There are 33 different aftermarket shim thicknesses available, ranging from .307 to .241 inch (in .002-inch increments). One of the biggest frustrations in building these axles is that the thin shims in an aftermarket pack are very difficult to install without damaging them. And if you use the OEM shims you have a bunch of different sizes laying around. If you build a lot of differentials, it makes sense to have a variety of OEM shim thicknesses in your shop; otherwise, take your time with the aftermarket ones. This is also a reason to save any old shims for your next build.
The Ford base part number is 4067; you can purchase the exact thickness that you need from there. The Ford shims are pricey and that is why most people use an aftermarket shim pack, which consist of a “sandwich” of three shims. The inner and outer shims are thick, while the middle shim is thinner. The pack can be driven in with a brass drift without any concern for cracking. A final check of pattern and backlash is a good idea.
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You can see how the parts fit in order. The pinion has a gear or head on one side and the spline and threads on the other. The head shim and then the head bearing are pressed in place on the pinion. (This shim is what sets the pinion head mounting depth. If you do not have the original shim as a baseline, a .030-inch-thick shim is a good starting point for the 8.8- inch axles. If you are using another Ford gear set, all are really close and the original shim is a great starting point. I prefer the Ford gears as they are readily available, more consistent, and higher quality compared to aftermarket gears. If Ford does not produce the needed ratio you have to use aftermarket gears.) Next come the pinion head and tail bearing cups that are pressed into the axle housing. The collapsible spacer goes in between the head and the tail bearing cones. The head bearing cone is a press fit on the pinion shaft but the tail bearing cone is a slip fit. The collapsible spacer provides resistance to the tail bearing cone and allows for manufacturing differences in the housings. There are solid shim kits available in the aftermarket. If you go this route, always add a shim .012 inch thicker than the original collapsible spacer height and continue decreasing the shim thickness until you achieve the correct torqueto- turn measurement. This makes certain that you do not overload and damage the bearings by not having a thick enough shim in place. Next is the pinion seal oil slinger and seal that are pressed into the housing; and finally the input flange and nut.
These are the pinion head shims that come in a typical rebuild kit. The total shim range is .010 to .038 inch. These shims can be stacked to achieve the correct height. These shims are used to compensate for variations in production tolerances, specifically the variation of the housing machining and gears. As mentioned earlier, Ford gears are very consistent and the main source of variation is from the housing machining.
Step-1: Install Head and Tail Bearing Cups
Again, make sure the housing is clean, and rinse out any debris from the oil feed and return ports. From the backside of the axle, drive in the pinion head bearing outer race, or cup. Make sure that it is fully seated and exercise extreme care to not hit the tapered race. Drive on the thin outer diameter of the bearing cup. If you have an old cup or the correct size bearing driver, use one of them. These are strongly recommended for driving the new cup, but you can also get away with a punch, but exercise care not to hit the tapered surface. Make sure that the cup is seated squarely into the axle housing. Follow the same procedure to install the tail bearing cup in the front of the axle. Set in the tail bearing cup and drive in the pinion seal. The pinion seal actually traps the tail bearing in place, so don’t install the pinion seal without the bearing in place.
Step-2: Set Pinion Bearing at Correct Depth (Special Tool, Save Money)
Here the pinion head shim is in place. The head bearing is propped up to show the shim. Press fi t this bearing onto the pinion shaft. If the shim is incorrect, you must remove the bearing in order to remove the shim. When you remove the bearing, make sure that you grab the bearing inner race and not the cage to make the shim adjustment. A bearing spreader (such as the one shown in Step 13 on page 37) is required to properly grab the bearing race. If you grab the cage and remove the bearing, you will distort the cage and ruin the bearing. Make or purchase a set-up bearing so you avoid bearing damage when you remove it from the pinion shaft. If you perform a number of axle rebuilds, this tool comes in very handy. You can get a bearing with the inside diameter opened up to allow for a slip fi t (for about $70) or you can simply take a die grinder to the inside diameter of a new bearing to make it larger. When you have determined the correct shim thickness, use the correct pressfit bearing and a new collapsible spacer to replace the set-up bearing. While this may sound like a hassle, you need to do this correctly because you need to achieve the correct gear position for the best differential performance and reliability.
