In recent years, rear axles have become so robust and reliable that most mechanics have long forgotten how to diagnose and repair them, much less modify them. The rear axle is one part of the vehicle that has evolved over time into a variety of designs but has never been replaced. We can’t say that about many other technologies in the vehicle. While ignition points, carburetors, and lever shocks have faded into obscurity as more precise and efficient devices gradually superceded them, today’s rear axle designs are not much different than they were 30 years ago.
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Yes, we have made huge advances in materials to increase strength, developed better mounting structures to control deflection and noise, and designed even better bearings and seals to ensure long-term durability. We have even developed suites of sophisticated tools to analyze and predict the math and physics behind gear reaction loads. But the same basic function and implementation is evident.
The most frequent complaint made of any rear axle is a gear-generated or bearing-generated noise. The next most common complaint is a fluid leak. Fortunately, actual gear failures are very uncommon on modern vehicles, so gears are typically serviced only when the vehicle owner desires a ratio change. Altering the axle ratio or swapping a limited-slip differential in place of an open differential can greatly change the performance and feel of the vehicle.
For instance, a 500-hp big-block with a 2.73:1 ratio axle will not launch anywhere near as hard as the same car with a set of 4.10 gears. However, you need to resist the temptation to install 4.10 gears simply because it’s a larger numerical ratio. First you must understand the effect of tire size on the overall ratio. And you must understand the basic structure and maintenance of axles.
The typical beam axle has to provide the structure to support the gears and differential. It also needs to provide mounting points for the brake hardware and suspension, and must react to loads imparted by every interfacing vehicle system. This chapter covers most of the in-depth processes required to rebuild the complete rear-axle assembly. I will also include references to specific topics that are covered in other chapters, namely hypoid gear set contact pattern (Chapter 6) and limited-slip differential rebuild (Chapter 4).
Common Axle Problems
To best diagnose an axle problem, it is always preferable to road test the vehicle before tearing it apart unless, of course, the part failed catastrophically and made its own exit through the axle housing. Most issues can be lumped into one of the following categories: noise, vibration, or leakage. Try to isolate the exact circumstances that exhibit the noise. Is it occurring during mild acceleration, heavy acceleration, during deceleration, or all the time? Does it get worse as you drive faster or slower? Listening is an important step as it can help guide your pursuit to answer the question, “What broke?”
Typically, noises that get worse as speed increases are related to bearings or to the wheel/tire assembly. Tire noise is usually most prominent around 60 to 80 mph. Noises that change with acceleration and deceleration are typically gear noises. (Vibrations usually point to the universal joints or to driveshaft balance issues.) Clunk noises or thumping noises in the vehicle occurring during direction changes (when shifting from drive to reverse, or when applying and releasing the throttle, or when cornering) are typically from excessive backlash in the hypoid gears, differential gears, or excessive wear in these components or bearings. Severely worn universal joints in the driveshaft can cause similar disturbances, which are often accompanied by clicking noises.
Clunks or noises that get louder during turns usually suggest that something is wrong with the differential gears or case. Believe it or not, even brand-new axles as well as improperly set up “rebuilt” axles will make noise that only gets worse with time. Interestingly, the blame for such issues is often shifted toward the gear manufacturer especially on Internet forums when the true cause can usually be traced to a flawed gear setup.
Whining noises or high-pitched sounds that vary with a vehicle traveling at speed in a straight line typically emanate from worn hypoid gears and bearings. Worn bearings usually have a lower pitched sound while worn or misadjusted gears generate a higher pitched sound. If you hear a whine during acceleration that gets noticeably quieter on deceleration, a worn pinion head bearing could be the cause. If the noise is present during deceleration and not acceleration, the pinion tail bearing could be the cause.
Wheel-end bearings are unique for semi-float axles (see Chapter 2) because the inner bearing race is the axle itself. Over time, this surface can wear considerably, and when it has significantly worn, this type of bearing or surface problem produces a rumbling or roaring noise while the vehicle is in motion. You may also notice that the tone changes as you turn back and forth.
