How to Rebuild Factory Limited Slip Differentials
The main reason to have a limited slip differential in your rear axle is to maximize traction on slippery surfaces, such as snow, ice, or mud, which can be exacerbated with an open differential. Another important reason is to help distribute the torque to the wheels from a high-performance engine and transmission combination. The limited-slip differential transfers torque to both wheels even if one wheel is spinning. This is a huge improvement over the traction-limited open differential. Limited slip differentials also maximize acceleration of the vehicle.
Determining Limited-Slip vs. Open Differential
To determine whether your vehicle has a limited-slip or an open differential is one of the easiest driveline checks. The first step is to place the vehicle on a flat and level surface, block the front wheels, and raise the rear wheels off the ground with a jack. As always, practice safety first and place the vehicle on jack stands. Shift the transmission into neutral and make sure that the parking brake is released. Now just rotate one rear wheel in the forward direction and pay attention to the rotation direction of the other wheel.
If it rotates in the same direction (forward), the rear end has a limited-slip differential. If the other wheel rotates in the reverse direction, then the rear end has an open differential. It is that simple.
However, there is one exception: An extremely worn clutch-plate-style limited-slip differential may act like an open differential because the plates are so severely worn. So it’s not a totally foolproof test. But this method works for most cases.
You may wonder why one wheel rotates in the opposite direction. There is a lengthy engineering reason for this, but it can be summed up by the following simple equation:
2 x Ncarrier = Nleft + Nright
Ncarrier = rotational speed of the differential carrier
Nleft = speed of the left side gear, which rotates at the same speed as the left tire
N right = speed of the right side gear (or right tire)
If the carrier speed is zero, which basically means that the pinion and prop shaft are not rotating, the left side of the equation is equal to zero.
You rotate one side; let’s say the left, at 10 revolutions per minute. In order for this equation to work, the other wheel, in this case the right, must rotate at the same speed but in the opposite direction:
2 x 0 = 10 + (-10)
0 = 10 – 10
0 = 0
The equation is satisfied. Basically the differential gears are free to rotate relative to one another inside the differential case while the differential case is stationary.
If a limited slip differential is in the vehicle’s rear axle, a preload should be applied between the side gears, which would cause both gears to rotate in the same direction (until the torque generated by the friction of the preload has been overcome). One of two things can happen. The clutch plates can slip relative to the differential, and the differential case would not rotate. If the differential case does not rotate, the hypoid ring gear and the pinion will not rotate. Basically, it would be the path of least resistance. Hopefully, the ring and pinion rotation torque is lower than the torque to slip the clutches, and therefore, the pinion rotates.
If this sounds confusing, don’t worry. This is a little difficult to understand and is only meant as additional information. The most important thing is to understand how to test for the presence of an open or limited-slip differential.
With the wheel adapter tool you can rotate the torque wrench at the centerline of the axle shaft. You just need the correct bolt pattern and nut welded in the center. This home-made tool is round and spans all five lug studs; a triangle-shaped plate that spans three of the studs works fine too.
The limited-slip checking tool just fits over the wheel studs. You just need to stop the other wheel from rotating.
With the tool in place and the torque wrench in place, you can rotate the tool and tighten the bolt. You just need to read the torque values for break-away and slipping torque.
Once you have determined that you have a limited-slip differential, you can check the preload or break-away torque relatively easily. You will need to fabricate an adapter to allow your torque wrench to line up with the center of the wheel bolt pat-tern. This homemade tool can be as simple as a steel plate with the bolt pattern drilled in, and a large nut welded in the middle that is aligned with the center of the axle shaft.
Now stop one wheel from rotating (such as jacking only one tire off the ground), remove the other wheel, and put the adapter over the lug studs. Make sure that the parking brake is not applied; you don’t want to be fighting the brakes at this stage. Now just put your torque wrench on the large nut in the center of the adapter and take a reading of the torque before the axle begins rotating. This can be accomplished with either a beam-style or click-type torque wrench. If you use the beam style, it is helpful to have an assistant read the numbers. If you use the click type, set a torque of 100 ft-lbs initially. If the axle does not rotate, keep increasing the torque until it breaks away.
Typically, if the breakaway torque value is below 50 ft-lbs, the differential needs to be rebuilt or simply re-shimmed. A good range for street use that still provides decent performance is between 110 and 140 ft-lbs of torque. If the clutch pack is shimmed tighter, it produces poor on-road manners during slow, tight-turning maneuvers. These poor manners are exhibited by tire wind-up during a low-speed, tight turn maneuver (such as pulling into a parking spot). Also, the differential will act more like a locked differential and the vehicle can tend to under steer or plow.
The above information is true for all traditional factory type-differentials. You should keep in mind that the helical style differential behaves like an open unless the torque bias ratio is very high.
