The axle assembly work begins when the differential components are fully disassembled, cleaned, and ready for reinstallation. If you are reusing the same ring and pinion gears and differential carrier, re-assembly is relatively simple because the mesh pattern does not have to be reset so the shims remain the same. Nothing really moves. If you’re replacing the ring or pinion gears or the differential carrier, you have to start from the beginning for gear mesh and depth.
This Tech Tip is From the Full Book, CHEVY DIFFERENTIALS: HOW TO REBUILD THE 10- AND 12-BOLT. For a comprehensive guide on this entire subject you can visit this link:
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Before proceeding, verify that you have the correct parts. Check that the gears are actually the ratio that you ordered. This is the final safety check. Two main settings are critical to the operation of the axle assembly: pinion depth and backlash.
Project: Installing an Axle Assembly and Differential
Step:1 Measure Width of Shims
You use the same procedure for properly setting up the gears for a 10- or 12-bolt, regardless of the actual version you have. The only differences are a few of the settings specs, which vary by version and year. You start the procedure by setting pinion depth. You should begin by using the stock shim when installing a new pinion gear. Use a caliper micrometer to measure the width of the shims. It doesn’t matter if you use one shim or five, just as long as they are clean and measure to the right spec.
Step:2 Calculate Pinion Depth (Documentation Required)
You need to correctly figure out the pinion depth and therefore the required shim stack. Two shims were used here to match the chosen spec, which for this 1969 12-bolt passenger car was set at .025 inch. The original shim was .020, the stock spec is .030, so I went in the middle, knowing that the previous setup was a little loose. The starting point is usually a rough guess between the factory spec and the previously installed shim.
Step:3 Install Pinion
Following the procedure detailed in Chapter 6, install the pinion with the gear and bearing assembled and loaded into the housing.
Step:4 Install Outer Pinion Bearing
To set the pinion depth, install a new outer pinion bearing, without a crush sleeve, over the new pinion shaft.
Step:5 Lubricate Pinion Yoke Washer
Use gear oil to lubricate the original pinion yoke washer. You should also prelube the bearings.
Step:6 Install Pinion Yoke (Important!)
Use an impact wrench or ratchet to install the pinion yoke to the pinion. Be careful; there is no crush sleeve to take up the slack. The nut should be tightened slowly until the bearings contact the races and then tightened just a little bit more. Spin the yoke by hand; you should feel some drag. It should not be loose or tight.
Step:7 Install Carrier
You have installed the new gears per Chapter 6. The carrier has new bearings, to which the new race and original shims are positioned.
Step:8 Insert Carrier Assembly into Housing
Slide the assembly into the housing. You can feel whether it’s a tight or loose fit within the case. If there is a certain amount of slop between the carrier bearing and the case, you need to add more shims. If it is too tight, try pushing the shims in after the carrier is in. If that doesn’t work, you need thinner shims. This is all done by feel; pack in as many shims as you can.
Step:9 Inspect Carrier Shims (Critical Inspection)
You want the carrier shims as tight as possible. You can’t get too much carrier bearing preload. If the carrier is loose or too tight, a new shim pack is in order.
Step:10 Install Shims
Use a caliper micrometer to measure the width of the shims. If your case needs more shims, measure the originals with calipers. Make sure they are clean, as any dirt or grease can change the reading.
Step:11 Install Shims (CONTINUED)
The new shims are two-piece shells with an inner ridge for locating additional shims.
Step:12 Install Shims (CONTINUED)
Step:13 Install Shims (CONTINUED)
Step:14 Install Shims (CONTINUED)
You may use a stock shim on one side and a new shim on the other. This is just the initial alignment, you may need more.
Step:15 Torque Bearing Caps (Torque Fasteners)
To get the right pinion depth setting, torque the bearing caps to spec. Tighten both bolts until they are snug, then torque. Some housings have locating features for the caps. If this is the case, use a soft hammer to tap them into place first; don’t use the bolts to pull them into place.
Step:16 Lock Pinion Yoke to Platform
Working on the bench makes this process easier. Lock down the pinion yoke with a pair of bolts against a 2×4 platform.
Step:17 Mount Dial Indicator on Housing
Position the dial indicator on the ring gear as inline as possible with the rotation of the gear. Refer to “Project: Setting Up a Dial Indicator” on page 105 for correct setup of the dial indicator. Zero the gauge with the ring gear pulled away from the gauge. Do this to set the initial backlash.
