DATSUN/NISSAN LIMITED SLIP DIFFERENTIALS

A Semi-technical Description

DISCLAIMER

This document is a description of the limited-slip units used in Datsun/Nissan H190 and R160/180/200 differentials AS I UNDERSTAND IT! I am not an engineer nor do I claim to have a full and complete understanding of these units. There seems to be no published information on these units and I have been unable to find anyone who can explain them to me.

In order to understand these things, I have done some research into the theory and parts, and have successfully rebuilt a few of them. My experience has been exclusively with the H190 unit, but the R-type units operate on the same principles. Please view this document in that light. It should be considered an introduction to the workings of these units, a base from which to do your own investigations. I make no claims to total accuracy. If you find mistakes in either the theory or practice, I would appreciate hearing from you so I can learn, too.

I also assume that the reader already has an understanding of a regular differential and how it works.

STRUCTURE OF THIS DOCUMENT

This document begins with a general description of some types of limited-slip differentials. It then defines the part names as they are used in the Nissan Competition Catalog and gives a brief description of each part. Following that is a section showing the relationship of the parts as they are assembled inside the unit. The next section is a description of how this unit functions. The final section provides information on rebuilding and testing techniques. The Appendices contain cross-references of part names to part numbers for the H190 and R160/R180/R200 units along with some interchangeability information. I make no claims of completeness for this information; I have done the best I can to cross-reference part numbers from the numerous Nissan Comp catalogs in my collection.

GENERAL INTRODUCTION

In a standard differential, if one wheel loses traction, it will get all the power and will spin, while the wheel with traction gets nothing. The idea of a limited-slip differential is to prevent all power from being applied to only one driving wheel when traction is lost. There are numerous types of limited-slip, positraction, locker, etc. units.

Lockers

This term can refer to two types of differentials. The simplest is a regular differential that has had the spider gears welded to the side gears, resulting in a completely "locked" assembly that has no differential action. The circle-track people often use a specially built unit that has no spider or side gears at all, just a carrier for the ring gear that has splines for the axle shafts. This unit is also referred to as a "spool."

A second type of locker is the "Detroit Locker" which is the brand name for a ratchet-type unit. When no power is being applied, there is full differential action. As soon as power is applied, the ratchet mechanism effectively locks the axles together. It is an all-or-nothing deal. Nissan supplies Detroit Lockers for the pickups that run H190 rear ends, but I don't know about interchangeability into other Datsun applications nor will they be discussed further in this document.

Limited-slip

The feature which distinguishes a limited-slip from a locker is its progressive action. It provides some degree of differential action even as power is applied until at some point it may lock up completely, depending on how it is set up. I have heard the terms "limited-slip" and "positraction" used interchangeably and I have heard other people claim there is a difference. No one has been able to explain the differences, if any, to me. The biggest difference seems to be that "Positraction" was a particular brand of limited-slip.

The Nissan limited-slip is a Salisbury-type unit consisting of a set of discs and clutches inside a housing that provide a friction coupling between the two axles. This same type of unit has also been used in E-type Jaguars and Cobras.

Note: at one time, another type of limited-slip unit, called a Gleason-Torsen, was available for the R200. At last report it is no longer available, which is a shame, because it had all the advantages of a limited-slip with none of the drawbacks. It was a very unusual design, using worm gears, worm wheels and spur gears to limit wheelspin while still providing full differential action. Like the Detroit Locker, it will not be discussed here.

TERMINOLOGY

There are a lot of parts in a limited-slip not found in a regular differential, so we had better start off with some names and definitions for these parts.

Case

This is the big cast piece to which the ring gear bolts. It is actually two parts, the major portion of the case and an end-cap to hold all the parts in. The end cap is held on by four countersunk Phillips-head machine screws. The inside of the case has four large grooves in it that run parallel to the axle centerline. The ring gear bolts go through the end-cap and the case and are threaded into the ring gear.

Side Gears

The side gears in the LSD are a bit different from regular side gears but they do the same thing. They have a splined hole in the middle that mates with the splined ends on the axles. They also have a "shoulder" on the back side (away from the teeth) that has six grooves or notches parallel to the axle centerline. The tabs on the Friction Disks and Spring Disks fit into these notches.

Pressure Rings

These are two large cast pieces that sit inside the case. They have four tab-type protrusions around the outside that fit into the grooves in the case. They also have v-shaped slots where the ends of the Pinion Shaft sit (sorry, I haven't been able to come up with a decent drawing of one yet).

Pinion Gears

These do very much the same thing as regular pinion gears. They pivot on the pinion shaft (at Section B-B in the diagram below) and sit between the side gears. In a regular differential they are also referred to as "spider gears."