Every rear end has a differential (see Chapter 1). There are many types available, but I will concentrate on the most common types, which are the open (or standard) differential and the limited-slip differential.
Every manufacturer has its own unique proprietary name for these limited-slip rear ends, but all of differentials essentially function in the same way. I chose the Ford Traction Lok as an example to rebuild, but keep in mind that it is very similar to the GM Eaton-style differential and others. These are relatively simple mechanical devices that are quite effective when compared to an open differential. If your limited-slip differential is worn out, you will end up laying down one of the most monstrous, single wheel peels possible.
The first step is to remove the differential from the axle housing (as outlined in Chapter 3). Consider removing the ring gear for the rebuild process. Some differentials can be rebuilt with the gear still attached (depending on gear ratio and manufacturer), but the differential is a bit heavy and bulky, and the ring gear only adds to that. Additionally, the ring gear can partially block the already small differential case windows. Usually the tear down is much easier than the re-assembly process because in most cases the clutch pack is worn and loose.
Once the differential is removed from the axle and the ring gear is off, the next step is to remove the differential pin bolt and push the differential pin out. The differential pin bolt is the 5/16-inch head bolt with the long shoulder on it that keeps the differential pin from falling out.
It is also a good idea to replace the bolt, if possible, or at least re-apply a thread locker before final installation.
Next, remove the preload spring. To make it easy, use a hammer and soft punch or suitable piece of wood to tap the spring out.
Be careful as you push the last portion of the Ford S-style spring out. It tends to follow the differential side gears’ tooth path and may shoot out of the housing. To minimize the chance of the spring flying out, place a rag over the opposite end of the differential case window.
Once the spring has been removed, take note of how much play there is in the gears. Keep in mind that the thickness of the clutch packs and shims controls the position of the side gears. If the clutches are worn, the side gears are allowed to move farther outboard and the gears have more backlash or clearance between the teeth. (This is the same type of backlash or clearance that we will check when setting up the hypoid gears in Chapter 6.) A typical acceptable backlash range is 0.005 to 0.015 inch.
Since we are talking about high-performance rear axles, you should shim the pack tighter for your hot rod and driving preference
Before you remove the internal gears and clutch pack, mark and keep track of the side gears so they can be returned to the same position or purposely swapped if excessive wear is visible. This is not absolutely necessary, but you want to make certain that you keep track of the parts and have plenty of bench space to lay out the parts.
As you inspect the gears for wear, don’t mistake machining marks for wear. Revacycle is an older machining process for producing the tooth profile, and it produces machining marks from the inside diameter of the tooth face to the outside diameter. This process is still used today but is not very common.
The differential pin retention bolt threads into the differential case to hold the pin in place.
The differential pin retention bolt threads into the differential case to hold the pin in place.
Drive out the Ford-style S-spring between the side gears and through the differential window. The spring will pop out the opposite side. As it is under some compression, be sure no one is standing in front of it when it finally releases.
In order to prevent the spring from shooting across the shop, a rag is placed over the end of the differential case to catch the spring. This sounds like common sense, but many folks forget it and the spring flies across the shop. More importantly, you don’t want to risk injury.
With the gears in place with the clutch, we can check the backlash with a dial indicator.
The Revacycle gear is a cut gear as opposed to a net-forged gear. There are serrations from the cutting process on the gear tooth surfaces and there is no webbing on the inside or outside of the gear.
A net-forged side gear has a much smoother tooth surface. Since the tooth profile is forged in place and not cut, it allows for webbing to increase the strength.
Just like old bearing races, these can be helpful as press tools. Old axle shafts can be used to help rotate the side gears. Here we cut the shaft down in length and welded a plate to the end.
We can also use a complete axle shaft; we just need an old wheel to bolt to it. This takes up more space in the shop, but is easier if you do not have access to a welder.
The differential pinions have spherical washers on the ends. These washers are very important to help with wear, so be careful not to misplace them.
Once we are ready to rotate the differential housing, we can use the differential pin for this purpose. We partially install the pin and rotate the housing to walk the gears out.
Once the pinions are out of the differential, the side gears can be removed as well. Carefully remove the clutch pack and shims with each side gear. Note that the last shim tends to stick in the housing, from the lube film.
Using either net-formed or forged-tooth profiles is a more current process, and this process does not have signs of machining on the gear teeth. The net-formed process is becoming the standard for all new differential gears. The process allows for additional webbing in the gear profiles and therefore stronger gears.
The differential pinion gears (a.k.a. spider gears) can be rotated out. There are a few methods to do this. If the clutch pack is worn, it can be as easy as simply rotating the side gears, and the pinion gears will turn relative to the side gears inside the differential case and “walk” toward the window.
If the approach of rotating the gears with your fingers doesn’t work, there are tools available to help do this. In fact, you can make your own tool. An old axle shaft, with the spline end cut off and a square plate welded to it, is a good makeshift tool.
