A fairly simple mechanical device is used in many different applications to transfer rotational forces. In a car, the differential maintains drivability in varying conditions. It alters how the vehicle negotiates turns as opposed to having both rear wheels spinning at the same speed; the inside wheel is allowed to rotate at a slower speed than the outside wheel. Several types of differentials are used in automotive applications. Each has its own benefits and drawbacks. For example, an open-type differential is suitable for common highway use, but on the drag strip or rock crawling it wastes power and zaps momentum. Figuring out what kind of differential you need for your application is a key aspect of your build.
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The available differentials on the market are open, limited-slip, locking, and spool. The axles are tied together permanently with the spool, so technically, it is not a differential at all because there is no differentiating rotation between the axles. However, spools are used for certain applications.
Open differentials comprise the following components. The housing (or case) carries all of the gears. Side gears are the gears that are connected to the axles. Pinion or spider gears ride on the side gears, allowing the differential rotational action. The center (or cross) pin performs several functions as it runs through the pinion gears. This is the shaft on which the pinion gears spin. In addition, the cross pin keeps the axles from sliding inward, locking them in place. Differential side bearings allow the carrier to spin in the case. The ring gear is part of the main gear set, and it bolts to the carrier.
When the vehicle is traveling straight, both tires are spinning at the same rate. This means that the force on the differential side gears is the same. When the differential is operating in this manner, the pin-ion gears do not spin, so the power is transferred equally to both wheels.
When the vehicle enters a corner, the loads begin to vary and the outside tire travels farther than the inside tire. At this point, the differential pinion or spider gears spin to allow smooth turning without any tire noise or chirping. For most vehicles, this is just fine and rarely presents a problem, until you find yourself on a slippery surface such as snow, ice, or mud. When an open differential senses low traction, it simply allows the slipping tire to spin; if the other tire has good grip, it won’t matter, as the mechanical gearing has no kind of locking feature.
In performance applications, the open differential does not supply enough traction. Accelerating quickly overloads the traction capabilities of the tires, resulting in spinning one tire down the road, earning the open carrier names such as “one-tire fryer” or “peg-leg.”
Some folks try to solve this condition by welding the spider gears to the side gears. This essentially turns an open carrier into a fully locked carrier. But this is not the right solution. You should avoid doing this because a street car needs the rear tires to spin at different rates in turns, otherwise cornering is severely compromised. Welded spider gears are not very safe. If the welds brake, it could lead to a catastrophic failure of the complete axle assembly, causing a serious accident.
Another way to alter the open carrier case is by using a drop-in or “lunchbox” locker. This replaces the spider gears completely. The drop-in unit locks the axles together when in a straight line and then unlocks through turns with a ratcheting action. These will be discussed further in Chapter 5.
Limited-slip differentials (LSDs) perform as the label implies. This type of differential allows either side to slip a limited amount before locking the axles together so that they spin at the same rate. A limited slip is installed in most muscle cars and high-performance vehicles because this affordable differential effectively transmits torque to the ground. Best known by the GM names Posi-Traction (Chevy), Positive Traction (Buick), Anti Spin (Cadillac and Oldsmobile), and Saf-T-Track (Pontiac), the LSD has been used by just about every manufacturer at some point. This style of differential is designed to transfer power from “the wheel that slips to the wheel that grips,” or so the commercials used to say. To do this, the limited-slip uses either gears or friction clutches and cones to allow the axles to spin at different rates. Both styles have their advantages and drawbacks, depending on how the vehicle is to be used.
Two types of LSDs are used in automotive applications: clutch (or cone) and gear-driven. These designs have been around for more than 50 years. Over the decades, better manufacturing processes and advanced materials have increased strength and durability, but the principles of operation remain essentially unchanged. GM factory Posi-Traction units found in 10- and 12-bolt axle assemblies were all clutch-type units. The clutches and springs wear out over time but they are rebuildable. Gear-driven LSDs have been used in factory applications, but not in the 10- and 12-bolt housings. They are available in the aftermarket for these housings however, and have become increasingly popular because they are stronger and do not wear out as the clutch/cone units do.
Gear-driven LSDs are becoming much more popular. Eaton’s Truetrac was the first LSD to use helical gears over clutches, thus eliminating the wearable clutches and further reducing the chattering clutch noise.