Step-3: Shim Pinion Head
Some tools make shimming the pinion head easier, but they only work if the new pinion head is marked with its deviation from the ideal condition. Some aftermarket gears are marked, but the Ford Motorsports gears and original equipment gears are not marked. The tool (left) comes with good instructions if you are fortunate enough to have the pinion head markings. I actually own this tool and as you can tell by the lack of wear on it, I never use it. A typical aftermarket gear set (right) has the mounting distance offset and backlash values engraved on it.
Step-4: Press Shim and Head Bearing into Place
Press the pinion head bearing and shim into place (left). You can use an old bearing race as a press tool because you are sure to only press on the bearing race and not the bearing cage. Tapered bearings are very sensitive to roller positioning and it is crucial that you never press on the cage. Any damage to the cage misaligns the rollers and the bearings fail. On the right are some scrap pieces of various diameters are useful for pressing bearings and seals in place.
Step-5: Install Pinion in Axle Housing
Always lubricate the bearings with fresh gear oil prior to assembly in the housing. Place the collapsible spacer on the pinion and put the pinion in place. Add some gear lube to the bearings and spin the pinion to check for smooth running. Install from the back side of the housing.
Step-6: Inspect Pinion Installation
This is what the front side of the axle housing looks like. You can see that the seal is in place along with the tail bearing behind it and the pinion stem is partially sticking out. Be careful, especially when working with the axle still in the vehicle that the pinion does not fall out. There is really nothing holding it in place until the flange is installed.
Step-7: Set Flange in Correct Position
Position the flange in place. I recommend applying grease to the seal and putting a small amount of grease on the seal surface of the flange. This flange also has a brown plastic dust excluder installed to shield the seal from road debris.
Step-8: Install Pinion Nut
Install the flange nut and start the process of achieving the correct bearing preload. Always use a new pinion nut on the final installation because it has fresh Loctite on the threads and sealant on the flanged surface. During the bearing preload process, make sure that you rotate the bearing frequently to ensure proper torque-to-turn readings. Also, never back off the pinion nut to reduce preload. If the preload is too high, you need to replace the collapsible spacer and start over. I usually purchase extra collapsible spacers just in case this happens.
Step-9: Install Flange (Torque Fasteners)
A few different styles of tool can be used to hold the fl ange in place while you are tightening the pinion nut. A Ratech tool (bottom) can be purchased inexpensively or a simple homemade tool (top) can be fabricated. Keep in mind that achieving a torque of 250 ft-lbs is not unheard of in order to get the collapsible spacer to begin yielding. A long-handled tool on the breaker bar are usually required to hold the fl ange in place. Check the pattern to be sure there is no need to set the preload; rather, you just need to take up any clearance in the bearings. If you need to set the preload, be careful not to over tighten. You want the torque-to-turn to be between 16 and 28 in lbs for new bearings and 8 to 14 in-lbs for used bearings (see page 94 for details on the torque to-turn measuring process).
You can assemble the axle on a stand (shown) or in the car. If you assemble the axle while it is installed in the car, it is important to either have a hoist or get the car as high as possible on jack stands, so you have room to swing the long-handled tools while the collapsible spacer is installed.
This homemade flange tool is used for U-joint–style flanges. The tool does not have to be fancy, just attached firmly and long enough for adequate leverage.
Step-10: Clean Ring Gear
Make certain that there are no burrs on the mounting surface of the ring gear. Run a fine file across the face just to make sure. Use solvent to clean the ring gear thread holes and the entire gear. The entire gear as well as the holes need to be clean of any oil and debris or any metal shavings prior to installation on the differential case.