Another way to accomplish this (if you don’t have an axle shaft that you are willing to destroy) is to place the axle shaft from the vehicle vertically in a bench vise and place the differential over the splined end. You can even take the original axle shaft and bolt it to the wheel. Then just set the wheel on the ground with the axle pointing vertically upward.
Now that the differential side gear is essentially locked to the bench vise via the splined connection, rotate the differential case. The pinion gears will walk around inside the differential case, allowing you to remove them from the access window. In addition, pinion thrust washers are typically located between the pinion gears and the housings and usually these come right out with the gears. If you don’t see them immediately, look closely, as they may be stuck to the pinions or inside the differential housing from the lube film.
If, for some reason, the differential case won’t rotate by hand, do not worry. Some of these can be very tight. I recommend partially installing the differential pin into the differential case. Install it just enough to engage the housing, but not the inside diameter of the pinion gear.
The pinion gear needs to be free to rotate relative to the case, and the pin restricts that if inserted too far.
You can put a “cheater bar” on the end of the differential pin to get additional leverage. Be very careful; the pin is hardened and the case is just cast iron. Never pound or bang the pin during this process. Evenly pull on the bar until the pinions rotate. If you yank and bang on the differential pin, there is a good chance that the differential pin hole will be damaged.
Once the pinion gears have been removed, the top side gear and clutch pack will want to fall out of the case because the differential is vertical. Be prepared for this and catch the gear and the clutch pack. Again, note the order of these parts and their condition; you don’t want these parts falling and bouncing all over the shop. Make certain that the shim between the clutch pack and differential case has also come out.
Now the other side gear, clutch pack, and shim can be removed. At this point, the differential case should be completely empty. This is the perfect time to clean the differential case, either in a parts washer or rinsed thoroughly with brake cleaner.
Checking the Parts for Wear
Lay out the gears and clutches on a bench, in the exact order that they came out of the differential. Just remember which side went toward the ring gear flange side of the case and you are all set.
In my discussions with the axle and differential experts at Drivetrain Specialists (DTS), they reported observing wear on the side gear teeth. Take a good, long look at the surface of the teeth and look for signs of wear. The wear is usually concentrated on one side of the teeth because the teeth are loaded more severely in the drive direction relative to reverse. If the gears are worn on only one side, you can switch them from left to right and that will load the other less-worn side of the teeth in the drive direction. Your other option is to replace the differential gears.
Take a good look at the clutch plates and reaction plates. The tabs on the outside diameter of the clutch plates easily identify them, and you should be able to see friction material bonded to them. Also, inspect the differential pinions and the thrust washers. If wear is observed in the thrust washers, check the beveled surface between the thrust washers and the differential case for signs of wear or scoring due to a seized thrust washer that has spun in the case. There are new thrust washers avail-able, and you should replace them during your rebuild anyway.
The reaction plates are the steel plates with the spline on the inside diameter. These plates absorb most of the heat generated during slip events. This heat is dissipated in the gear oil and the differential case.
One tooth is actually chipped on this excessively worn side gear while the others show significant amounts of pitting and wear.
Here is a typical tabbed-style clutch plate, which happens to have woven carbon fiber as the friction material. The carbon-fiber material resists wear and can handle higher temperature when compared to traditional paper-based friction.
The reaction plates usually have an internal spline, special coating, and surface finish for proper interface with the friction material. These plates should not be mixed with different friction materials, the surface coating and preparation on a microscopic level has been optimized to work with the mating friction material.
The Ford 8.8-inch differential gears are actually painted at the factory, so they can be easily identified in the manufacturing plant. This side gear has a clutch pack with the latest carbon-friction-material-plates installed.
On the left is the typical amount of wear that you will find on the first and second plates. The plate on the right shows very little sign of wear.
Depending on the mileage and usage of the differential, the clutch pack probably shows signs of wear and scoring. It is a safe bet that the entire clutch pack should be replaced, especially if the differential, when assembled, had excessive play. This is the best way to restore proper function to the Traction Lok differential.
This is a great time to upgrade to the carbon friction plates that are used on the 2003 to 2008 SVT Mustangs. This is Ford part number M-4700-C, and this kit comes with a new preload spring.
The Ford stock-type clutch pack rebuild kit is part number M-4700-B. For some reason, the carbon style clutch pack rebuild kit does not include a friction modifier; the stock-style plate kit does include it. A friction modifier is required and needs to be purchased separately for the carbon-style pack. Also, both of these packs come with the correct clutch pack height and shims to match the factory specifications.
Typically, the first and second plates wear the most on any clutch pack. This is because, when the separating force from the gears is applied to the clutch pack, the first and second plates start transferring the torque first and then the other plates come into play. If they transfer adequate torque, the other plates see less speed difference and subsequently wear less. If you are going to re-use your old plates, which I do not recommend, you may want to move the first couple of plates to the center of the clutch pack. This should put the least-worn plates in the first and second position next to the side gear.