Step:18 Measure Backlash (Precision Measurement)
Rotate the gear forward until it contacts the pinion gear. With the pinion positively locked, this is easy. You want to be within the specs listed in this chapter. For the 12-bolt, the spec is .006 to .010 inch. This dial indicator reads .011, which is too loose. The shims need to be adjusted, more on the driver’s side, less on the passenger’s side, to move the carrier toward the passenger’s side. One shim change will get it to .008 inch.
How deep or shallow the pinion gear rides in the housing is critical for proper mesh between the pinion and the ring gears. The factory specifies the starting point for the pinion shim, but this must usually be adjusted in or out to achieve the proper depth.
When the pinion gear is too deep in the ring gear, the mesh of the pinion teeth rides deep into the ring gear. This results in gear whine under acceleration. The solution is to move the pinion gear closer to the housing.
If the pinion gear is too shallow, the gear mesh rides high on the ring gear teeth. This can result in a whine when the vehicle is decelerating. The solution is to move the pinion gear closer to the ring gear centerline.
There are two ways to set the pinion depth: with a pinion depth gauge and by checking the gear pattern. Both methods work; however, the pinion depth gauge is a bit cumbersome.
Pinion Gear Patterns
Pattern checking is the tried-and-true method of setting pinion depth. It is the process of installing the pinion gear (complete with inner and outer bearings), depth shims, pinion yoke, and pinion nut. However, you do not want to use a crush sleeve yet. You install the pinion gear and pull it tight with the pinion nut just until the bearings contact the races and there is no play. Then you install the carrier into the housing with the ring gear mounted.
Apply colored grease or gear-marking compound to a section of the ring gear. If the grease is too thick, you can mix it with a little oil to thin it. Gear-marking compound is designed for this specific task without running or smearing. Machinist’s dye and other marking agents do not provide a clear contact patch and therefore it’s difficult to interpret the contact patch.
Project: Pattern Checking
Step:1 Use Gear-Marking Compound
Once you set the initial backlash, check the mesh pattern. This shows the pinion depth in relation to the ring gear. You can thin the supplied gear-marking compound with a little gear oil.
Step:2 Apply Gear-Marking Compound
Use a brush to apply the compound to several teeth in a couple of places along the ring gear. Try to make the compound smooth.
Step:3 Allow Pinion to Rotate
Now the pinion is freed up, and one hand (or a helper) puts pressure on the pinion yoke while the other spins the ring gear several times in both directions.
Step:4 Check Mesh Pattern
You can see where the pinion gear hypoid has contacted the ring gear. You want a center-weighted mesh. Checking the pattern requires close examination. This is an ideal pattern and about as good as it gets on the coast side.
Step:5 Check Mesh Pattern (CONTINUED)
The drive side of this gear is also correct. You want smooth patterns; no sharp edges or saw-tooth shapes. Where the pattern falls on the gear heel to toe is not important; that is a function of the housing itself.
Step:6 Check Mesh Pattern (CONTINUED)
Here is a bad pattern; the pinion is too close to the ring gear. Note the hard edge on the coast side; it looks like a shark fin. This would be very noisy and generate a lot of heat. The solution is a thinner pinion shim.
Rotate the carrier and not the pinion to check the contact patch. Use one hand to hold the pinion yoke, creating a load, and rotate the carrier four or five times in both directions. The pinion must have a load in it for the pattern to be accurate. (If holding it by hand is not feasible, wrap a shop towel around the yoke and pull it tight.) This shows the contact patch between the pinion gear and ring gear. Then read the pattern.
When reading the patterns for pinion depth, you should not be overly concerned about where pattern falls on the gear’s heel and toe. Instead, you should focus on the position between the top land and root. Where the pattern lies heel to toe is dependent on the cut of the gears themselves and the left to right position of the ring gear carrier, as well as the housing itself. Trying to perfectly center the pattern is an exercise in futility. If the pattern is reasonably centered, it is sufficient.
The pattern should be checked at least twice during the installation procedure. First, an initial check should be done, and then the final check after the rebuild has been completed to make sure nothing has changed.
When the pinion depth is correct, the pattern shows smooth oval markings on both the coast and drive sides of the gear.
If the pinion is too close to the ring gear, the top of the gear pattern will be smooth and rounded, while the base of the pattern will be close to the root of the gear, cutting off in a sharp line. Certain patterns may show on one side as a teardrop or oval, with the other side cut off, even saw tooth–shaped.
