In 1983 the Ford 8.8-inch axle first appeared in a Ford Ranger truck, but since then, the 8.8 has been most commonly installed in the Mustang. Of course, you want your Mustang to perform at its best and have maximum longevity. Therefore, if you’re building 700-hp street car or 10-second quarter-mile car, you’re either going to perform a highperformance build of an 8.8-inch axle or you’re going to replace the axle completely with a larger, stronger 9-inch unit. People have been saying for years that you just need to put a 9-inch axle in your car, and that’s certainly an option, but it’s not your only option.
This Tech Tip is From the Full Book, FORD DIFFERENTIALS: HOW TO REBUILD THE 8.8 AND 9 INCH. For a comprehensive guide on this entire subject you can visit this link:
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This chapter reviews the steps required to fully disassemble an 8.8-inch axle and some common areas to inspect for wear. Specific details for Mustangs (but not all of the years) are also reviewed. The general steps apply to all axles even if you are working on an F-150 axle.
The 8.8-inch can certainly be used for high-performance and racing applications, so there are good reasons to stick with the factory 8.8- inch axle, but if you do you should make some upgrades.
If you are running sticky slicks and a manual transmission, both of these cause severe stress on the drivetrain, and care should be taken with them. Upgrading to 31-tooth-spline axle shafts is a must for any highperformance build because they are significantly stronger. In fact, the 31-tooth-spline axle shafts provide about a 30-percent increase in strength over the 28-tooth parts.
While you are ordering new axles, it is a great time to upgrade from the four-lug hub to the fi ve-lug wheel pattern to add more strength to the axle assembly. Of course, you are running sticky tires for a reason: to apply gobs of horsepower to the ground and avoid wheel spin. With a manual transmission and said sticky tires, you need a strong rear axle. Typically the axle shaft fails first in the low-400-hp range and with repeated drag strip launches.
When the axle shaft fails on a C-washer-style axle, such as the 8.8 axle, nothing is holding the wheel in place, except for the disc brake caliper brackets, if your vehicle has rear discs. These brackets were never designed to withstand axle failures and this high level of loading, and therefore they end up failing as well. Consequently, the National Hot Rod Association (NHRA) rules call for C-clip eliminators for car running 10.99-second or faster ETs. The real robust fi x is to have the 9-inch style, 3/4-fl oat-style wheel end bearing arrangement welded to the 8.8- inch axle housing. Most aftermarket rebuilders, such as Strange, Moser, and Currie, can perform this highperformance upgrade. You just need to send them the axle.
The automatic transmission offers some protection depending on whether there is a manual valve body and transmission brake installed. Typically, automatic cars do not start snapping axle shafts until 550 or more horsepower is produced. Keep in mind that the differential is next as the weak link, and it can fail before the axle shafts. It is good insurance to use a housing brace and rear-cover girdle at these power levels. When the power exceeds 600 hp and with good traction, the actual housing itself and tubes tend to bend. At this point, it is time to upgrade to the stronger and heavier 9-inch axle. Some owners used the 8.8-inch axle in high-horsepower applications, but the driver has to be careful not to bend the axle and smoothly apply the power, which is not always possible when drag racing but works in other types of racing.
If the axle is going to be abused, it is also a great upgrade to replace the collapsible pinion spacer with a solid spacer kit from manufacturers such as Ratech.
Driveshaft and U-Joints
In addition the driveshaft and U-joints should be upgraded. A good upgrade is to go to the stronger 1350- style fl anges and universal joints.
Disassembly Preparation Step by Step
The fi rst step is to get the vehicle raised in the air as high as possible. Larger six-ton jack stands are recommended to support the vehicle by the subframe. This allows the axle to hang as low as possible and gives more room to work underneath the vehicle. If you have a hoist, then you have plenty of room. Depending on the amount of work you are going to perform, it may make sense to pull the entire axle from the vehicle, especially if you are going to paint the axle.
Step-1: Jack Up Car(Professional Mechanic Tip)
First, place a hydraulic floor jack on the center section of the axle just forward of the cover flange and then jack up the vehicle. Do not apply any load to the cover itself; doing so may damage it. Some of the 8.8-inch axles had plastic covers (typically found on the Ford Rangers) and you can damage the cover by placing the jack on it. Also, the vibration damper is installed on the axle just in front of the jack. It is a cast-iron weight that straddles the pinion area of the axle housing. Keep in mind that this damper can get in the way of the fill plug on some vehicles. Remove the wheels and brake drums or calipers as required. If you have a disc-brake arrangement, you never want to let the calipers just hang from the fl ex hoses. Use zip ties or mechanic’s wire to hold them out of the way. Also, mark where the driveshaft flange connects to the axle flange so you can return them to the same orientation to avoid any potential vibration issues later. Once they are both marked remove the driveshaft.