The assembly process is the opposite of the tear-down process, but there are a few special things to watch out for. Depending on the type of preload that you want across the differential, you can shim the clutch packs tighter or looser. The tighter the clutch packs are, the more windup and potential chatter. You can also upgrade the spring to an F-150 spring. The F-150 spring is approximately 1/4-inch wider than the typical passenger car spring. Its free height measures around 1.765 inches, as compared to the car spring at 1.510 inches.
But let’s keep in mind that we are building a high-performance rear end, not just a stock rebuild. If you cannot assemble the differential by hand, the clutch pack is too tight.
The first step is to soak the friction plates in friction modifier for at least 20 minutes. You can still expect a few initial “pops” out of the clutch pack when the vehicle is driven. Normally, you should only hear about a dozen or so. The longer the plates are pre soaked, the less initial popping will be present. Overnight would be the maximum amount of time before there is no additional benefit. There is certainly no need to soak the plates any longer than 12 hours.
Be sure to have a clean, shallow, plastic or steel pan to hold the modifier and plates in. A cookie sheet works great for this, but don’t use it to bake with afterward. You are only soaking the friction plates. The steel reaction plates do not require this step.
The friction plates need to be soaked in the correct friction modifier prior to installation. This soaking process actually conditions that friction as the modifier is absorbed into the fibers of the material. Here we have three of the plates soaking for 30 minutes.
Also, if you are going to use the latest carbon fiber style plates, you should properly dispose of the friction modifier used for soaking the plates and only use fresh modifier when you fill the axle.
While the plates are soaking, make sure that you have all of the parts you need and that they are clean and dry. During the assembly process, lubricate all of the parts before assembling them into the unit, making certain that they aren’t dry when you first start driving the car. Therefore, make sure that you have some fresh gear oil nearby (the same oil to be used for the final axle fill).
Lay out the clutch pack assembly in the correct order. You should have a total of three friction (F) plates, four reaction or steel (R) plates and one shim (S) per side. The order is as follows: side gear (G), reaction plate, friction plate, reaction plate, reaction plate, friction plate, reaction plate, friction plate and shim (G-R-F-R-R-F-R-F-S).
Some enthusiasts use a different order and even install additional clutch plates for a tighter differential. Your strategy depends on your level of expertise and what the vehicle’s application (high performance, drag racing, road racing, etc.) is. There are even rumors of people grinding the friction plates to get more in the total clutch pack stack.
I like to stick with the factory order and just shim the pack tight. If you are working on an early axle (1980s vintage), notice that the clutch plate tabs are square and the new style has rounded tabs. The square tabs are no longer available. Think of it as a weight savings and your car will be that much faster.
Also, the order of the plates has changed over time. The order for the 1980s-style differentials as stock was: side gear, friction plate, reaction plate, reaction plate, friction plate, reaction plate, reaction plate, friction plate, and shim (G-F-R-R-F-R-R-F-S).
Note: An axle of any vintage should be rebuilt with the updated clutch pack order (G-R-F-R-R-F-R-F-S). If the friction plate is placed directly against the side gear face, a couple of undesirable things happen. First, wear is imparted to the back face of the gear. Second, the clutch friction surface is larger than the mating surface on the back of the gear.
We have arranged the clutch-pack stack and shim so that you can see the proper order of the steel and friction plates. It is important to get the order right to achieve the correct performance. This stack requires a 0.020-inch shim.
Close examination of this gear and pack assembly reveal that the clutch plate actually is larger than the gear. The clutch plate hangs over slightly past the gear.
Close examination of this gear and pack assembly reveal that the clutch plate actually is larger than the gear. The clutch plate hangs over slightly past the gear.
This smaller area results in a pres-sure increase on the clutch face and more wear occurs on the gear and plate. Some people even try adding more friction plates to the pack. This does allow for more friction surfaces and additional torque capacity in theory. The problem is that the one clutch surface has to run on the back of the gear surface. My opinion is to avoid this altogether and stick with the latest recommended order of plates (G-R-F-R-R-F-R-F-S).
It is also very helpful to add gear oil to each plate during the assembly process. This makes it easier for the pack to stay together and again alleviates any dry running concerns.
On the assembled clutch, notice the two reaction plates next to one another in the third and fourth position. The main reason for this odd order is to provide a good heat sink to help out the first clutch plate and to allow for full face contact. Each assembled clutch pack consists of seven plates and the appropriate shim.
Now is the perfect time to align all of the friction plate tabs. This will make it easier to drop the assembly into the differential case. These tabs fit into the pockets in the differential housing. Once you have one complete side gear assembled with the clutch pack and shim, it can be loaded into the differential housing. Double check that the clutch plate tabs are all located in the windows in the differential housing.