If the pinion gear is too far away from the ring gear centerline, the pattern rides high on the teeth, showing a smooth rounded base with a sharp line on the top. Sometimes, there is one clean contact patch on drive or coast side, but the other side is cut off at the top.
Checking the pattern on used gear sets requires a different procedure. Ignore the drive side of the pattern; instead, concentrate on the coast pattern only. This is because the drive side already has wear from use, and the markings look like new patterns.
Project: Setting the Base Pinion Depth
To start, the pinion bearing needs to be pressed off the pinion. Any machine shop can do this for you for less than $10. Try to save the old bearing. Keep the original shim as well. If it is in good shape (not bent), it can be reused; otherwise you need to measure it with calipers and match it up with a new one from your installation kit.
If you have a hydraulic press, you can remove the bearing yourself using a bearing separator or a bearing clamshell. Pinion bearing removal requires a lot of pressure; you can’t just beat the pinion out of the bearing with a hammer.
If the bearing came off the pinion in one piece, it can be reused for checking the pinion depth. Because you want to be able to remove the bearing to change the shims, the inner diameter of the bearing needs to be enlarged. You can do this with a die grinder and either a flap wheel or carbide cutting tool. Holding the bearing tightly in one hand (please wear gloves for this process), remove material evenly from the center of the bearing until it slides easily onto the new pinion gear.
If you are changing the gears, the pinion depth needs to be set. GM 10- and 12-bolt units use shims behind the inner pinion bearing (the large bearing inside the case).
Fortunately, you don’t have to play the guessing game when it comes to setting the pinion depth. The factory provides the pinion depth specs. You just need to do the final dial-in.
You can reuse the original shim or select a new shim that matches the factory specs. If there is a large difference between them, you can simply go with a shim that splits the difference. This occurs quite often, as the factory spec and the actual gears and housing do not always work together.
After the pinion depth has been set, use a hydraulic press to install the new bearing onto the pinion gear. If you don’t have the special pinion tools, you can use a piece of heavy pipe and the old pinion bearing to match the size. Make sure the bearing is fully seated.
Place the shim on the pinion gear, and then slide on the bearing.
If you have not already installed new bearing races for the inner and outer pinion bearings, now is the time. Do not proceed with the original races in the housing.
Clean the pinion gear with brake cleaner or parts wash to remove any protective grease.
Slide the new pinion gear into the housing, install the outer pinion bearing and then the pinion yoke, followed by the pinion washer, and finally the original nut (if available). Do not install a crush sleeve at this time.
Sometimes the outer pinion bearing is very tight on the yoke, and has to be pushed onto the shaft with the pinion yoke. (This is a variance in the parts.)
Slowly thread the pinion nut onto the pinion, pulling the pinion bearings into their races. Once the bearings are fully seated and the pinion does not move in or out, apply a little more tension on the nut for a minimal amount of bearing preload. Remember, there is no crush sleeve between the bearings, so too much load could damage the bearings.
The crush sleeve installs as shown, only after the pinion is in the housing. It rests between the inner bearing and the outer bearing inside the case. This image is for reference only, the sleeve is only installed after the pinion is in the case.
Step:8 Install New Front Bearing
The new front bearing frequently fits quite tightly to the pinion bearing. You can use the yoke to help seat it if necessary.
Step:9 Install Front Bearing Seal (Performance Tip)
Once the bearing has been seated, install the new seal using a soft hammer or a seal driver. Passenger car 12-bolt seals are no longer available new, but the truck seal fits and works. Make sure that the seal is lubed with gear oil; otherwise it could tear.
Step:10 Install Pinion Yoke
The yoke goes on next and then you place the thick yoke washer on the pinion. Not much thread is visible; this is normal, as the pinion may not slide easily onto the new pinion gear.
Step:11 Install Pinion Yoke (CONTINUED)
Apply high-strength thread locker to the pinion threads before placing the new pinion.
Step:12 Install Pinion Yoke (CONTINUED)
The best tool for this job is a 1/2-inch electric impact wrench. Hold the yoke by hand then crank on the nut until the bearings contact the races. Once they contact, work slowly until there is a slight drag on the yoke. It takes 300 to 400 ft-lbs to crush the sleeve. But once it crushes, it has to be right. Too much preload means starting over with a new sleeve.