Vibration can be one of the trickier issues to diagnose. You want to first rule out a driveshaft problem (see Chapter 8); or it could even be something as simple as a wheel out of balance. You may remember that really cool, sideways burnout you did coming around the corner you remember, the one that ended with a hard slide into the curb. Well, in this situation, you may have bent your axle. This is far more common than you might think. One easy way to check if the axle flange is bent is to measure runout with a dial indicator.
If the axle shaft is bent, it is harder to detect. One indication is that the shaft is difficult to remove. You can also support the axle with V-blocks and measure shaft runout. Do not try to straighten axles that have been bent, as they were overloaded in the first case, and now you are just going to overload them more trying to get them straight. This continued overloading weakens the shaft. This is similar to bending a paperclip back and forth a few times; it ends up breaking in half. They need to be replaced.
Remember that time spent diagnosing a problem is time well spent. The reason to isolate the source of the noise allows you to eliminate the culprit for good. Even if you have already decided to replace all of the bearings and seals (which is highly recommended), you need to look for other problems that may have caused the bearings or seals to go bad in the first place, such as a bent axle or a worn seal surface.
Leaks are easy to spot; just look very closely for any signs of fluid leakage. The easiest leak to repair is the rear-axle housing cover. Any of the radial lip seals that are on the pinion flange or wheel ends require the axle to be partially disassembled. Usually dirt and debris sticks to the leaking fluid, making it relatively easy to spot the source of the leak. As always, exercise common sense. I have worked on some vehicles where the engine has leaked so much oil that the entire floorpan is soaked with 30-weight engine oil. Most of the time, the source of the leak is obvious. Other times it may not be as obvious, such as when oil is pumped out of the axle vent. Unintentional overfilling of the axle, or ingesting water during operation, can overfill the axle and cause it to leak.
As discussed previously, the only minor repair is an axle cover leak. Just about any other repair requires removal of the axle shafts for access to the item(s) that need to be repaired or replaced. Before getting into the removal of these components, I need to address some basic items. First off, you need to have the vehicle supported on a flat surface with enough room around the sides of the car to allow the axle shafts to be completely removed.
Each axle shaft is typically about 3 feet long and needs to be removed to access the wheel-end bearings and seals. You would hate to have the vehicle parked close to one side of the garage and have the axle shaft hit the garage wall before it completely clears the axle housing. It’s similar to removing a camshaft with the motor still installed in the vehicle; you need to clear the radiator and air conditioning condenser. So give yourself plenty of room, ample time, and patience. Place a jack on the center section of the axle and raise the rear wheels off the ground. Be careful, as each axle housing is unique and you want to avoid damaging the stamped steel cover if so equipped.
Alternatively, you can simply raise the vehicle from the frame rails and support the vehicle with jack stands. I like to get the vehicle as high as possible and use jack stands to support the frame rails. Then the axle will hang as low as possible. Just one quick comment about complete axle assembly removal: If you are going to remove the entire axle assembly to work on it separately, leave the wheels in place so that you can roll the axle around the garage. Otherwise, you will fight to balance it on a floor jack. Completely removing the entire axle and servicing it on a stand preserves much sanity.
Or you can use a few jack stands to help hold it in place. If you have access to a vehicle hoist, then the above doesn’t apply to your situation and you should consider yourself fortunate that you don’t have to crawl around on the ground and work on your back. If you are just going to be repairing items and not completely stripping, sand blasting, and repainting the axle housing, it is best to avoid the hassle of removing brake lines and parking brake cables and suspension to remove the axle. The hoist allows you to comfortably work under the car, so take advantage of that.
With the vehicle safely supported, you can get underneath. Carefully examine the axle, propshaft, brake lines, and even the suspension mounting points. I have worked on some vehicles that have a horrible clunk noise, only to find that the suspension bushings are excessively worn or the sub-frame mounting brackets have ripped out of the sub-frame. Never look past the obvious culprits. I am not going to cover every axle here, but rather the foundations of a complete “typical” axle disassembly.