Step-11: Install Ring Gear in Case (Torque Fasteners)
The ring gear is a clearance fit over the differential, except for the last 1/2 inch, which is the press-fit pilot diameter. One method to install the ring gear in the differential case begins with setting the ring gear in place and starting all of the ring gear bolts. I always re-use either the factory bolts with fresh Loctite or get new fasteners from Ford directly. If you are going to re-use the factory bolts, make sure they are clean before applying fresh Loctite. You do not want any traces of oil that makes the Loctite fail later. Tighten the bolts in stages in a crisscross pattern to achieve the final torque of 100 ft-lbs recommended for Ford Motorsports gears. I do this in five steps: snug the bolts down by hand, then torque to 25, 50, 75, and then 100 ft-lbs. Start with a low torque to evenly pull the gear in place. You never want to excessively load the bolt threads when they are partially engaged because you may strip the threads.
Another method to install the ring gear on the differential case begins with putting the differential case in a bench vise with soft jaws to avoid knicking the differential case. You install the bolts to act as guide pins and then use a press to force the ring gear in place. I am not in favor of this method because you risk chipping or cracking the gear teeth, but with care and a soft surface such as blocks of wood to press against, it is acceptable.
Step-12: Compare Factory and Aftermarket Ring Bolts
Here is a comparison of the factory ring gear bolts (left) and the typical aftermarket bolts (right) that are supplied in most rebuild kits. The aftermarket bolts always seem of marginal quality to me, with the exception of a premium aftermarket brand such as ARP. I always choose to re-use the factory bolts over using the aftermarket ones. Of course, the ideal situation is to get new bolts, as most of the new applications use torque-to-yield fasteners that really should not be re-used.
Step-13: Press On Differential Case Bearings (Special Tool)
Similar to pinion head bearings, differential case bearings are a press fit. Use an old bearing race as a press tool and press just on the inner race. No shims go underneath these bearings. Just make sure that the bearings are fully seated. Shims that go between the bearing cup and the axle housing establish the ring gear position and bearing preload. Re-use the original shims and check backlash and pattern (see Chapter 7). Just as with the pinion shim, some adjustments may be required. Install the old left-hand (ring gear) side shim and then use a thinner shim on the right side to make it easier to install the differential for this first fit up. No play side-to-side can be present in the differential. You can take care of bearing preload shimming at the final installation, once you have the gear pattern and backlash setting correct.
Step-14: Mate Axle to Differential (Documentation Required)
Here, the differential shim is partially in place. It is the cast-iron steel ring with the light green and black stripes on it. The beveled edge goes toward the outside of the axle. It is an interference fit to re-install the differential. Be careful; the differential is heavy and awkward to install, and you don’t want to drop it. Partially install the differential with the bearing cups and angle the shims in place. Note the two punch marks above the bearing cap surface inside the axle housing. These are the marks for the bearings caps.
Step-15: Select Shim Driver Tool
Since the shim is press fit, use the shim driver tool to install it. Do not hit the cast-iron shims directly with a hammer because they are very brittle and will crack. You can purchase aftermarket steel “super shim” packs that are not cast. You can use a mill or a belt sander to make your own tool like the one (left) I cut out of fl at plate. Be sure the curved shape accurately matches the shim diameter. You can also purchase a tool (right) that is actually a series of different curved shapes to fit just about any axle. Yukon Tool makes versions of both of these.
Step-16: Install Bearing Caps and Bolts
Once the shims and bearing cups are driven in place (left), install the bearing caps and bolts. Remember, during the disassembly process, the caps were marked so they are oriented correctly from left to right and top to bottom. Ensure they go back into the original place and orientation. The bolt torque is 90 to 100 ft-lbs; tighten them in stages, just like ring gear bolts (right). I always use 90 ft-lbs to be safe.