Step:13 Install Pinion Yoke (CONTINUED)
It can be difficult to apply the amount of torque required to crush the sleeve. Here the yoke is locked down on the bench. If you have to use a breaker bar, you can install an axle and either lock it down with a wrench or have a helper hold it in order to crush the sleeve.
Next, the carrier and the gear should be cleaned with brake cleaner or parts wash to remove any protective grease.
If you have not replaced the bearings on the carrier, now is the time to do it. The ring gear should drop right onto the carrier, and; with just a little effort, seat onto the gear flange. Secure two bolts to the ring gear and thread them in by hand.
Apply a small amount of thread locker to the rest of the bolts and thread them into the gear. Remove the two bolts used to secure the gear in position and apply threadlocker. Reinstall the bolts.
If you have an assistant, have him (or her) hold the carrier. If not, you can use a bench vise. The ring gear bolts must be torqued in a criss-cross star pattern to ensure that the gear is properly seated and torqued.
During disassembly, you removed and labeled the carrier preload shims. Retrieve them and prepare them for installation. If you are reusing the original carrier, it should work as is.
Step:17 Check Pinion Preload
Once the preload is close by feel, check it with an inch-pound torque wrench. You need a wrench that measures 0 to 20 in-lbs in 1-inch increments. These are small and usually have a 1/4-inch drive. You can see the shop-made tool that adapts a 1/4-inch drive to a 1/2-inch socket. The large adapter shown here is a 3/4- to 1/2-inch adapter with a small 1/4-inch socket welded to the inside. It works.
Step:18 Check Pinion Preload (CONTINUED)
Without axles installed, rotate and measure the yoke. The initial torque is not the spec you want; rather, it’s the rotational force needed to spin it. The measurement here is 6 in-lbs, which is within spec for used bearings, but new bearings need to be 14 to 19. Once the preload is set, you can use a punch to tap the pinion nut to help keep it from backing off.
Install the carrier into the housing and hold it in position with one hand and use the other hand to slide the preload shims into the housing between the carrier bearings and the case. They should be tight, but not so tight that they don’t slide in.
If you are using a new carrier, you may have to adjust the shim package. As long as the carrier is not loose side to side, the originals should be sufficient for this process. Replace the carrier end caps and torque them to spec, which is 60 ft-lbs for all GM 10- and 12-bolt units.
Mix up a little gear-marking compound and follow the procedure for marking the ring gear and check the pattern (see “Project: Pattern Checking” on page 98). In the unlikely event that the pattern is dead-on this time around, pat yourself on the back. This does not happen often. More likely, you will need a series of trial-and-error test fittings. Once you have a baseline pattern, you can adjust it.
You want to make large changes that intentionally move the pinion to the other extreme so you can see which direction you need to move (if you are too close, push the pinion deeper, or vice versa). Once the pattern is in the ballpark, you can make small adjustments to fine-tune it. A large adjustment is .005 to .015 inch, and small changes are .002 to .004 inch. If you start out making small adjustments, the process takes much longer. As you reach the correct pattern, the smaller adjustments center the pattern between the top and root of the gear teeth.
Once you reach the correct pinion depth, remove the pinion gear from the housing and install the new inner bearing to the pinion gear with the correct shims. This can be done with a basic hydraulic press and a piece of heavy-wall pipe or a bearing clamshell. If you don’t have the ability to do this in your shop, any local machine shop can install the bearing for you. Remember, do not install the new bearing until you are sure that the pinion depth is correct.
Ring Gear Backlash
Next in the process of setup is setting the initial backlash for the ring gear. This is the amount of forward and backward headspace between the ring gear and pinion. Backlash is measured with the pinion gear installed in the housing, with minimal preload. You need a dial indicator with a magnetic base to check the backlash.
Backlash is adjusted by moving the carrier left or right with shims between the carrier bearings and the axle assembly housing. Moving the carrier to the right (passenger’s side) pushes the ring gear into the pinion, decreasing backlash. Moving the carrier to the left (driver’s side) away from the carrier, increases backlash.
Most gear sets follow these basic parameters: Moving .010 inch toward the pinion gear yields a decrease of .007 inch in backlash. Moving .010 inch away from the pinion gear yields an increase of .007 inch in backlash. The sweet spot for GM 10- and 12-bolt backlash is between .006 and .010 inch. If the backlash is too tight, the gears generate a lot of heat and burn up. If the backlash is too loose, the gears are clunky and noisy.