A little trick here to see how tight the pack is without completely assembling the unit is to install the two pinions and cross pin without the other side gear. Insert a screw-driver between the differential pin and the face of the installed side gear and lightly pry to simulate the apply force of the preload spring. With the force applied, carefully rock a pinion back and forth to check backlash. If you have no back-lash, the pack is too tight. If the backlash is much beyond 0.020 inch, the clutch pack is too loose.
These are all of the internal components of a limited-slip differential. We have arranged them outside of the differential to show the relationship of the parts. Once installed in the differential case, it can be difficult to see all of the components.
The clutch plate tabs must align with cutouts in the differential casting. This semi-circular notch can be seen near the bottom of the access window.
It may be tricky to get set up with a dial indicator without an assistant. You can wait until the entire differential is built without the pre-load spring installed and check it again. Of course, if you want a real tight differential, you need to shim it tight. A good range for back-lash is 0.001 to 0.006 inch.
You can purchase complete clutch packs that are already shimmed and ready to go. If you go that route, be sure to keep everything together as packaged. Be careful not to mix things up while the friction plates are soaked in modifier. This is a quick and easy way to go because you can skip the whole clutch pack shimming and backlash setup.
You can measure the clutch pack assembled height in advance to get an idea of how close you are. The Ford style’s typical range is 0.640 to 0.645 inch. The performance kits can be as thick as 0.655 inch. The easiest method to measure the pack is to lightly clamp the pack with a C-style clamp and measure the overall thickness with a set of calipers or a micrometer. It is also recommended that the clamp thickness of the left and right packs is kept close to one another. If there is too much variation, it can be difficult to install the differential pin later in the assembly process.
Again, if the pack is too tight, the pinion gears are extremely difficult, if not impossible, to assemble. Also, keep in mind that you want backlash or clearance in the differential gears when they are completely assembled.
As stated above, this comes down to personal preference. If you are planning on running slicks and racing often, shim it tight. The differential is shimmed too tight if you cannot get it put together. The tighter the pack the more windup or bind you will notice during low-speed, tight-turn maneuvers such as pulling into a parking spot.
This preload that is being built into the unit and the force provided by the S-spring is all static preload. Once the differential gears (not hypoid) begin rotating relative to one another, the separating forces from the gears apply additional thrust loads to the clutch pack to resist this spinning. As mentioned earlier, this is the speed difference as compared to between the left and right wheels, and not vehicle speed.
The chart shows speed difference plotted on the horizontal axis against torque transferred on the vertical axis. You can see that the line does not begin at the zero point, where there is no speed difference across the differential gears. It is actually offset up on the torque curve. This amount of offset is based on how tight the pack is assembled and the force applied by the S-spring.
There are many “old tricks” that work, but aren’t always the best idea. Some folks build the clutch pack tight and omit the S-spring. This allows the pre-load to be zero and the differential will allow speed difference under light torque easily, just like an open differential. Then, when higher torque is applied, the separating forces from the side gears further compress the clutch packs. In this situation, if one wheel begins spinning on a slippery surface (ice, gravel, etc.), the torque is very low in the system as the resistance between the tire and the slippery surface is very low. Stepping on the gas pedal harder does not produce more torque in this case. The S-spring helps in this specific driving condition. If you are concerned about too much preload, don’t substitute the stock spring with a stiffer spring.
In order to measure the overall pack thickness, we lightly clamp it together and measure it with a micrometer. In this case, we used a pair of vernier calipers to measure the overall pack thickness or height.
The Ford service kit comes completely assembled and pre-shimmed. Although this takes the shim adjustment part out of the equation, we still need to disassemble the clutch pack in order to soak the plates in friction modifier.
This graph represents a typical preloaded clutch-pack-style limited-slip differential. The clutch-pack preload shifts the graph vertically so this is no speed difference across the differential, but there is still a resistance to differentiation from the preloaded clutch pack. Once a speed difference occurs, the side gear separating loads applies additional forces to the clutch pack.
The nut and bolt with large washers provide an assembly aid. While typically not required, it can be helpful. Once the pinions are installed, you can remove this nut, bolt, and washer arrangement.
Although pinion washers can be difficult to re-install in the differential, make certain to get them in place during the assembly process.
It is possible to rotate the pinions in place by yourself, but it’s much easier to have a second person help. Notice that the pinion washer is correctly in place.
As long as the pinions are lined up correctly, the differential pin will slide smoothly in place. It is best to check this alignment now before the ring gear is installed.
It is very easy to install a pinion one tooth off. When this happens, it is impossible to correctly line up both pinions through bores with the differential case. If this situation occurs, you need to remove the pinions and start over.
