limited slip rebuild instructions

dfink · 05-30-2007, 02:41 PM

Anyone have instructions on how to rebuild a limited slip from a 915 tranny. I checked and can get parts from the dealership. Also what items should I need. I was going to replace the friction disks and what is called an outer spline disks. There are 4 of each 2 on each side. There doesn't appear to be a gasket for differential to case connection. The dealer's mechanic said it is just silicone.
There are also different thickness plates 1.9mm 2.0mm and 2.1mm I assume this is how the lockup rate is adjusted. I was planning on just getting 1.9mm plates.
Any help on this would be appreciated there doesn't seem be much documentation on rebuilding these. From what I have read they are an item that requires periodic maintenance.
Part numbers I was going to order are

4 ea - 92833255100
4 ea - 91733255210

Bill Verburg · Bill Verburg's Garage

The factory disks are carbon coated and don't last long, contact Guard transmission for their plasma coated disks.

You are correct the actual breakaway torque is varied by varying the stack thickness.

You will want to be sure that the wear components areflat and true

Chris Walrad is doing a 964 lsd rebuild here

dfink · 05-30-2007, 03:22 PM

Well I called guard transmission and they do stock most parts for the limited slip units so that is a plus. I am still a little confused on the quality of the disks. He basically said the disks he has are equivalent to an earlier molybdenum disks but the price is not that much more and if Bill says they are better I will believe him.

It sounds like there is really no way to pre-order parts as I won't know the thickness of what is required until I take it apart. I really hate paying shipping fees from California to get things here quick but that may be required. Next auto-x is June 10th local points race don't want to miss that and June 23/24 is divisionals at Kansas City can't miss that.

Is this a fairly easy job to do. Can it be done a weekend tear down , get parts, weekend put back.

The rennlist thread is interesting but I need details man "Details".
How do you measure for the plates?
Is there a tolerance or do you just shove in as many as you can get in there?
Should the thrust washers and spring plates be replaced as a matter of practice?
Do I really need to regrind the ramp plates?

Wayne where is that transmission rebuild book??????????

Grady Clay · 05-30-2007, 05:43 PM

Dfink,

There are a lot of issues rebuilding a ZF LSD. This is a situation where you can’t order up the exact spare parts in advance. A rebuild requires measurement and then install the correct parts. In between there are measurements and more adjustments.

One of the reason shops charge money is they have all this stuff and info available. This allows them to complete the process without “wait time” getting parts. I did this 20-40 years ago and had all the parts and tools.

I want all the details. Perhaps I can offer some direction.

Best,
Grady

dfink · 05-31-2007, 12:50 PM

Tthanks l will probably have to wait until July to get enough time.

dfink · 06-25-2007, 11:57 AM

OK it is time to rebuild the Limited slip. Grady and Bill don't fail me now. The LS unit is out of the car and on the bench. All went pretty well here.

Removed the axle connections then put two bolts back in the axle flange, wedged a baseball bat in the bolts put the car in gear and used a long extension and socket to remove the bolt holding the axle flanges to the differential. The flanged then just slipped right out.

Loosen the clutch cable and move it out of the way and remove the clutch helper springs and levers. Be sure to drain the trany and then remove the 4 bolts that hold the connection flanges for the springs and clutch cable.

Remove the remaining bolts around the differential cover. I used a plastic mallet on the small square protrusion at the bottom of the cover to start removal of the cover. Then used a putty knife to wiggle the cover loose. If you didn't drain the tranny this is where you remember.

Any how after the cover is off the limited slip unit just slides right out. Be careful or it will be on the floor. I would suggest a pan to catch drippings and perhaps some cardboard so if it does slip it might cushion the hit a bit. JUST DON"T DROP IT....

So the unit is out and on the bench. Now what do I do. I have attached a couple of pics but it looks like I have to remove the the bearing and the ring gear. Two things I didn't want to do...

dbanazek · 06-25-2007, 12:23 PM

You do need to remove the bearing & the ring gear. Make sure you have the right puller so you don't mess up the beating or will be reseting your preload. If you measure your disks you should be able to tell which ones you need. They won't be worn that much. You probably just need the inner splined disks in 2.0mm. Gaurd makes nice stuff but I tried their inner splinned disks with Porsche outer splined disks and wrecked the outer splinned disks real fast. They say I need to use all gaurd discs.

dfink · 06-25-2007, 01:58 PM

There appears to be a washer below the bearing is it OK to pull on the washer with a two jaw puller.

Also I got the retainers off of the ring gear bolts. Guess where the only 20mm bolt is on the car.....

dfink · 06-25-2007, 08:06 PM

Bumpity bump bump

dbanazek · 06-26-2007, 06:43 AM

You can pull on that washer. Just make sure as you are pulling you do not start to crush the cage around the bearing. Check it every few turns of the wrench.

Geary · 06-26-2007, 06:53 AM

Do NOT remove the bearing unless you are replacing it. Without the proper puller, you will likely damage it. Only remove the ring gear.

dfink · 06-26-2007, 07:53 AM

I found that all I had to do was remove the bolts from the ring gear. The ring gear did not come off but slid down far enough I could get to the 2 small bolts holding the diff plate. Then the unit just came apart with a little persuasion. The bearing stays put. After the unit is apart slide the ring gear back up and put in a couple of bolts in to hold it.