I have chosen the Ford Mustang 8.8- inch axle, which is similar to the Chrysler and GM axles. The first step is to drain the axle fluid. Some axles actually have drain plugs, but these are few and far between; you might get lucky, so be sure to check the bottom for a drain plug. Most V-8-powered vehicles do not have drain plugs.
Without the advantage of a drain plug, you need to remove the rear cover in order to drain the fluid; this is similar to removing the transmission pan. As will be discussed more later (in Chapter 6), gear oil is extremely smelly and nearly impossible to get off your clothes and skin. Make sure that you have planned for that; some folks like to wear latex gloves for this part of the work. First, remove every other bolt, and then remove all of the bottom bolts. Next loosen and back out the remaining few top bolts but do not remove them completely, to prevent the cover from falling into the drain pan. The covers are usually held in place and sealed with a paper gasket or a bead of Room Temperature Vulcanizing (RTV) sealant. Slip a standard flat-blade screwdriver between the cover and the axle and lightly pry the cover away. Be careful not to score or damage the mating surface of the housing and the cover because this would prevent these two parts from sealing properly.
The fluid will begin pouring out of the gap that you have created. Have a drain pan in place to catch the fluid. Now you can remove the remaining bolts and the cover can be completely removed. At this point, you should see the ring gear and the internals of the axle itself.
The next step is to rotate the differential and ring gear assembly in order to gain access to the differential pin retaining bolt.
Loosen and remove the bolt in order to allow the differential pin to come out. On most axles the differential pin can be lightly pushed out. With some axles subjected to hard use, and as a result the differential pinion gears have worn into the pin, removal is difficult, but the differential pin nevertheless needs to come out for servicing. Worst case scenario is to get in there with a torch and cut it out. This is extremely difficult, so take your time and try to force stubborn pins out. You may need to get creative. Usually if you tap the opposite end of the pin where the bolt went in you can drive the pin out slightly and then rotate the differential 180 degrees and insert the bolt in the pin hole to try to pry it out of the case.
With the differential pin out of the way, the axle shafts can now be pushed inboard to give access to the C-washer that holds them in place. First, you need to remove the wheels.
Then the brake hardware needs to be removed. This can be as simple as removing the drums or as involved as removing the brake calipers, caliper brackets, and rotors.
At times, especially on off-road vehicles that have had the wheel ends submerged in water and mud, the brake drums are corroded in place. There are a few methods to coerce the drums off. You can drive them farther on with a hammer by striking them on the flat part between the stud holes. A trick that works quite well is to place one hammer on the flat surface of the drum between the studs and hit it with another hammer. You can use a puller to yank them off or you may have to resort to heating them with a torch to get them off. I have seen some drums and rotors that actually end up cracking before they come off. In those extreme cases, the drum or rotor needs to be replaced.
Depending on what year Ford 8.8-inch you are working on, you may have to deal with the ABS tone wheel sensors first. This style first went into production on the Mustang SN95 chassis.
The sensor needs to be moved slightly inboard so that the axle can travel inboard the correct amount to expose the C-washer. Remove the sensor retaining bolt from the back side of the sensor, and then push the sensor inboard about 1/4 inch. The plastic sensor is very delicate and easy to damage. Since the sensor may be corroded in place, be very careful because it may be difficult to move. If you break it, just replace it with a new one. In the case where a torch was required, we ended up melting the sensor.
Now the axle should freely slide inboard. It will go in about 1/2 inch or so. This is enough to expose the C-washer from its retention pocket in the side gear. In some cases, the washer simply falls out. In others, it may be held in place with an O-ring or just stuck from the lube film. The clip is tricky to remove if you are working on a limited-slip differential, so it’s helpful to use a magnetic pick-up tool to reach in and grab the washer.