If you have temporarily installed the pinions and differential cross pin, they need to be removed. Now you can install the other side gear with clutch pack and shim. Double-check that you have the clutch pack order correct. This is where things get tricky. You need to hold the side gears in place while putting the pinions in place and getting all the teeth to line up. You can use a bolt with large washers to temporarily hold the side gear(s) in place. You can get by with-out this aid, but it can be helpful.
Do not forget the spherical pin-ion washers that go between the pinion gears and the differential case. Take your time and be patient; these can be tricky to install. At this stage, assistance from a second per son can be helpful, although it is not required.
Once the pinions are in place, you need to rotate one side gear relative to the differential case to get the pinions to rotate into place.
If the gears are all lined up correctly, the differential cross pin should drop right in.
If the pinion teeth are off by one and yes, it is possible one pinion hole may line up correctly, but the other may not.
Unfortunately, in order to correct this misalignment you need to rotate the pinions back out and index one pinion over to the next side gear tooth. This is where the homemade splined axle shaft tool is so helpful. Again, be patient and get it right.
Once the gears are in, installing the S-shaped pre-load spring is the last step. There are many ways to do it. You can compress the spring in a vise or use plates on either side with a C-style clamp, or just tap it in. I have done several of these and am comfortable with just tapping them in with a hammer. Be careful, this is made out of hardened spring steel, and you don’t want to crack or over-compress it.
Next, I like to put the differential pin in place and start the cross pin bolt a couple of threads. This ensures that everything stays together until I am ready to put it back in the car. A trick to help align the cross pin bolt is to put the bolt in the pin before pushing the pin in all the way. Then, use the bolt as a handle to turn the pin and align the bolt with the hole in the differential case.
The final step is to remove the bolt, push the pin in flush, and the bolt hole should line up perfectly.
Final Assembly and Performance
When you get the axle completely re-assembled, don’t forget to add the correct amount typically 4 ounces of friction modifier to the lube fill. This is very important to make certain that the clutch remains quiet over time. You should plan on using traditional non-synthetic gear oil during break-in. After you have 500 miles or so on the gear, feel free to change the oil and switch to synthetic if you like.
If you have replaced the ring and pinion set, check to see if the manufacturer has added a phosphate coating to the pinion and/or ring gear to help the gears break in properly. Think of this like flat tappet camshaft break-in lube that is added during engine assembly. Always follow the instructions that came with your new gears or differential if they vary from the information above. Some gear and clutch manufacturers recommend synthetic lube during the break-in process.
The preload S-spring can be com-pressed with channel-lock-style pliers or a C-clamp to help with installation. Once you have rebuilt several of these units, you develop the feel for how much to tap the spring to get it started in place.
Orient the differential pin retention hole before the pin is completely installed. We like to use the retention bolt to help us visually orient the pin before we push it in all the way.
Once the pin has been completely installed, you should put the retention bolt in place and lightly start the threads. This will make certain that the pin does not fall out before you are ready to install the differential.
Now that the Traction Lok is rebuilt and installed, you should no longer have that embarrassing single wheel peel. You can confidently put down two black ribbons of tire rubber. Of course, before you try this, make sure that the hypoid gears are properly broken in (if they were replaced at the same time as the differential rebuild).
The typical amount of time required to perform the rebuild is four to five hours depending on your skills and how many you have done before. It is not a difficult task, even for the beginner. It just takes time and patience.
GM-Style Limited-Slip Differential
The GM-style limited-slip differential doesn’t utilize the S-style spring for preload. It instead uses a series of wound-coil springs trapped between two steel plates. Eaton Corporation produced these differentials, which are typically referred to as Positraction rear ends. The Eaton Posi was first introduced as optional equipment in 1960s-vintage Chevrolet rear axles.
The Traction Lok assembly and disassembly instructions apply except for a few minor changes relative to removing and installing the springs and plates. Basically you follow the same steps until it is time to remove the S-spring. Then you tap the spring pack partway out through the differential housing window with a brass punch (not all the way, as you want the pack to remain intact and not fly apart). Usually 1 inch or so will do.
With the proper preload across the differential and a good set of clutch plates, the rear axle acts like a spool during hard acceleration. This is evident because both wheels have left a black patch during a burnout, and those patches start at the same time. If one patch starts before the other, it may be a sign that the preload is not high enough.
The GM-style limited-slip differentials use four compression springs as opposed to the single S-style spring Ford uses. Eaton builds these differentials both as factory-installed and aftermarket units.
The spring pack needs to be tapped out like the Ford S-spring. You want to make certain that you alternate between the spring plates and do not hit the springs themselves.
Once the spring plates are partially pushed out, you can grab them with a suitable set of pliers. Just compress the springs, and the plates and springs should pull out.
The 200-, 400-, and 800-pound spring and spring plates are shown from left to right. The spring plates become thicker as the spring rate increases. Also, the spring wire diameter and number of active coils vary and, consequently, it is easy to visually tell which springs have been installed in the differential.