Pinion Shaft

This is the pivot shaft for the pinion gears. The middle part of the Pinion Shaft is flat on two sides (Section C-C in the diagram). The pinion gears sit on the round portion (Section B-B). It is different from a regular pinion shaft in that the ends have flat areas machined on them in a v-shape (Section A-A). These flats sit in corresponding notches in the Pressure Rings.

Friction Disks

These are round flat ring-like steel disks that go inside the case.The disks have 6 round tabs towards the inside that fit into the grooves in the back of the side gear. They come in several thicknesses, 1.75 mm, 1.85 mm. and 2.00 mm being the most common.

Friction Plates

These look very much like the Friction Disks, except that they have four round tabs towards the outside. These tabs fit into the grooves inside the case. The plates come in the same thicknesses as the Friction Disks.

Spring Disks and Spring Plates

Some LSD units have these and some do not. They are like the Friction Disks and Friction Plates except that they are dished instead of flat. The technical name for this is a Belleville spring. I believe their purpose is to give the LSD a more progressive action. When they are installed, there is one of each on each side, and they are the outermost pieces in the stack (farthest left in the diagram below).

This is a detail of how the Friction Plates, Friction Disks, Pressure Ring and Side Gear stack together.

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ASSEMBLY SEQUENCE

The sequence in which these pieces occur inside the case is as follows:

• Friction Plate or Spring Plate
• Friction Disk or Spring Disk
• Friction Plate
• Friction Disk
• Friction Plate
• Friction Disk
• Pressure Ring (shaded)
• Side Gear (not shown in drawing)
• Pinion Shaft with Pinion Gears (center of drawing)
• Side Gear (not shown in drawing)
• Pressure Ring (shaded)
• Friction Disk
• Friction Plate
• Friction Disk
• Friction Plate
• Friction Disk or Spring Disk
• Friction Plate or Spring Plate

This is an expanded view of the internal components. Note: none of these drawings are to any kind of scale.

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FUNCTIONING

This arrangement creates a stack of alternating disks and plates that sits between the end of the case and the pressure ring on each side. The disks are "connected" to the side gear and the plates are "connected" to the case, both by the tab-in-groove arrangement. If a side gear tries to rotate at a different speed than the case, it causes the disks to drag between the plates. This friction is what provides the limited-slip action.

The amount of resistance to slip is determined by the amount of power being applied - the more power, the more resistance. Here is how it does it.

The power is actually applied to the ring gear by the pinion gear (the main pinion gear, as in ring and pinion). The ring gear is bolted to the case, so the case turns. The pressure rings inside the case are forced to turn with the case because the tabs on the pressure rings are inside the grooves in the case. The pressure rings transmit the force to the pinion shaft, which is trapped between them in those v-shaped grooves. The pinion shaft pushes on the pinion gears and thence on the side gears.

When the pressure rings push on the pinion shaft, the weight of the car resists. The fact that the pinion shaft is sitting in the v-groove causes a spreading force to be applied to the pressure rings. This pressure squeezes the friction plates and disks together, increasing their resistance to slippage. If your unit does not have the Belleville spring plates and disks, then this action will be quite sudden. The Belleville springs allow for a gradual increase in slip resistance before lock-up occurs.

How much slip are we really talking about here? Well, if you do the math on turn radii and tire travel distances, you will find that in ANY 180 degree turn, the outside tire travels about 2 revolutions more than the inside tire, regardless of the radius of the turn. For a car with a 50" rear track, the difference in a 300 ft radius turn is 13.09 ft. For the same car in a 30 ft radius turn, the difference is 13.09 ft. A 23" diameter tire has a circumference of 72.26" or just barely over 6 ft. So when you take that 180 degree sweeper going onto your favorite freeway, the outside tire goes an additional 2 revolutions. And when you take that 180 degree around the cone in your favorite parking lot, the outside tire goes an additional 2 revolutions.

So what's the difference? The difference is the amount of time it takes to do those two revolutions. On the freeway ramp, it may take 10 or 15 seconds, while at the autocross it will take maybe half that. So the relative speed of the surfaces in the LSD unit is different and you will feel it as being tighter at the autocross.

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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.

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APPENDIX A Roadster/200SX Limited Slip (H190)

Legend
NLA - No Longer Available
n/a - not applicable
n/l - no listing of number
Notes:

1. The Roadster used two different cases based on the sizes of the ring and pinion. To use a 4.375, 4.625 or 5.125 gear set with the 38420-A9980 limited slip unit, you must use a 4.0 mm spacer.
   
2. This is the factory setting for breakaway pressure.
   
3. Can be used with modifications.
   

APPENDIX B 510/610/Z Limited Slip (R160/R180/R200)

Legend
NLA - No Longer Available
n/a - not applicable
n/l - no listing of number
Notes:

1. This is the factory setting for breakaway pressure.
   
2. Indicates interchangeability with H165B unit in HL510, 200SX and 710 from 04/73
   

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