Repeat this process for the other side. Then the axle shafts can be removed from the axle housing. It is best to support the shaft at the wheel end because this helps protect the bearing and seal from damage. You need to keep in mind that the seal has likely been damaged from the years of brake dust and rust, accumulated on the axle shaft, that was forced into the seal surface to get the C-washers out. It is good practice to always replace the seals with highquality replacements any time the axle is disassembled.
The end of the axle has a relief or notch machined into it to accommodate the C-washer. Keep in mind that this is the only thing holding the axle and wheel in place laterally on most beam-axle vehicles.
Carefully inspect the axle shafts in the seal and bearing area, especially if you are going to re-use them. Refer to Chapter 7 for more details on this inspection.
At this point, you may want to check the gear pattern and backlash before the axle is disassembled any further. (Backlash and pattern checks are discussed in detail in Chapter 6.) This is only required if you are planning to re-use these components, and this information can be helpful to identify a problem area. Now you can re-install the differential pin and thread the retention bolt a few threads just to keep everything together. This step is not required, but it helps keep clutter to a minimum, which is especially important if you are working on multiple projects at once.
The ring gear and differential bearing caps are next in line to be removed. These caps are like engine main bearing caps and are machined in place, so mark them before removal to make certain they are returned to the correct orientation.
Now remove the differential carrier and ring gear assembly. Be careful here because this is a heavy piece. If you are uncomfortable doing the removal yourself, get some help. Compounding the difficulty, the differential should be preloaded or have an interference fit to the axle housing that prevents it from being easily removed. If you look closely, you should be able to see the shims between the bearing races and the housing. Now pry the differential case out of the axle. You can typically place the end of a prybar in the window of the differential case to help pry the assembly out. Or you can pry on the ring gear mounting bolts and the axle housing. Just don’t pry on the ring gear teeth because you may damage them. Also, be prepared for the weight of the unit when it comes out. You don’t want it to fall on the ground.
You need to keep track of which side the shims came from. This is important later when you re-assemble the axle. At this point, you should have a good view of the last internal gear left in the axle housing.
In order to remove the pinion, first remove the driveshaft. On most vehicles, this is bolted to the pinion axle flange and its opposite end just slips into the back of the transmission. It is a good idea to mark the position of the propshaft flange relative to the axle flange. From the vehicle assembly plant, the axle and propshaft imbalances are offset to cancel any vibrations. (For more information on propshaft joint styles, see Chapter 8.)
Now simply unbolt the propshaft from the axle flange (some prying may be required for removal) and then slide the yoke out of the transmission. When the yoke comes out of the transmission, chances are that the transmission will begin leaking fluid. Be prepared for this with your drain pan. You can slide a plastic cap tool in place to stop the transmission from draining.
You can even leave the propshaft in the transmission; just move the axle end portion of it out of the way and secure it there.
Before we remove the last part of the geared axle assembly we need to loosen the pinion flange nut.
Once the nut is removed the flange can be removed. On some axles the flange slides off, but most require some force. There are two methods for removing a stubborn flange. The first is to use a puller tool.
The second method is to use a punch on the pinion to drive the pinion into the axle and subsequently off the flange. This will probably damage the bearings but you should replace them anyway. If you use this method, make certain that you support and grab the pinion from inside the axle. As always, do not let these parts to fall on the ground, which will surely damage the gear teeth. Once the flange is removed, you can slide the pinion out with the head bearing and the crush sleeve still in place.
Now that the pinion is out, you have access to the bearing cones and pinion seal.
The best way to remove the bearing races from the axle housing is to use a punch and drive them out from the opposite end. The pinion seal can be removed with a pry tool or driven out with the pinion tail bearing race.
Now the center section of the axle should be completely disassembled. The remaining parts installed on the axle are the wheel-end bearings and seals. The seals can be pried out, just like the pinion seal above.
Now you can access the wheelend bearings, which are cylindrical roller bearings. The axle shaft is the inner bearing race, so with the axles removed, you can see the rollers. We are using a tool with a foot inserted behind the bearing to apply the force necessary for removal.