Next grab the spring pack from the opposite side window of the differential housing. Use a pair of locking pliers and remove the entire pack.
Inspect the plates and springs for cracks; clean and replace as required. If you want a higher preload torque, now is the time to replace the spring pack with a stronger set. Typically for these units, there are three spring-preload options available: 800-, 400, or 200-pound. The higher the spring force, the higher the preload and torque bias ratio.
Spring Force Math
There are rumors out there suggesting that the advertised force from these springs is not correct. I decided to perform some reverse engineering to verify which is true:
k = G x d4 / (8 x Na x D3)
k = spring rate expressed in lbs/in G = shear modulus of elasticity (a constant value of 11,312,516 psi for steel)
d = wire diameter in inches
Na = number of active coils. The term active coils or a turn of a compression spring refers to the number of turns of the wire material that actually deform during compression. Most springs are designed to have the top and bottom coils ground flat to allow for better seating. The ground section of the spring is not considered active in this case.
D = mean coil diameter in inches, which also can be found as (inner diameter + outer diameter) ÷ 2
Once we have the spring constant calculated, we can find the force per spring by solving the following equation:
F = k (Δx)
Δx = (spring free height) – (spring installed height)
We made some estimates for the number of active coils and rounded to the nearest number. We also considered the different spring plate thicknesses and came up with the following:
From this we get the spring rates (k) as 80, 364, and 1,485 lbs per inch, respectively.
Now we assume that the total compressed height is 1.32 inches and take into account the two spring-plate thicknesses. We used the above force equation with the following parameters:
This yields forces of 45, 90, and 163 pounds, respectively. Since there are four springs in parallel, we multiply the forces by four to yield 180, 360, and 650 lbs per inch, respectively.
These are very close to the advertised values, especially when you consider that we made some approximations for values such as the number of active coils, compressed height, and wire diameter. Keep in mind that you cannot simply take the 200-pound numbers and multiply by four to get the 800-pound spring numbers. There are differences in plate thicknesses and, therefore, in the compressed heights of the springs.
Differential Gear Removal
Follow the Traction Lok procedure to remove the rest of the differential gears and plates. Always keep track of shims and the order of friction plates and steel plates so you can reassemble them in the same order. A new friction plate is approximately 0.070 inch thick for this style differential. Also, this style clutch pack uses small hardened caps, upon which the clutch plates ride inside the differential case. These simply prevent the plates from wearing grooves in the case and inhibit free motion of the clutch pack.
When it is time to re-install the spring pack, you just need to squeeze the pack together between some plates with a C-style clamp and then tap them in between the side gears. Once the spring pack is started, you can remove the clamp and finish tapping the pack home.
As with the Traction Lok procedure above, once the pack is in, double-check that the differential pin assembles easily without interfering with the spring pack.
The clutch pack tabs run on these hardened caps. There are two caps per side-gear-and-clutch combination. Therefore, you need to install four caps per differential.
A C-clamp with some steel plates can be very helpful to compress the spring pack. Then we can tap the plates in place. Remember, you want to tap the plates and not the spring themselves. Make certain that the springs stay aligned on their seats.
The compression-spring-style differentials have different options for preload springs and clutch-plate quantities as well as clutch friction material (as described in the “GM-Style Limited-Slip Differential” section above). As a result, you can set up different preload torques. The following describes some of the combinations for this type of differential.
If you stick with the typical stock springs that produce a net force of 200 pounds, you have a couple of choices: The stock steel plates with a nine-plate pack setup per side (made up of five friction plates and four reaction plates), or a seven-plate pack per side setup with carbon-fiber friction plates (made up of four friction plates and three reaction plates). The carbon-fiber plates are a great upgrade over the steel plates and significantly reduce, if not eliminate, clutch chatter. (Remember as the preload increases, the tendency for chatter increases.)
If you switch to the 400-pound spring setup, you can use the seven per side arrangement with carbon plates, which is the standard for new Eaton differentials, or you can use steel with a nine-plate pack per side like above.
If you switch to the 800-pound spring setup, you can again choose between a seven-per-side carbon-style pack or a nine per side steel-pack arrangement. As always, the steel pack produces a fair amount of clutch plate chatter. The most aggressive kit is the 11-plate-per-side arrangement with steel plates and 400-pound springs, or carbon plates with 800-pound springs. These two arrangements wear extremely quickly and are meant for drag racing only.
Just how much bias torque is theoretically achieved from these different arrangements? The torque capacity of any clutch can be represented by this simple equation:
T = μ x N x Rm x F
T = torque in ft-lbs
μ = coefficient of friction (a dimensionless number, typically 0.10 for these clutch plates in gear oil)
Rm = mean diameter of the clutch plate surface in feet (0.2458 foot for the GM-style plates)
F = apply load in pounds
N = number of friction surfaces
Here are the results of our calculations:
The bias torque value is a linear relationship to the spring force. As the spring force doubles for a given clutch pack’s total plate count, the bias torque also doubles. (See Chapter 5 for more about clutch pack preload and side gear separating force.)