A gear drive is technically not a limited slip; rather, gears are actually torque multipliers instead of controlling slip through a clutch/cone system. As one wheel loses traction, the gears spin and transfer power to the wheel that is gripping. Once a tire has completely lost traction, the LSD operates as if it is an open differential, except that the slipping tire receives no torque. The benefit of the geared LSD is that the application of torque is moderated by engine speed and tire slippage, which makes it easier to get the vehicle out of stuck situations.
The geared LSD is well suited for drag racing, drifting, and road racing. Not all geared LSD units are created equal. Icy conditions can cause traction problems for street cars equipped with geared LSDs because these differentials free wheel in no-traction situations, and this can result in a loss of control. In this situation, applying light pressure to the brakes or using the emergency brake puts enough load on the axle to send torque through a particular wheel. This can be considered a drawback of the gear drive, but not all geared limited slips have this issue.
Some units feature a bias plate that maintains a supply of torque to both wheels. The Truetrac, a helical-gear-type limited-slip, pro-vides smooth power delivery in poor traction situations and makes an excellent choice for hauling heavy payloads and trailering. It is also an excellent front axle application choice for a vehicle that is both a daily driver and a regular off-roader. The biggest benefits are the low-maintenance design as well as smooth and quiet operation. When the clutch-type differential chatters and alters the ability to steer the car, such as with a 2-way-clutch LSD, the gear-drive differential does not.
Clutch and cone-type differentials are commonly installed in cars from the factory. In a clutch/cone differential, pressure rings on the clutch stack are forced outward as the pinion cross shaft tries to climb the ramps of the gear teeth. As the torque increases, more compression is put on the clutches, coupling the axles, and reducing slip; it functions similar to slipping the clutch on a manual transmission. The Eaton Posi (a clutch plate type) is the original Posi-Traction differential. It provides efficient transfer of power to the rear wheels equally so that acceleration is maximized yet controlled. Some of these units can be tuned with shims, different clutches, and even metal-on-metal clutches.
Clutch-type limited-slips are the most common and popular. Many owners opt for this differential because they are inexpensive com-pared to other limited-slip differentials and they deliver consistent performance for high-performance street cars. There are three different configurations: 1-, 1.5-, and 2-way. Each is suitable for a particular application; there are enough options to allow you to get the right style for your application.
Replacing the clutches is not difficult, but you need to correctly break in the clutch pack. See “Project: Rebuilding a Clutch-Type Differential” on page 65). Each manufacturer has its own specific break-in procedures that must be followed to obtain optimum coupling. Although not difficult, the break-in is crucial. Another caveat to the clutch LSD is oil; all clutch LSDs require an LSD– gear oil additive.
The LSD operates on three input torque states: load, no load, and over-run. Under load, the axle coupling is in proportion to the torque load. Therefore, heavy torque loads yield full coupling while light torque yields a partial couple. Under no-load conditions, it operates in static couple similar to an open carrier. Overrun operation classifies the limited-slip type. Overrun is the sudden release of torque, such as hard on the throttle and then jumping off.
The manner in which the LSD reacts to overrun situations deter-mines whether the LSD is a 1-,1.5-, or 2-way. It’s an important aspect for any street car.
The type of limited slip has a big impact on the driving characteristics of the vehicle. A 1-way LSD releases the coupling. The LSD releases the coupling as soon as the throttle lifts. This is the safest type of LSD, as it allows the rear tires to spin as needed.
It is a 2-way differential if it increases the coupling regardless of forward or reverse torque upon throttle release. Drift racers prefer the 2-way LSD, because it does not open the coupling when the driver lets off the gas, going from 100- to 0-percent throttle. This does two things: It allows the driver to be in control of the wheel spin, the differential is engaged, and in terms of drifting, it keeps the wheels spinning throughout the drift. However, an inexperienced driver can fall into an unwanted spin with a 2-way LSD.
The middle ground is the 1.5, which has less deceleration lock-up than the 2-way but retains the coupling, unlike the 1-way.
Most performance applications should consider a 1.5- or 2-way unit to maintain vehicle control. Road course, auto-crossing and aggressive street driving are best suited for a 1.5–LSD. The 2-way differentials remain in a hard lock during aggressive cornering. Drift racers and experienced road racers often use a 2-way LSD, but these drivers have the experience and skill to control the car with a 2-way on the limit. A 2-way should only be used by a skilled and experienced driver on the street, otherwise its use can lead to loss of control; many drivers find the 2-way differentials challenging. Imagine turning a corner with your foot deep in the throttle. As the axle assembly breaks away and the vehicle goes into over steer, you let off the gas.