This tool has a slide hammer on the other end to allow removal of the bearing.
Usually, bearings pop right out with a few strikes from the slide hammer.
At this point, the axle’s internals have been completely disassembled and are almost ready for cleaning. Before I cover cleaning, let’s remove the final bearings from the differential case and pinion shaft. The tapered roller bearing can be removed in a few ways. The destructive method is to cut the cage with a set of side cutters and remove the cage and rollers. Then cut off the inner race with a small cutoff wheel tool. Cut the race at a 45-degree angle to avoid the cutter wheel nicking the gear or differential case. Another method is to use any one of a variety of bearing tools.
Or, you can remove the pinion bearings with the clamshell-style tool. This tool is very helpful but not everyone has one. If you are going to be working on many axles, you can probably justify the expense of such a tool.
The main reason to remove the bearing, besides the obvious goal of replacing the bearing, is to gain access to the pinion head shim.
Most pinions have the shim behind the bearing to properly locate the pinion. Dana axles are an exception, where the shim is located between the pinion bearing race and the axle housing itself. Even some of the newer Dana axles have pinion shims behind the head bearing on the pinion and behind the head bearing race in the housing. Now is a good time to measure the thickness using a micrometer, and record it. (For more information on shims and gear patterns, see Chapter 6.)
The differential bearings can typically be removed with a bearing spreader or cut off as described above. (For instructions on ring gear removal, see Chapter 6.)
Cleaning and Inspection
Start from the inside and work your way out in the cleaning process. I use a liberal amount of cleaning solvent in this process. Brake cleaner is a good choice because it does not leave a residue. Spray the entire inside of the axle housing, taking special attention to clean the lube ports for the pinion and all of the bolt holes.
Now clean off the cover sealing surface. It is important to get this surface clean and free of any old gasket material and oil. This helps ensure a good seal surface for the gasket or RTV. Perform a similar operation on the wheel end and inside the axle tubes. Thoroughly rinse the seal and bearing bores on the wheel ends and then rinse the inside of the axle tubes. Wet down rags with solvent and push them down the axle tubes and retrieve them from the center section of the axle. Do this a few times on each axle tube to make certain that they are clean. You can use a long screwdriver or even a broom handle to push the rag down the tube.
Give the internal section another final rinse of solvent and blow it out with shop air. You can’t make things too clean here.
The wheel ends can be tackled next. First, drive each bearing into the axle housing until it is completely seated.
Then the seals can be installed. Grease or liberally apply gear lube on the inside of the seal lip before installation.
Then apply RTV sealant to the outside surface of the seal.
Now the seal can be driven into the axle housing.
With the wheel end completed, concentrate on assembling the gears and differential. First, drive the pinion bearing races into the axle housing.Just line up the bearing races and, with a suitable size punch, drive them into place. Tap evenly around the race in order to install it straight.
You need to repeat this procedure for the pinion tail bearing race on the front of the axle. Now set the pinion tail bearing in place. Be sure to lightly lubricate it, along with every subsequent bearing you install, with the correct gear oil. At this point, the pinion seal can be installed.
Notice that the pinion seal keeps the bearing in place. You can no longer remove the pinion tail bearing without removing the seal first. Put the correct shim on the pinion and press the pinion bearing in place. Be careful here to make sure that you press on the bearing inner race and not the cage.
Next, install a new crush sleeve. Now you can reinstall the pinion into the axle housing.
Apply a small amount of RTV on the flange spline area. This keeps oil from wicking past the splines and leaking later. Then slip the flange on the pinion, install the nut, and tighten until you achieve the correct pinion bearing preload (see Chapter 6). One method to make certain that the bearing races are completely seated is to tighten the pinion nut until the correct preload is achieved and then with a soft punch (usually brass or aluminum), tap the pinion head, not the teeth, and the opposite side of the pinion stem into the axle housing. Then check the pinion torque-to-turn again and adjust accordingly.
Next, install the bearings on the differential case. There are no shims behind these bearings.