Keep in mind that the higher the preload torque, the tighter the differential, or the less likely the differential is to unlock and allow a speed difference across the wheels. The tighter the clutch pack, the more windup and noise can be expected from the rear axle, especially in slow, tight-turn maneuvers. Basically, the on-road street manners of the vehicle will be compromised as clutch pack preload increases.
Cone Style Limited Slip Differential
Chrysler Sure-Grip limited-slip differentials are typically found in 83⁄4 -inch rear ends. There were two variations available: the plate type (described earlier) and the cone type. Models from 1958 to 1969 used the plate-style Dana Power Lok. Borg Warner originally manufactured the cone type from 1969 to 1974. The cone type cannot be rebuilt; your best option is to purchase an aftermarket unit if yours is not working correctly.
The cone-style limited-slip differential has the same function as the plate-style but uses cone-shaped friction surfaces. If you look closely at the side gear near the circular window on the left, you can see a portion of the cone surface. (Auburn Gear)
The cone-style limited-slip differentials perform the same function as the plate style. There are just a couple of differences in design. The cone style utilizes a conical-shaped friction surface, much like a manual transmission synchronizer, and there is typically only a single surface per cone. Borg Warner originally produced them in the 60s and 70s under the name of Spin Resistant. Many of the Big Three vehicles (GM, Ford, Chrysler) used them. In the early 80s, Auburn Gear began manufacturing differentials with the cone-style design and still produces them today. The units have a set of thrust plates with preload springs between them that look similar to the GM preload springs.
The cone-type surface reacts against the differential case and they end up being a matched set. Some enthusiasts mix and match different parts from multiple differentials in an attempt to repair a worn-out unit. This is not recommended and the performance cannot be predicted. You may get lucky, but it is not worth the risk. Fortunately, these units are replaceable. Auburn Gear actually offers a direct exchange pro-gram. You can go to the nearest distributor and, for the typical price of a clutch pack rebuild kit, you can exchange your worn unit for a new differential that’s ready to go. A nice aspect about this program is any updates or improvements to the parts are already included in the replacement hardware at a reasonable price.
Ford Mustang Differentials
There are several differentials that have been offered for the Ford 8.8-inch axles typically found in the Mustangs over the years since the introduction of the Fox chassis. I am not going to review all of the older-style (pre-Fox) designs but will cover most of the different combinations that have been available for the 7.5-, 8.5-, and 8.8-inch axles.
The first thing to be aware of is that all of them are two-pinion differentials.
Remember that all 28-tooth-spline side gear differentials require a 3/4-inch differential pin, while the 31-tooth side gear differentials utilize a 7/8-inch-diameter differential pin.
The 7.5-inch ring gear axle has a 28-tooth spline and a 10 x 16-tooth combination. There are 10 teeth on the differential pinions and 16 teeth on the side gears. The typical application for this axle is the V-6 Mustang.
Here are the various Ford differential gears that are available. From left to right: 7.5-inch 28-tooth spline 10 x 16 tooth combination (blue), 8.5-inch 28-tooth spline 10 x 16 tooth combination, 8.8-inch 28-tooth spline 10 x 14 tooth combination, 8.8-inch 31-tooth spline 10 x 14 tooth combination, 8.8-inch 31-tooth spline 9 x 13 tooth combination.
The 8.5-inch ring gear axle has a 28-tooth spline and a 10 x 16-tooth combination also. Some applications are the Fox chassis (1979 to 1993) Fairmont, Zephyr, Mustang, Thunderbird, Granada, etc., and Explorer/Ranger.
One 8.8-inch ring gear axle has a 28-tooth spline and a 10 x 14-tooth combination. This was a hybrid differential. It has an upgraded differential case that is stronger than the previous differential. This is the differential that you want if you are not going to upgrade to 31-tooth-spline axles and you broke your 8.5-inch 10 x 16 differential. These are found on the SN95-chassis (1994 to 2004) higher-powered V-8 applications, such as the Marauder, Bullitt, and Mach 1.
Another 8.8-inch ring gear axle has a 31-tooth spline and a 10 x 14-tooth combination. Some applications for the differential are the F150 and S197 (2005 present) V-8 Mustangs.
The third 8.8-inch ring gear axle has a 31-tooth spline and a 9 x 13-tooth combination. These differential gears are stronger, which results in a slightly heavier differential assembly. This differential can be found on the S197 (2005–present) specialty Mustangs only, such as the Shelby GT and KR.
At this point, you know how to tell the difference between an open differential and a limited slip differential. And you know why you want a limited slip differential in your vehicle. You should also be aware and confident of how to disassemble and reassemble your limited-slip differential and what areas to look for regarding wear.