After cleaning I did the measurements and came up with a chamber difference of 5.5mm

The outer spline disk is 2.0 mm and the friction disk is also 2.0 mm. One possible problem found is the spring washers. One has a nominal thickness of 2.5 mm with deflection in the center to 2.75mm or .25 deflection. the other side has a nominal thickness of 2.5 with a deflection to 3.5 or a 1mm deflection. I believe this indicates at least one bad spring plate.

So my current plan is to order (2) new friction plates, (2) new 2.1 mm outer spline plates and (2) new spring plates. Pictures when I get some time.

dfink · 06-26-2007, 10:35 AM

Well I talked with Guard Transmission and they had some helpful information. They suggested that there should be 1.2mm difference between the stack and the chamber. This allows for .6mm each side. On my setup after double checking measurements I had .2mm total clearance. One spring was collaped causing an un-equal bias in the differential.

As for parts we decided to go with a plasma coated friction disk that is 2mm, I am going to flip the plain disk (outside splined) as they are still 2mm with one side un-worn. We are then going with 2.2mm spring disks. This will add .3mm of play on both sides and will allow the diff to unload properly and have sufficient travel to engage when needed. The thinner disk however will mean that the diff will need to be serviced sooner than with a 2.5mm spring disk. It was suggested that I use swepco 201 fluid with the more aggressive friction disk.

In any case it would appear that 80K miles is a bit much before checking things out. I was told the plasma disks should be more agressive than the originals but since mine was probably never working since I got it I won't be able to compare other than hopefully it will now hookup both wheels on launch.

dfink · 07-02-2007, 06:13 AM

Well the new friction disks and spring washers arrived and have been installed. For those curious the the two small bolts that hold the diff cap on are 10 ft lbs. The ring gear bolts are a difficult 116 ft lbs.

To hold the diff while tightening the ring gear I used a chain wrench. This is basically a chain with a long handle that wraps around the diff housing then locks when pressure is applied.

After assembly I attempted to check the friction level and it is definitely more than before. I put the flanges back in and put two bolts in each flange. A very long screw driver on one side and a baseball bat on the other to attempt to turn the unit. At first I thought it was locked but finally gave way and started to turn. I am guessing there is about 60-70 lt lbs of torque required at this point. Stock is 7 to 25 ft lbs so it should be quite a difference. I am sure that after things get broken in a bit that value will come down significantly.

I have new retainers clips for the ring gear bolts and new bolts for the axels coming hopefully today. Also some swepco 201. There is an auto-x this weekend so should get to try it out. I also have 2 new A6 hoosiers coming for the back and also had the rear swaybar re-drilled with an inch spacing on the hole placement for drop link. I hope that adds maybe 20% to the sway bar.

Will it finally hook up both tires?
Will it push like a pig?
Will it still have horrible oversteer?
Will I have to downshift just to get around a corner?

We shall soon find out.

Geary · 07-02-2007, 06:46 AM

A breakaway measurement is meaningless at this point. You will have a terrible time at the autocross without first breaking this diff in. If it's primarily autocross racing you're doing, a 2-friction disc LSD might suit you better than a 4-disc unit.

dfink · 07-02-2007, 09:13 AM

Quote:

Originally posted by Geary
A breakaway measurement is meaningless at this point. You will have a terrible time at the autocross without first breaking this diff in. If it's primarily autocross racing you're doing, a 2-friction disc LSD might suit you better than a 4-disc unit.

This is a 2 disk unit. There is one friction disk and one plain disk on each side. We shall see how it goes after break-in.

dd74 · 07-03-2007, 10:00 AM

Subscribing - Dfink: any preliminary results? Have you driven the car around a bit?

dfink · 07-03-2007, 11:02 AM

Quote:

Originally posted by dd74
Subscribing - Dfink: any preliminary results? Have you driven the car around a bit?

Not yet the final parts just arrived. I was waiting for retention clips for the ring gear bolts. I will put it togeather later today and hopefully get a nice drive in on the 4th.

dfink · 07-04-2007, 11:56 AM

Well the unit is back in the car and has around a 100 miles on it. No noise from the diff at all so everything must have went back the way it was. I was expecting some drag when turning corners for the first few times but nothing. The car drove perfectly right from the start.
I have not really dropped the hammer from a standing start yet but from a rolling start I don't even get spin it just goes. Also jumped on it a couple of times going around corners where before I would have really gotten alot of wheel spin now really not much of anything just go. It does feel like it may have a bit more tendancy to push.
All this testing has been on my toyo T1S street tires. They are old, hard and worn out. The real test will be this weekend when the hoosiers go back on. I have a brand new set of rear A6 hoosiers but I am keeping my old ones on for this weekend autocross. I want to see what differences I can attribute to the diff and not new tires. To help compensate for the increased tendancy to push, I also had the rear sway bar redrilled 1 inch inward to increase the power of the sway. Just in driving around town it feels like this had quite an effect. Perhaps all in my head but I figure maybe around 20% now that it is about 20% shorter arm. I also moved the mounts for the sway from the trailing arms to spring arms. This lines up better for the adjusted sways.
A few more specs.
The bolt for the flanges 32 ft lbs
The case bolts 17 ft lbs.

dfink · 07-04-2007, 12:04 PM

Also forgot to mention for those reading this to do their own rebuild. In the first post I mentioned the dealer mechanic said there is no seal for the case cover. This is not correct. There is an O-ring that makes the seal for the diff cover to case. The housing is very thick and sets down in the case probably 3/8" there is a groove in the cover for the o-ring. Mine was in good shape and I had not ordered one so I reused mine we will see if I live to regret that one. As hard as I am on the tranny I will probably have to pull and rebuild again soon anyhow so I will put on a new 0-ring then.