A 1.5- or 1-way differential allows the spinning tires to slow as needed, but a 2-way remains in a hard lock, making it much more difficult to control the slide.
The shape of the ramp on the spider gears determines the LSD type. The spider gears climb the side gears as the carrier spins in the case. The ramp of each gear tooth controls how the rotational torque is applied. Under load, the drive-side of the gear is climbing the front edge of the side gear teeth, controlling the clutches.
When torque is suddenly removed, the backside of the gears takes over as the side gears are running up on the back of the spider gear teeth.
If both ramps of the gear teeth have symmetrical slopes, the differential is a 2-way. If they look like saw-teeth (one vertical, one sloped) then the LSD is a 1-way. If both sides have sloped ramps but are asymmetrical, it is a 1.5-way differential. A 1-way LSD has the quietest, smoothest, most noise-free operation. The 2-way tends to be a little noisy, the cut of the gears and increased clutch pressure tend to “chirp” around corners. This is not tire chirp, but rather the clutches slipping.
Project: Installing Plug and Separator Block
Step 1: Install Plug and Separator
This plug and separator block for the Eaton Truetrac is also used for the Eaton Detroit Locker; it is much stronger than stock. The separator block installs between the axles, and provides much better reliability than the stock pin, especially in high-horsepower applications. These typically do not back out or break.
Step 2: Install Snap Ring
Place the plug in the carrier (it goes in easily) and use snap ring pliers to place the snap ring into the retention groove in the carrier.
Step 3: Install Snap Ring (Important! CONTINUED)
Make sure the snap ring is fully seated. If it comes out, the axle will suffer substantial damage.
Step 4: Separate Ring Gear and Carrier
In most cases, this step is fairly easy, but some-times it takes a bit of effort, especially if the gears have been on the carrier for years. You can lightly tap the gear to break it free from the carrier. Tap the gear in several places until you have a slight gap between the gear and the carrier.
Step 5: Separate Ring Gear and Carrier (CONTINUED)
Once there is some room between the carrier and ring gear, you can insert a prybar, but be careful when you use it because you don’t want to damage the carrier or the ring gear. If you have a soft vise, you can put the assembly in it. Otherwise, having a second set of hands to support the gear is a good idea.
Step 6: Inspect New Gear
Inspect the new ring gear for any signs of damage, burrs, or problems in the casting. You should also clean off the shipping grease. The new gear should slide on nice and easy.
Step 7: Bolt Carrier to Ring Gear
Place the carrier against the ring gear and thread two bolts into the back of the ring gear at opposite sides of the gear. You just need them finger tight so the ring gear and carrier are attached. This allows you to hang the ring gear upside down.
Step 8: Install Carrier Bolts
Apply red Loc-tite or another high-strength thread locker to the bolt threads. Thread them into the ring gear but do not torque them down yet.
Step 9: Install Carrier bolts (Torque Fasteners ,CONTINUED)
Tighten the bolts in a star-pattern and then torque them to spec. For GM differentials, 55 to 65 ft-lbs is the general rule. You will need a helper for this task.
Project: Installing Ladder Bars
When it comes to building a street/strip drag car, you need to make some compromises. Big power coupled with a stiff chassis and stock leaf springs is a recipe for no traction. Leaf springs, like traction bars and installing stiffer springs, offer a Band-Aid fix. But the inherent problem remains; there is no static connection to the chassis. The axle assembly is hanging off the chassis on a stack of steel leaf springs that shift, twist, and wrap up. This allows the tire to leave the payment so traction and control are compromised. In addition, the leaf springs sit outside the subframe rails, seriously cutting into the wheel well area. Because I am running slicks on the Royal Scamp, I need a better solution.
In addition to independent rear suspensions, you can choose a leaf spring, ladder bar, or four-link suspension. Leaf spring rear suspensions perform well for some applications, but a serious drag car doesn’t use the stock leaf springs. Instead, most drag cars use a four-link–style suspension because it offers the maximum amount of adjustability and allows for chassis tuning at the track.