Almost all factory carriers are shimmed with a solid spacer that has been machined specifically for that housing. If you retain the carrier, it may work great, but it usually needs adjustment. A shim kit that includes a pair of master spacers and a series of shims of varying thicknesses does the trick. The master spacers lock together in the center and hold the shims in the pack, making installation easier. The shim pack also provides the carrier bearing preload, but that is the last step.
For now, you want the shim pack to be snug, but not tight. Use calipers to measure the original spacer and select shims from the kit to match. Try this first and add or remove shims from there.
Project: Setting the Ring Gear Backlash
Install the carrier into the housing, and then slide the shim packs between the bearing races and the housing. Bolt the end caps in place and torque to spec, 60 ft-lbs per bolt. Set up your dial indicator, taking care that it measures near 90 degrees to the ring gear (see “Project: Setting Up a Dial Indicator” below).
Secure the pinion yoke from moving; this is a key component to checking backlash. If possible, mechanically secure the pinion so that it can’t move at all. Rotate the carrier until it contacts the pinion gear, and then zero the gauge on the indicator. The indicator dial can be zeroed anywhere on the dial.
Once the zero point is determined, rotate the carrier in the opposite direction until it contacts the pinion gear again. Read the dial. If the measurement is between .006 and .010 inch, you are good to go. If not, the shim pack must be adjusted until the correct backlash range is reached. Once you find the correct shim load, check the backlash in at least three places on the ring gear to be certain it is correct.
Record the shim sizes for each side, just in case you need to rebuild the shim pack.
Project: Setting Up a Dial Indicator
Proper setup of the dial indicator is crucial for getting accurate measurements. In the case of reading backlash, you need one with a magnetic base. This allows you to position the indicator anywhere there is a ferrous (magnetic) surface. The Summit Racing kit shown here costs about $30. A typical kit will contain the indicator, the magnetic base, a couple of adjustable arms, and a case. As long as the unit is accurate to .001 inch, it will work for this task.
The magnetic base must be stable and locked to the housing. If it moves at all, the measurements will not be accurate. In addition, you need to properly tighten the clamps. Loose clamps mean the indicator can move when pressure is put on the needle and the gauge’s readings will be inaccurate.
Of course, the gauge needs to be properly zeroed. Don’t zero the gauge at the actual end of the gauge’s movement because this can lead to false readings as well. Push the needle in a bit and then zero the scale to that point. This allows you to get an accurate reading at the true zero point.
Make sure you double-check your readings. The large indicator scale has hash marks in .001-inch increments. A smaller second gauge indicates tenths of an inch, up to 1 inch.
The indicator’s scale rotates on the gauge. Don’t worry about tightening the lock knob. The scale is not loose on the gauge; it won’t move. The act of tightening the knob can take the gauge out of position. The magnet on the base is strong, but it slides easily with moderate pressure.
To properly position the indicator on the housing to take readings, follow these steps.
Step:1 Install Magnetic Base
A magnetic base is the first part of the indicator. Your kit must include one for ease of securing it to the housing. You can mount the dial indicator on any part of the center section that is clear of your work area.
Step:2 Mount Second Arm
The second arm slides onto the base’s ﬂ agpole. You can slide it up and down as necessary.
Step:3 Mount Second Arm (CONTINUED)
The mount on the back of the indicator attaches to the second arm. Install this loosely until the final adjustment is complete.
Step:4 Mount Dial Indicator on Housing
The base has an on/off lever that moves the magnets to the base to secure it to the housing. The base also has a V shape on the bottom for round surfaces.
Step:5 Mount Dial Indicator on Housing (CONTINUED)
An adjustable angle fitting on the second arm helps get the indicator in the best position.
Step:6 Mount Dial Indicator on Housing (CONTINUED)
This is a good position for taking the backlash reading. The base is solid and not wobbly and the indicator is inline with the ring gear. You want to read the gear with the indicator inline with the rotation of the gear.
Step:7 Position Gauge Needle
The needle of the gauge should sit on the edge of the tooth without slipping off.
Step:8 Zero the Gauge
Once the indicator is positioned, it has to be zeroed. The outer ring of the gauge rotates so that you can set the “0” mark where ever it needs to be. Each hash-mark is .001 inch. Move the ring slowly so you don’t throw off the alignment.