Now you just need to install the ring gear. Notice that the ring gear slips over most of the differential case; only the last 1/2 inch is a slight press fit that locates the gear relative to the carrier. Slip the ring over the differential case, start threading the bolts to make certain that the holes are lined up, and press it into place. (Refer to Chapter 6 for the detailed steps required to properly install the ring gear.)
With the ring gear bolted to the differential, install the differential assembly with the proper shims in the axle housing. As it was to remove, the differential assembly can be a little tricky to install because of its weight and the slight interference fit between the shims and the housing. It is helpful to first place the differential loosely in the axle housing, and set the bearing cups and shims in place.
Push the differential in place and be careful that the shims don’t fall out. This can be difficult and usually the shims need to be tipped in slightly.
Once everything is in place, use a tool to tap the shims and bearing into place.
Now install the bearing caps in their correct location and torque them accordingly.
It is a good idea to double-check the backlash and pattern at this point, just to be on the safe side.
There are some companies that sell aftermarket billet bearing caps for popular axles. The cap should be installed on the ring gear side (driver’s side) of the axle where the highest forces are reacted. These are not direct bolt-up parts. Just as the axle manufacturer machines the OEM bearing caps at assembly, a replacement cap must also be machined to fit properly.
If the differential pin is installed, temporarily remove it. Carefully slide the axle shafts into the axle tubes, making certain to guide them over the new wheel-end seals and bearings. You don’t want the axle shaft spline to damage the new seals. Once the axle shafts are lined up with the side gears and installed deep enough to expose the C-washer groove, simply install the C-washers and push the shafts back outboard. This allows the C-washers to fully seat in the counter bores in the side gears. Repeat for the other side and then install the differential pin and differential pin retaining bolt. Make sure that you use thread locker on the differential pin bolt and torque accordingly.
Adding a replacement structural cover helps support the bearing caps and the axle housing. Think of the axle as a box without a lid on it. The box is not very strong until there is a lid properly attached to it. Castaluminum aftermarket covers can provide that additional support and structural integrity. The stock stamped steel covers provide some support, but very little when compared to the aluminum covers. Furthermore, many of the aftermarket structural covers are equipped with a drain plug for convenient fluid removal.
Whether you choose to re-use the old cover or upgrade to a structural cover, you need to seal the cover back to the axle. This is a great upgrade that you may want to perform on your axle during a normal lube change. The structural covers typically provide better fill and drain plugs, better cooling, and more lube capacity plus the additional structure. These covers are factoryinstalled on many of the larger truck axles. Most kits come with a replacement gasket, but keep in mind that most axles from the factory are just sealed with gasket sealer. Make sure that both surfaces are clean of any old sealant, gasket material, and gear oil.
For the structural cover, we chose to use the gasket. We first loosened the bearing caps preload fasteners, Later, these preload bolts will be lightly tightened to provide additional support between the bearing caps and the cover, after the perimeter bolts are tightened.
We sprayed a gasket adhesive on the axle housing and cover to help hold the gasket in place during installation.
Now it is simply a matter of installing the cover and bolts. Don’t forget to put washers under the heads of bolts. Then torque them to the required specification.
Apply sealant to all of the threads on the cover. Remove the drain plug and apply a dab of RTV sealant on the threads. You need only a pea-size amount to do the trick.
Now set the bearing cap preload. Apply some RTV to these threads.
The last step is to fill the axle with the correct amount and type of lubricant. Don’t forget to add friction modifier if your differential requires it. (See Chapter 6 for more information on gear oil and friction modifiers along with proper break-in procedures.)
Re-install the fill plug and install the drums and wheels. Torque the lug nuts and lower the vehicle to the ground.
You have been shown the major and minor steps required to completely disassemble, clean, and reassemble a typical beam axle. You can see that it is a fair amount of work, and this work needs to be performed carefully in order to obtain ideal results. The details are very important but the task is very achievable with the correct tools and time for the job.
Written by Joe Palazzolo and Posted with Permission of CarTechBooks