Step-17: Check Backlash and Pattern
Set up a dial indicator and verify backlash and pattern one last time. The dial indicator base has a magnetic portion that is temporarily attached to the axle housing; the dial position is adjusted to get the dial pointer in the correct position. Once in place, the dial face can be rotated to align with the zero position of the gauge. Check backlash in at least four different positions that are about 90 degrees apart from one another; there should not be more than .004-inch variation. The backlash should be .008 to .012 inch. New gear sets come with a backlash recommendation in the instructions. Always use the backlash that the gear manufacturer recommends. If the backlash is not within specification, change the shim thicknesses accordingly to achieve the correct backlash. To increase backlash, move the ring gear farther to the left by decreasing that shim’s thickness. The amount of decrease on the left shim is the same amount that you need to add to the right shim in order to maintain the overall bearing width and zero endplay. Once the backlash is correct and you are satisfied with the pattern (see Chapter 7), then you need to achieve the correct bearing preload on these bearings. If you are using aftermarket shims, add .006-inch shim thickness per side. The aftermarket units are thin shims stacked behind the factory cast-iron shims. Install the thin shims first and then drive in the thicker shim. The other option is to have an assortment of OEM-style cast-iron shims on hand.
This chart shows the required change in shim thickness as compared to backlash change. It is not always a one-to-one relationship.
Step-18: Install Axle Shafts
First, remove the differential pin retention cross bolt that was partially installed earlier and then remove the differential pin. Remove the pin to allow the axle shaft to go in farther than typical (see Chapter 3). This gives access to the C-washer groove. A small O-ring was installed in the groove of the stock axle shafts to help hold the C-washer in place during the initial build at the axle assembly plant. These O-rings are usually long gone by the time the axles are repaired, so don’t worry if they are missing because they are not critical.
As during the disassembly process, always guide the axle shaft carefully through the axle seal. If the spline is dragged across the seal, seal damage will result. If the weight of the shaft drags across the seal, the garter spring can be knocked off the seal. The last couple of inches are a little tricky because the splines need to be aligned. Just lightly tip the flange up and down and the shaft eventually lines up.
Step-19: Install C-Washers (Torque Fasteners)
Carefully reach in behind the S-spring and install the C-washer. Use a small screwdriver to fully seat it and make certain that it is all the way in place. For clarity, this photo shows the installation outside the axle and without the ring gear. (This is actually done after the ring gear is installed and the differential is in the axle housing.)
When the washer has been fully seated in the axle shaft groove, pull the axle outboard; the washer becomes trapped in the pocket that is machined in the side gear. Although not readily apparent, the gold colored washer is nested in the pocket. Repeat this procedure for the other side and then install the differential pin. Torque the retention bolt to 26 ft-lbs. I like to use a new retention bolt and most rebuild kits come with one, so be sure to use it. The new ones already have Loctite on the threads but if you re-use the old bolt, make sure that it is clean and dry and add Loctite to the threads. You never want any of the bolts to come loose inside the axle housing.
Step-20: Install Axle Cover on Housing
You can use RTV sealant to seal the axle cover on the housing. Use a bead of black RTV sealant and let the sealant cure, overnight if possible, prior to filling the axle with oil. RTV is a condensation cure product, but if there is too much humidity (70 to 80 percent), it actually slows the curing process. So you need to work fast! Apply the RTV and get the cover bolted down within 5 minutes to be on the safe side. Using a paper-style gasket is another way to seal the axle cover on the housing. Apply a thin layer of gasket adhesive on both sides. The sealant needs some cure time before it is exposed to the gear oil. I recommend waiting at least 60 minutes before you add oil; overnight is better.
Step-21: Install Axle Cover Bolts (Torque Fasteners)
Use a crisscross pattern to tighten the bolts. First snug up the bolts, then torque them to 33 ft-lbs for steel covers and 24 ft-lbs for aluminum covers.
Step-22: Fill Differential with Fluid
If you have a limited-slip differential, add 4 ounces of correct friction modifi er. Typically for the 8.8- inch axle, the fill amount is about 2½ quarts to the bottom of the fi ll hole when the vehicle is level. This photo shows the fill hole with the 3/8-inch square drive plug. This plug and housing are brand new; most are rusty and the square drive recess is full of debris. Make sure that you take the time to clean it out and fully seat the drive tool or you will strip the plug. (There is a picture of this in in Step 1 on page 61). The fill plug is on the driver’s side on the forward-facing, vertical surface next to the pinion.
Written by Joe Palazzolo and Republished with Permission of CarTech Inc
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