Once the pinion depth and backlash have been set, check the gear pattern again. If all is well, you can move on to adjusting carrier bearing preload and pinion bearing preload, as well as installing crush sleeves. Apply a coating of fresh gear oil to the bearings before proceeding with the final installation.
Carrier Bearing Preload
The carrier bearings must have some load on them for the unit to operate correctly. All GM 10- and 12-bolt housings use an outside shim design for the carrier bearings. After the backlash has been set, add equal shims to both sides until the preload is as tight as possible without damaging the shims. Carrier bearings rarely fail because of too much preload; in fact, it is difficult to load too many shims in this design without using tools. The shims should be tight, and it may help to pack the shims with the carrier halfway out of the housing and roll the assembly into position all at once.
Pinion Bearing Preload
This is the most critical component of the final assembly, and the GM design uses a crush sleeve, which is the trickiest of them all. The pinion preload is achieved by tightening the pinion nut until the bearings contact the races and then continue loading the bearings until they have reached a range of rotational torque that is measured in in-lbs.
Many experienced axle assembly builders don’t use a gauge for this; instead, they rely on their sense of feeling for the right range. Although this works for the experienced builder, the novice should rely on the proper tools to get the job done right. Too much preload kills bearings; not enough makes for a noisy ride.
This is a special metal sleeve that crushes with 300 to 400 ft-lbs of torque. That means that to properly set the pinion bearing preload, you need to put that amount of force onto the pinion nut. If you are working under the car on the ground, this task is not easy without an impact wrench. Many air-impact wrenches are not capable of this kind of torque; you need at least a 1/2-inch-drive impact, if not a 3/4-drive unit. Electric impact wrenches can provide the torque you need. You have to have the right tool for the job before you start.
If the vehicle is on a lift or the pieces are on the bench, you can use a breaker bar with a long pipe to achieve the best results. This is the hardest part of any 10- or 12-bolt rebuild, simply because of the torque required.
Installing crush sleeves requires a lot of patience and finesse. If you overload the bearings, you can’t just back off the torque; you need a new crush sleeve. Most rebuild kits come with two crush sleeves for this reason.
You must check the preload several times as you tighten the pinion nut. If you use an impact wrench, watch the socket for movement as you hit the trigger. As you reach the point of preload, the socket does not move much; use very small increments. Check the preload often, until it reaches the range for your unit.
Pinion Bearing Spacer Kits
Although the factory design uses crush sleeves, you can use an aftermarket spacer and shim kit instead. This removes the potential for ruining crush sleeves by replacing the sleeve with a solid spacer and a set of shims. The best thing about these kits is that the results are easily reproduced every time you remove the pinion from the center section, as long as you use the same shims.
To increase preload, you remove shims; to reduce preload, you add shims. You determine the correct shim pack for the preload range for your axle assembly by using the trial-and-error method of adding and removing shims.
One key point on installing pinion spacer kits: Make sure that the spacer and the shims are very clean. Any dirt or dust on the surface can affect the preload measurements. These kits are available for most GM axle assembly designs.
Complete the Rebuild
Once you have set up the ring and pinion, you install the rest of the components, starting with the axles. The outer axle bearings, or wheel bearings, should be replaced (see Chapter 6 for more details).
Lubricate the axle seals with grease or gear oil. Skipping this step could result in a torn seal, which means a big mess and having to tear down the housing to fix it.
The axles slide into the housing; take care not to gouge the seal. Once the axle ends reach the carrier, the splines must be rotated until they slide into the carrier. With both axles in place, push them as far as they will go toward the carrier.
Unless you are working with a Buick or Olds 8.2 or 8.5 10-bolt, the axles are retained by a C-clip. Slip the original C-clips (or replacements if the originals are unavailable) into the groove in the axle. Once the clip is in, pull the axle outward, locking the clip in place. Repeat for the other side. With both C-clips in place, reinstall the cross-shaft and cross-shaft retainer bolt.
The housing is now ready to be reinstalled into the vehicle.
Cover and Fluid
The differential cover must be sealed to the housing. You can use a paper gasket, RTV silicone, or a combination of the two; any of these methods is sufficient. As long as the housing and cover are clean, you should get a leak-free seal. If the rear cover is tweaked or bent at all, a paper gasket will likely leak.
Most builders apply a light coat of silicone to both sides of the gasket and then install the cover to the housing.
The cover bolts are next. Thread them into the housing and torque them to spec in a star-pattern. If you are using an aluminum cover, snug all of the bolts first and then torque to 20 ft-lbs.