The four-link uses an upper and lower bar on each side that connects the chassis to the rear end. These may run parallel or be triangulated. The top bars run outward from the center of the chassis to the rear. The bars connect to the chassis and rear via rod ends that bolt to plates. These plates typically have 8 to 12 potential mounting points so there are quite a few tuning options.
The ladder bar suspension pro-vides adjustability on the front side (bar to chassis) and a static mount to the rear. For the novice, ladder bars are easier to tune than the four-link, and when set up correctly, they pro-vide the same benefits. As with the four-link, ladder bars use a coil-over shock to support the weight of the vehicle. This also allows for more tuning, such as ride height; with adjustable shocks, the dampening factor can be tuned.
Beyond tuning, the thing that makes ladder bars (and the four-link) so effective is the controlled motion of the suspension arc. The motion through the range of travel occurs the same way, every time. This static connection to the chassis controls wheel hop and promotes better weight transfer to the tires for optimum traction. Leaf springs simply cannot do this; there are too many variables. What one side does can differ from what the other side does. The choice for the Royal Scamp is ladder bars.
Installing ladder bars is a big task. Although a few bolt-in kits are avail-able, most kits are weld-on systems. I chose a 36-inch ladder bar for a custom drag car from Chris Alston’s ChassisWorks. The kit came with the ladder bars, coil-over shocks, track locater, and tubing for mounting the system. I went with VariShock double-adjustable shocks for maxi-mum tuning. I could have installed 32-inch bars, which would have pulled the front crossmember rear-ward about 4 inches (this would have put it on the factory subframe), but the longer bars provide more stability and because I was adding subframe connectors, it was not a problem.
The rear coil-over mounts are one key area that added difficulty to this project. Factory shock mounts are typically not suitable for sup-porting the entire back end of the car. The shocks have a maximum extended length of 14 inches, and minimum travel of 13 inches. The recommended travel is right in the middle at 13.5 inches.
With this setup, I need to mock up a set of shocks. With the rear ride height of 10 inches and the shock mounted in the middle of the shock mount, the top of the shock mount was above the stock floor. I could have dropped it down, but would have lost some adjustability. The shock crossmember was welded to the rear down-bars on the roll cage. This might not be acceptable for some builders, so choose your shocks and springs carefully. A shorter 10-inch spring and matching shock would probably fit under the stock floor, but take your own measurements first.
It took about a month to get it all together, which was mostly evenings and weekends, including all the measuring and cutting out the floors. There is probably 100 hours’ worth of work on this project.
With a set of Toyo 12-inch slicks mounted to 15 x 10 centerline “Fuel” drag wheels, I had about an inch of clearance on either side (stock frame rail to the outer wheel well lip). Gaining this much room is worth the effort of the swap, not to mention the superior traction.
You must take your time and measure five times. This is one area where you don’t want to rush things.
Step 1: Install Threaded Rod Ends
Thread the tie-rod end into the ladder bar. Thread each end 3/4 of the way so you can adjust it once it is hung on the suspension.
Step 2: Measure for Fitment
You can use a sheet of plywood on the ﬂoor for protection when you assemble the bars. Square the bars 90 degrees to the vertical center of the rear differential mounting point and measure the actual length of the bars. The long bar with the rod end fully threaded is the upper bar. It is important to ensure that the bars are located correctly front to back in the wheel well.
Step 3: Measure Suspension Arms
During the mockup, check where the bars should sit at ride height, which is 8 inches from the axle centerline to the top of the wheel arch, providing 12 inches of ground clearance. The lower portion of the bar (the long side) should rest parallel to the ground at ride height. Using a magnetic level, the lower bar should read close to 0, or 90 degrees. If the 0 is on the top, the needle should point to 90. If 90 is at the top, the needle should point to Also measure and mark the wheel center-to-front ladder bar mounting point at ride height.
Step 4: Measure Subframe Connector Mounting Points
You marked the subframe connector at 38 inches. Double-check the mark with a measuring tape. The overall length of the ladder bars, including the mounting brackets, to the center of the forward crossmember mount is 38 inches.
Step 5: Locate Front Mounting Points
Once you are satisfied with the position of the ladder bars use the front ladder bar brackets to mark the subframe. With the bracket rotated so that the pick-up points run 90 degrees to the ground (straight up and down), and the upper edge just touching the ﬂoor, mark the subframe connector with a half-moon. You will weld in a round crossmember here. Note the roll cage tube coming through the ﬂoor; you will tie it to the crossmember tube for extra stiffness.
