REBUILDING TECHNIQUES

In this section, I'm not going to go through the entire process of rebuilding a limited-slip. The basics of taking one apart and putting it together are pretty obvious when you look at it. First I am going to explain breakaway pressure and tell how to adjust it. Then I will give a few disassembly/assembly tips, and mention some things to look for. Finally I will show you how to test the breakaway pressure on the bench.

Breakaway Pressure
What the heck is "breakaway pressure" anyway? The breakaway pressure is the amount of torque required to make the plates and disks slip. In practice, it is the torque generated between the rear tires when they try to go different distances. If you put a torque wrench on one axle shaft and lock the other one in place, then apply torque, the axle won't move until a certain pressure is reached. Then the axle will start to rotate, showing a constant torque resistance. That's the breakaway pressure. More on how to test this later.

How much is enough?
There is no easy way to figure this out. If you set the breakaway pressure too high (set the unit "too tight") for the tires you have, you won't get any differential action at all. Both tires will lose traction before the clutches start to slip. The result is the same as having a welded rear end. The sensation will usually be something like this: when you start into a turn, the front end won't bite - it feels like massive understeer; then the front end will grab and turn and the rear end will try to come around. That's the extreme case. A milder case is moderate understeer with the rear end being overly-sensitive to throttle - as soon as you get on the throttle in a turn, the rear tries to come around or at least get very loose.

Since all of this is based on the traction of the rear tires, you can see why a race car can use much higher settings than a street car. The factory specs are listed in the parts tables. For a street 2000 Roadster I would go 60-80 ft/lb. We have used as high as 200-225 ft/lb. in our 2000 race car (SCCA EP) with Goodyear 23.0/9.0-15 race tires but I think that may be too high.

Adjusting the breakaway pressure
The breakaway pressure can be adjusted by putting shims between the end of the disk/clutch stack and the case end plate. Putting in a thicker shim increases what is called the "preload" or the initial pressure on the disk/clutch stack. If you have the type with all flat disks and clutches, don't get too carried away with shims, because a little bit will make a big change. If you really intend to do this right, you will need a micrometer or at least a very accurate caliper. Be sure you measure each friction disk and plate separately, and keep notes. If you can't dial in the right breakaway pressure with shims, you can selectively change disks and plates to get the right combination.

Tips on disassembly and assembly
Let's start with disassembly because that's probably what you'll do first. Getting the unit out of the housing is no different than with a regular diff; same thing with removing the ring gear. But now there's a difference. Underneath the ring gear are the four little screws that hold the case together. You might be able to take them out with a screwdriver, but if the LSD has been rebuilt with a higher breakaway pressure, there may be a lot of tension on those screws and they strip their slots quite easily.

You can relieve the tension on them by putting four bolts with washers through the ring gear bolt holes and tightening them down snug with nuts. Then try to remove the screws. Be sure to use this same trick when reassembling the unit. Once the screws are in, they will hold things okay until the ring gear is bolted on. It is the ring gear bolts which really hold the case together.

The case end plate may be reluctant to come off the case. Pry it carefully, making sure you don't damage the mating surfaces. Once the end plate is off, remove the disks and clutches, keeping track of the order. Clean and measure all of them. Look for notches in the sides of the tabs where they contact the case or side gears. If they show notches, they should be replaced. All other checks are pretty standard - wear patterns on gears, etc.

Testing the breakaway pressure
When rebuilding one of these units, it is important to lubricate all the parts EACH TIME you reassemble it. If you don't, the breakaway reading will get higher each time you test it. This is a hassle. It means taking the whole unit apart and relubricating every disk and plate in both sides of the case. Unfortunately it is the only way I know to get consistent test results. Use the same kind of gear lube you will be using in the car. Be sure you test it "wet". I tested one dry at 225 ft/lb, then lubed it and it tested at 50 ft/lb!

For working on Roadsters, I made a "test bench" from pieces of plywood as shown in the diagram. The one with the four holes is the end piece. The holes correspond to the wheel lugs in the axle. There are two of the other piece. The notch in the top is a support for the axle shaft. I lined this notch with aluminum sheet so the axles would turn freely. All three pieces are bolted to studs in my garage wall.

To mount the diff for testing, put one axle into the brackets with the wheel studs through the holes in the end bracket, then slip the case and ring gear assembly onto the axle splines. Slip another axle into the other side of the assembly and rest the axle shaft in the far bracket.

Other items you will need to test the breakaway pressure are a 4-foot bar of flat steel stock about 1 1/2-2 inches wide by 1/4" thick and a dial-type torque wrench (NOT a click-type). The bar will need to have two holes 4 1/2" apart near one end and two bolts welded to the other end. One bolt should be two feet from a point exactly between the holes and the other bolt should be another foot further down the bar. Use a good-sized bolt, like one that takes a 3/4" socket.

With the diff mounted in the test bench, slip the bar over the wheel studs in the axle. Put the torque wrench over one of the bolts (so that the wrench is parallel to the bar and the handle of the wrench is away from the axle) and gradually increase the pressure. Try to pull with consistent pressure. When the axle starts to turn, the pressure will bounce a bit then settle down. Record the steady reading. If it isn't right, take it apart and add or remove shims and try it again. The positioning of the bolts will allow you to test a wide range of breakaway pressures with a maximum of control. The near bolt has a multiplier of 3 and the far bolt has a multiplier of 4. For example, setting 80 ft/lb breakaway will be indicated by a torque reading of 20 ft/lb on the far bolt. Remember to re-lube all the plates and disks.

That's really about all there is to it. It's not magic; it's just time-consuming. Take your time, be careful and take good notes so you know what you did. Good luck.