Differential Support Covers
Installing a support cover is an available upgrade. Unlike dress-up covers, a support cover actually serves a purpose other than simply looking good. Typically made of cast aluminum, these lightweight covers feature carrier pinion preload bolts.
They add support to the open side of the housing and this makes a big difference in controlling rear-ward deflection of the differential. In most cases, just 5 ft-lbs does the trick.
Most support covers also have other features, such as cooling fins to reduce the internal temperature of the gear oil, which prolongs the life of the gears and bearings, as well as a small increase in fluid capacity. For high-torque applications, such as off-road and drag racing, these covers can make a big difference in longevity.
The manufacturer of the differential inside your housing makes the decision for you regarding which gear oil to use. That said, there are still a lot of choices to be made.
Modern gear oils are designed specifically for use in manual transmissions and axle assemblies. Gears have specific durability issues, such as work hardening, pitting, spalling, and scoring that must be taken into consideration when designing a lubricant. Additives are blended into the base oil to provide protection against these and other gear-specific issues.
You need to choose the correct viscosity for your gear oil. SAE (Society of Automotive Engineers) and API (American Petroleum Institute) provide an oil viscosity rating, which is based on the oil’s ability to resist flowing at specific temperatures.
SAE rating labels are in a format you have seen many times, such as 10W-30 or 75W-90.
Straight-weight oils are measured at 100 degrees Celsius, while multi-grade oils (anything with a “W” in the grade) are measured for a specific viscosity at multiple temperatures.
Multi-grade lubricants feature special modifiers that allow the molecules to alter how they react at different temperatures as the oil thins. This allows a single oil to cover a wider range of temperatures and working conditions.
The API ratings are not classified by viscosity, but rather by usage.
Although some gear manufacturers may suggest a different grade, choose your manufacturer’s recommendation for the grade for your differential carrier. This ensures that you have the best oil for the application.
The rating of GL-1 is intended for manual transmissions in mild conditions without additives or modifiers. GL-2 is used for worm-gear axles, which are not commonly found in automotive use. GL-3 is for severe-load manual transmission usage. It is not intended for hypoid gear use. GL-4 is for hypoid gears in moderate duty and load. Ring and pinion manufacturers commonly specify it. GL-5 is the most commonly used type for differentials; it is rated for hypoid gears in high-speed heavy shock loads and low-speed high-torque loads. This is what most differentials see, especially high-performance applications. MT-1 is rated for non-synchronized manual transmissions. It is used in heavy-duty vehicles such as buses and industrial trucks.
If your differential is a clutch-type LSD, you need to add a friction modifier. These are designed to reduce chatter and provide longer life for the clutches. Without it, clutches are noisy and do not function as well as they could. The rule of thumb is to add 2 ounces per quart of gear oil; that’s 4 percent for used clutches, and up to 8 percent for new clutches. Adding more does not really help, and in some instances hurts the over-all effect.
Always consult the manual for your aftermarket differential when it comes to adding LSD friction modifiers to the gear oil.
Gear Break-In Procedure
Drag racers tell you that there is no break-in period for gears. Well, for them, there is no break-in period. The only distance their vehicle travels is 1/4 mile of hard throttle, and then a 1/2-mile leisurely cruise back to the pits. There is not enough time for the oil to heat up and cause any issues. However, for street and off-road vehicles, nothing could be further from reality. New bearings are nice and tight, but that also means heat. Heat is what kills gears.
The most important time for a new set of gears is the first 500 miles. This is when the first heat cycles are made, and the wear patterns are established. Here are the established rules for gear break-in:
- First 15 miles: Keep the speed under 60 mph. Allow the gears to cool for 30 minutes before driving farther. Repeat until 100 miles have been traveled.
- Avoid heavy throttle, hard starts, and high speeds during the first 500 miles. After the first 100 miles, long trips are fine, but avoid high speeds for the remaining 400 miles.
Here is the advice that was provided by a major gear manufacturer after a fresh build two days before a long road trip that culminated in an evening of drag racing: “Just run the gears. Don’t worry about burning them up. If you did a good job putting the gears and differential together, there are not going to be any issues. Problems with gear installs usually show up in the first 100 miles, so just go out, drive the car like you stole it, and don’t worry about.”
Those are words to live by.
Written by Jeferson Bryant and Posted with Permission of CarTechBooks