Step 6: Cut Frame Rails for Front Mounting Brackets
Trim the rails with a plasma torch because cutting half-moons with a hole saw is difficult with the connectors on the car. Dress the cuts with a right-angle grinder and grinding disc.
Step 7: Measure Ride Height (Precision Measurement)
Use the adjustable rear end from Chapter 2 to set the height of the car. Measure both the vertical wheel centerline and the ride height using the center of the wheel well arch (which you marked for the original centerline before removing the original rear), and the lower quarter panel in front of the wheel. This unique device allowed you to mount the wheels and set the width of the rear end so you could narrow the housing.
Step 8: Measure for Position of Front Mounts
Once the rear end is built, the real work begins. Hold the ladder bar crossmember in the car with jack stands and set the position of the mounts. Measure 2 inches from the inner side of the subframe connectors to the outer mount. Bolt the ladder bar rod ends in place to keep every-thing lined up.
Step 9: Tack Weld Front Mounting Brackets in Position
Use a MIG welder to place the front mounting brackets under the car. Complete only one side; the other side must match, and that comes later. Mark the for location in the subframe.
Step 10: Weld Crossmember
Remove the entire crossmember and fully weld the brackets with a single bead. The outside and inside edges of the brackets should be welded up.
Step 11: Assemble Crossmember
Assemble the mounts for the other bar (inner and outer ladder bar mounts and the rod ends for each, bolted together) in the same manner. Place the entire assembly on the ﬂoor. The fully welded side ensures the correct geometry for the unwelded side. Measure each side to ensure the opposite mounts are in the right place. Nice and simple.
Step 12: Weld Crossmember to Chassis
Reset the crossmember into the car, aligned to the original marks. Measure for correct placement, and then weld in place.
Step 13: Install Rod Ends on Bracket
Reinstall the rod ends into the bars (with antisieze!) and bolt them in place. Set the ride height to the center position so you have some adjustment room. You will torque them down when the final assembly is on the ground.
Step 14: Weld Rear Mounts to Axle Assembly
Install the rear end mounts and place them up to the rear housing. The optimal position for the rear end is 2 to 3 degrees down angle to the front of the car. This provides the right geometry for the driveshaft rotation. A magnetic angle protractor is really helpful for this project.
Step 15: Weld Rear Mounts to Axle Assembly (CONTINUED)
Tack weld the ladder bar mounts and the shock mounts in place. You can put the shock mounts anywhere, but the farther apart, the better. You can keep it simple and place them between the ladder bar mounts.
Step 16: Weld Rear Mounts to Axle Assembly (CONTINUED)
Assemble the shocks (no springs yet) and bolt them to the lower mount. Position the upper mount ears vertically and mark the placement for the crossmember.
Step 17: Measure Rear Down Bars (Special Tool)
Measure the distance between the two rear down bars of the cage and transfer it to the 15⁄8-inch-diameter tubing supplied for the shock crossmember. Take the mea surement between the two down bars. You can use a PipeMaster notching helper to get the right shape for the notches, and then install the tube between the down bars.
Step 18: Weld Rear Shock Mounts
Slide one of the Miller welding sleeves over the shock to protect it and position the ears to the crossmember. The placement is not critical, so you can position the shock so that the lower shock mount is centered between the ladder bar mounts on the housing. It is always best to set the shocks as close to vertical as possible. Then weld the tabs in place.
Step 19: Assemble Shocks
Remove the shocks from the upper mounts and install the springs. Assembling the shocks is a little tricky. You may have to use a set of spring compressors to get the upper retainer in place, even with the lower retainer threaded all the way down.
Step 20: Bolt Lower Shock Mount to Brackets
Bolt the shocks to the lower shock mount using the supplied hardware and spacers. The ride height can be adjusted later using the threaded lower retainer on the shock.
Step 21: Bolt Upper Shock Mount to Crossmember
With the shocks in place, tack weld the lower shock mount to the housing. All that is left is to pull the rear end and finish welding the brackets in place.
Step 22: Mount Track Locator Bar
This bar centers the rear housing under the car. You can use a panhard bar, track locator, or Watt’s linkage. The track locator bolts to opposing sides (one front and one rear) for the ladder bar mounting bolts with rod ends. Turning the rod moves the rear to the left or right depending on your needs.
