, 2015 10:42 am

Very Nice Paul! :D

Sun Mar 01, 2015 11:14 am

nice thread,I plan on taking a swing super diff with screw in sides (not C clips)and turning it into a IRS LSD....some day when i get some extra time.

Sun Mar 01, 2015 4:52 pm

Some of these LSDs are almost beyond repair

Mon Mar 02, 2015 6:58 pm

Paul,

Do you offer rebuilds with the new parts as a service?

Mon Mar 02, 2015 10:03 pm

Probably best to PM me if you need help with a rebuild.

Thu Dec 17, 2015 8:12 pm

Forgive my ignorance, can you please clarify what zf and lsd are?

Thu Dec 17, 2015 8:35 pm

Bob Brugge wrote: Forgive my ignorance, can you please clarify what zf and lsd are?
ZF is the manufacturer and LSD is Limited Slip Differential.

Mon Sep 04, 2017 3:09 pm

Kits now offered to rebuild your own LSD:
https://www.thesamba.com/vw/classifieds/detail.php?id=2091555

Sun Dec 10, 2017 9:07 am

Will this fit the bus 3 rib trans which still uses type 1 internal parts?...this would be so nice to have in my baja!!

Sun Dec 10, 2017 10:23 am

No, that trans would require this ZF, installed with 091 side adjusters and flanges. We're also upgrading this model with our plates and discs, which raises the performance level of either new or old, worn units.

Fri Nov 29, 2019 4:15 am

Hi,

the ring grooves in my LSD body (vanagon 094) are damaged.

I wonder if there is a way to repair the grooves and think about either welding the nicks and reshaping the grooves or machine the grooves wider till the nicks are gone and adding spacer stripes to get the right dimensions.

Just brainstorming, maybe someone with more knowledge can chime in.

Fri Nov 29, 2019 10:47 am

I know Bruce has repaired this sort of damage before, but I find that at this stage of wear, the side gears and spiders have very little life left in them (making the effort of patchwork on a worn-out housing almost pointless).

When we redesigned the swingaxle ZF (and all the early Porsche LSDs), we went to a round-tabbed plain plate, which both minimizes this wear, and allows a fully billet 1-piece housing. (Machining square channels requires that an end plate be welded on after the machining.) We also use 8620 material, rather than mild steel like the ZF.

Sat Nov 30, 2019 8:18 am

Not what I wanted to read :D , but thank you. Iĺl have a look at them side and spider gears.

gears wrote: I know Bruce has repaired this sort of damage before

How? :D

Mon Dec 16, 2019 8:49 pm

The housing is mild steel, but the discs and pressure rings are hardened. That's why those notches appear in the sides of the slots.
Wash off the oil well then dry the diff. Go in with your MIG welder set to high, (or TIG) and build up enough so you can shape it back to what it looked like new.

After I weld, I start with a die grinder to remove the weld that protruded into the bore of the housing. The diameter is not critical, just make it clear.

The next step I do is to use a small diameter cutter in the die grinder to bring down the weld at the sides of the slots to about a mm or less from where you need to end up. If you grind away at the bottom of the slot, don't worry, that doesn't matter.

Now the fun part. Use a square file to work the high points down. If you have an old dull file, get a new one. It will take a long time even with the new one. Start with a 5/16" file then finish with a 3/8" square file. I use a M6 nut as a guage block to start, then use the pressure ring to finish.

Once you've restored the grooves so there's no rock of the pressure rings, you're not done yet. Like Paul hinted above, the side gear and spider gear wear badly. I think it's from lack of oil since the housing is closed. See the pics way up of the Type 1 ZF? It has 4 large holes in the side to let oil in. Type 1 spider gears and side gears rarely suffer the same wear as the 091/094 gears. I strongly recommend drilling holes in the housing. Drill them so they line up with the gap between the pressure rings. I'll see if I have a pic somewhere.

Tue Dec 17, 2019 9:15 pm

Whats are the LSD options for a 901/914 Porsche dogleg box ?, and is there a good source for one ?.
I have one I am putting in my 71 IRS Bug Convertible and an LSD would be good.

Wed Dec 18, 2019 5:25 am

Here are some pics of modifications I did for extra lube in an IRS ZF, I did these after talking with Bruce, picked up a used ZF years ago, like Bruce says not much room for lube when these units are assembled.

Surface grind a flat

Thu Dec 19, 2019 10:35 pm

gereonb wrote: Whats are the LSD options for a 901/914 Porsche dogleg box ?, and is there a good source for one ?.
I have one I am putting in my 71 IRS Bug Convertible and an LSD would be good.

Just two of the many Porsche LSD types still offered by my old company, guardtransmission.com :