General Motors has several names for its limited-slip differential unit. Commonly referred to as PosiTraction, the exact same clutch-based LSD is also called Controlled Traction, Saf-T-Grip, and Positive Traction. This is a clutch-type LSD. That means a stack of clutch discs are packed together to provide the grip.
In the standard pack, there are nine clutches per side: five locking discs with tabs on the outer side that lock into the case and four discs that are splined to lock onto the side gears. When the spider gears pressurize the side gears, the clutch packs press together, pro-viding the positive coupling of the axles. Because the unit uses clutches, they eventually wear out, essentially becoming an open-type differential.
Most of the factory clutch-type LSD units are based on the Eaton LSD, which means that the parts interchange across several brands and sizes. The clutches for the GM 10-bolt (8.2- and 8.5-inch) and both car and truck 12-bolt LSD units are exactly the same as the Ford 9- and 8.8-inch, and the Chrysler 83/4, making parts readily available.
Steel Clutch Packs
Four clutch pack options are available. Most stock LSD units use an 18-disc clutch pack, with 9 discs per side. These solid discs use high-quality steel with crosshatched lines on both sides. This is the friction surface, which is metal-on-metal. These do wear out, but it takes a long time for that to happen. Unless the steel discs are severely worn, you can shim the clutch pack to increase the pressure on the discs, regaining the limited-slip action. These discs are very strong and don’t break, but they tend to chatter, especially with the heavier center springs.
The GM service kit uses an 18-disc pack with a series of slots cut around the perimeter of the clutch discs. These slots reduce chatter, but they end up weakening the disc, which leads to breakage. In addition, they still chatter when used with 400- and 800-pound springs. Most builders prefer the solid discs.
For race cars, the Eaton 22-disc clutch pack is available. These clutches work well in extreme conditions, until the locking tabs on the intermediate discs wear out. These discs chatter with all spring combinations and require frequent service.
Eaton also makes a carbon-fiber clutch kit for LSD units. It is a 14-disc pack with seven per side, and uses a more traditional fiber-clutch and steel-plate configuration. Each of the three clutch discs has a carbon-fiber pad on either side that engages the smooth steel surface of the intermediate discs. In this pack, the clutches have tabs that lock into the case on the side, and the intermediate discs lock onto the side gear. The carbon-fiber material provides excel-lent grip without chattering when used with 400-pound springs or less.
Four spring rates are available for Eaton clutch-type LSD units: for Eaton clutch-type LSD units: 200, 300, 400, and 800 pounds, which indicates the preload on the clutches. These spring kits are available from Eaton or General Motors. For most street performance vehicles, the 400-pound springs are best. The 800-pound springs are very tight and produce more chatter and less slip in the corners. All Eaton Posi units come from the factory with 400-pound springs.
Project: Rebuilding a Clutch-Type Limited-Slip Differential
Rebuilding a clutch-type differential is not difficult, but it does require some patience. You need to use spring compressor or sliding clamps, the proper wrenches, and other materials. To properly assemble the clutch-type limited-slip differential, carefully per-form the following procedures.
For rebuilding a clutch-type differential, the first step is to remove the carrier from the vehicle. In most cases, the cross shaft is already out. However, in vehicles with bolt-in axles, you have to remove the cross shaft first.
You must secure the housing in a bench vise and the rest of the process can be done on the workbench. Place the ring gear pad in the vise and clamp it down, with the large hole in the case facing up. The springs and plates must be removed first. Using a pry bar or screwdriver, gently pry on the plates to slide them out of the housing.
As the assembly begins to come out, use a clamp to keep tension on the springs; otherwise, they will fl y everywhere. You can release the tension slowly once the assembly is fully removed from the housing, or keep them together.
Step 1: Remove Spring Plates
Before you start, place the differential in a soft vise so it’s firmly anchored to the bench and easy to work on. Rebuilding a clutch-type limited-slip differential is not difficult, but you do need to follow some specific procedures. First, carefully remove the spring plates with a screwdriver or prybar. Don’t pry too hard on the springs, if they come out of the plates, they can bounce anywhere. Work the plates out evenly, walking them out side to side.
Step 2: Compress Clutch Plates
Once you have pried up the clutch plates until the edges are exposed, use soft wood Irwin clamps or C-clamps to grab the edge of the plates and com-press the plates. The springs place tension on the plate and if not com pressed, they will fl y all over the workshop when removed from the differential.
Step 3: Remove Spider Gears
Slowly roll the spider gears by hand and remove them by hand from the case. If it has a thrust washer, keep it with its gear. You roll out each gear separately.
Step 4: Remove Side Gears
With the spider gears out of the case, simply lift out the side gears. Remember how the keepers slide out of the case. There are a total of four keepers.
Step 5: Inspect Parts
Be sure to inspect the clutch plates, side gears, spider gears, and all other miscellaneous parts. In this particular case, the gears in the set were good; just a new set of clutches was in order.
Step 6: Inspect Side Gears
The side gears are the last part of the clutch pack. Inspect the inner surface for hot spots, pitting, or bluing. If you have any of these issues, replace the side gears.
Step 7: Inspect Steel Plates
Stock clutches use steel plates with crosshatches on them. Although these clutches are not in bad shape, the housing had a lot of metal shavings in the oil and they were a little loose in the pack. These clutches don’t wear very much and can often be reused with a shim pack. If the discs have discoloration, they need to be replaced. You might want to change the stock discs to carbon-fiber clutches anyway.
The spider gears and side gears work together in mesh, so you need to spin them to get them out. Rotate the side gears until the spider gears reach the large hole and remove them. You have to rotate the side gears twice, as the one spider works its way 180 degrees from the hole.
With the spider gears out, the side gears slide right out of the case. Be mindful of the locking clip that holds the discs in the slot. There are two clips per side. Most rebuild kits come with replacement clips, but if you are just shimming your LSD, you need to keep them.
At this point, you should have all of the main components out of the case and on the bench. Remove the clutch packs from the side gears, and pay attention to how they are positioned on the gear, including the shims between the pack and the side gear. Keep this arrangement side to side.
Be sure to inspect the gears for excessive wear and any obvious dam-age. They should be smooth, without any chips, missing teeth, or rough patches. The spider gears should have smooth, shiny surfaces where they ride in the case and should not be pitted or discolored. The side gears should be clean and free of chips, scarring, or missing teeth.
Also inspect the inner splines for the axles and the outer splines for the clutches for damage.
The clutches should be intact, with no breaks, cracks, or discoloration. If you are reusing the clutches, make sure that the crosshatches are uniform and not significantly worn. If you are replacing the clutches, this inspection is not critical. Look at the case for obvious signs of wear. Replace any part that is suspect.
Clutch Pack Assembly
The clutch plates and wearable discs assemble in a specific sequence. From the inside of the side gear the sequence is: locking disc, spline disc, locking disc, spline disc, and so on until the pack is fully stacked and topped off with the original shims.
The number of discs varies by the style of clutch. For carbon-fiber clutches, the clutch material is on the locking disc, with the splined disc between. The stack sequence is the same.
These clutches are the wet type, meaning that they need to be lubricated. Apply new gear oil to each disc, including the inner side gear surface, before stacking the pack.
Step 1: Unpack Clutch Rebuild Kit
Limited-slip clutch plates are made from a variety of materials, including steel and carbon fiber. Aftermarket replacements are available in several versions. Yukon Gear offers this 14-disc pack with carbon-fiber clutches for about $250; it includes all of the clutch discs and new retainer clips.
Step 2: Inspect Clutch Plates
The carbon fiber clutches pro-vide ample grip and excellent torque transfer while maintaining a long life span. The steels between the clutches are new to match the clutches. Before you go ahead and install the parts, carefully look over the mating surfaces and tabs. Although these are brand-new parts, problems in packing and manufacturing can occur. You need to ensure the parts are not damaged.
Step 3: Clean Clutch Plates
Spray brake cleaner over the new clutch discs to strip away any shipping grease.
Step 4: Prelube Clutch Parts
Prelube all of the parts, including the clutches, with a product such as Royal Purple gear oil. Spread the oil onto the parts with your fingers. Synthetic oil is just fine for breaking in a differential but don’t use it for an engine build.
Once the clutch pack is assembled, the clutch pack and other parts can be installed in the differential housing. The side gears are fairly simple, but the locking tabs can fall off easily. Dab a bit of grease on the locking clip to hold them in place.
The spider gears are tricky. You have to compress the packs on both sides and spin the side gears at the same time. There is not much room for two sets of hands, so you will have to improvise. If you have a spreader clamp, use it. I use a bolt with a coupling nut to spread the load across the gears.
Place the spider gear in the housing and roll the gears until it is in the 180-degree location below the large opening. Reset your spreader device and place the other spider gear into the housing. If you have it just right, the two gears should line up with the cross-shaft holes. If you are a tooth off, you have to start over. Once the gears are aligned, rein-stall the spring pack along with the cross shaft.
Step 1: Place Clutch Disc on Side Gear
To begin the clutch pack assembly process, place all 14 discs in the correct order on the side gear. Place the clutch disc onto the side gear. Each clutch kit comes with specific instructions for stacking the pack.
Step 2: Place Locking Disc on Clutch Disc
Place the locking disc over the clutch disc.
Step 3: Coat with Oil
Make sure each side of the splined locking disc is coated with gear oil.
Step 4: Coat with Oil (CONTINUED)
Make sure to spread the assembly lube across the face of the disc. Repeat the process with the rest of the clutch pack.
Step 5: Add Keepers
When all of the discs are loaded, place the keepers onto the clutch stack. It may help to add a dab of moly grease to hold them in place.
Step 6: Install Pack in Carrier
Now that you have a fully assembled clutch pack, install it in the case. Carefully lower the side gear pack into the carrier, making sure that all discs remain in place. This part of the procedure can be tricky because the keepers tend to fall off. A dab of moly grease might help keep them in place.
Step 7: Roll in First Side Gear
Once both side gears have been installed in the differential housing, roll the first spider gear into the case by spinning one of the side gears. It has to be rolled into the position on the gears.
Step 8: Roll in First Side Gear (CONTINUED)
A spacer such as this one (made from a 1/4-inch bolt and coupler nut) helps push the side gears into the case so the other side gear can be installed. As you install the other spider gear, the side gears become tighter in the case.
Step 9: Roll in First Side Gear (CONTINUED)
Using the axle to spin the side gear helps roll the side gear into place. The clutches are very tight, so the added torque is helpful.
Step 10: Place Second Side Gear
You may have to experiment with a few pry tools to get the second gear into place. Using the center pin to stake the first spider is a good idea. The side gears become tight and spinning them is tricky. If the spider gear is off even one tooth, it doesn’t line up with the center pin. Be patient, you will get it.
Step 11: Place Second Side Gear (CONTINUED)
Drop the second gear into place and spin the side gear with the axle shaft to move it into position. The axle shaft makes it much easier to spin the side gear and roll the spider gear into position. It may take a few tries to position the spider correctly.
Step 12: Align Side Gears
Getting the two side gears into alignment with the center pin holes can be frustrating, but with some patience it will eventually line up. If the gear is off, remove it and spin it one tooth up or down on the side gear and try again.
Testing the Differential
At this stage, you can test the operation of your differential. You can use the axle shaft to engage one side gear and spin it or clamp the axle into the vise and rotate the housing. To do this, you need a bench and vise. If the housing spins easily, the clutch packs are not engaging and the limited-slip is not functioning. Thus, you need to add more shims to the clutch pack; try to add shims equally per side, one at a time. If it is tight, and requires a lot of effort to spin, it should be good. This is the method most builders use.
The spider gear clearance can also change how the unit locks up. Either steel or brass shims are available to tighten the spider gears. If your unit had shims between the case and the spider gear, reuse them. If the gears are not tight to the side gears, you can use a thicker shim. The goal is to have a nice, tight differential; if there is any play or it spins easily, add shims.
Axle Assembly Continues
In this chapter, I have explained how to rebuild the Chevy 12-bolt differential carrier. Specifically, I have detailed the steps for assembling the clutch pack and installing the clutch pack in the carrier. Before you install the carrier in the differential housing, refer to the installation of the pinion gear in Chapter There, I reveal how to install the pinion gear and set it at the correct depth in the case. The pinion bearing, seal, and yoke installations are also detailed in Chapter 8. After the carrier has been installed, you need to check ring and pinion gear mesh, which is also specified in Chapter 8. When backlash has been determined and is within spec, you’re close to completing the entire axle assembly so follow the remainder of the procedures in Chapter 8.
Written by Jeferson Bryant and Posted with Permission of CarTechBooks