Step-2: Support Car on Jack Stands
Securely support the car on the jack stands or hoist. The next step is to position a drip pan under the center of the axle, and remove the rear cover. Remove the 10 cover bolts with a standard socket and ratchet and then gently pry off the cover. I like to leave a bolt or two toward the top of the cover still in place but loosened. This allows me to pry off the cover, but not have it pop right off. This avoids some of the potential mess from the gear oil pouring all over the shop floor. Once the cover has been partially removed the oil flows out (shown). Used gear oil, and especially burned gear oil, is very smelly stuff. So make sure that you have plenty of ventilation and get the old oil into a sealed container for proper recycling.
Step-3: Inspect Differential Assembly
With the cover removed, inspect the axle internals. This Traction- Lok differential is based on the clutch pack and S-shaped preload spring. You can also see that this axle has been serviced before; the telltale sign is that the original assembly plant did not use blue RTV sealant but rather black.
Make certain that you have plenty of room on either side of the vehicle to remove the axle shafts. A typical single-car garage may be tight. The axle shafts need to clear the axle housing to be fully removed.
If you are going to perform a standard rebuild, which means replacing bearings, seals, and clutch pack, it is imperative to take good notes of the shims’ locations so you can return the axle back into service with minimal adjustments required. It has been rare that I have opened up an axle that did not need some replacement parts besides just the common wear items like bearings and seals so be prepared for a few surprises.
Axle Housing Disassembly
The 8.8-inch axle can be found on many different vehicles, with the Mustang being the most common. It can also be found on any of the smaller trucks with the 4.0-liter V-6 engine, such as the Explorer, Mountaineer, Ranger, and even the Aerostar. Some F-150s and Econoline vans with a V-6 engine also come with the 8.8-inch axle in the rear. Even some late-1990s trucks with the 4.6-liter V-8 engine used this axle.
The four-wheel-drive F-Series, Expedition, and Navigator use a special version of the 8.8-inch on the front axle. This particular differential is an aluminum, independent suspension arrangement with an opposite-hand spiral angle on the gears (commonly known as reverse-cut gears). The normal spiral angle for standard rear axle gears is a left-hand spiral angle on the pinion mating with a right-hand spiral on the ring gear. These are just switched for the reverse-cut gears, so there is a right-hand spiral on the pinion mating with a left-hand spiral on the ring gear.
Step-1: Measure Shaft Runout (Special Tool)
Depending on your rebuild plan, it may make sense to verify axle shaft runout by setting up a dial indicator. The dial indicator base has a magnetic portion that is mounted to the brake backing plate. Then the dial indicator stand portion is adjusted in order to achieve the correct position of the dial pointer. You can also remove the axle shaft and support it on V-blocks. (If you are simply upgrading the axle shafts, there is no reason to take these measurements.) Push the axle shaft inboard as far as possible until it contacts the differential pin. Rotate the shaft while observing the needle on the dial indicator. Be careful when rotating the shaft that the shaft does not plunge in or out. If it does, it artificially increases the runout values. Per Ford’s specifications, the axle shaft runout should not exceed .030 inch. If the shaft is out of specification, always replace it. Never attempt to straighten the shaft or flange because it will prematurely fatigue and break. The axle shaft holds the wheel on the vehicle for a semi-fl oat axle arrangement such as the 8.8-inch. If you are upgrading shafts and the differential for any performance type of build, go with the stronger 31-tooth spline axles.
Step-2: Remove Retention Bolt
To remove the axle shafts from the axle housing, remove the C-washers that are retaining the axle shafts. In order to gain access, remove the differential pin-retention bolt, along with the differential pin. The axle shafts are then free to be pushed inward for access to the C-washer. Remove the differential pin-retention bolt and the differential. Here, I have rotated the differential so that the differential pin is accessible. It is kind of tricky to get a socket on it. A closed-end wrench fi ts on the fastener head but it is typically too short to provide enough leverage to loosen the bolt. I have an 8-mm, 6-point, 3/8-inch-drive socket in my toolbox just for this situation. The retention bolt is partially removed in the photo.
Step-3: Remove Differential Pin
The differential pin should slide out easily. If the differential pin and pinion bores have excessive wear, there may be a slight step worn in the differential pin. If this is the case, the pin is very difficult to remove. Unfortunately, there is no way to continue the disassembly process if the pin cannot be removed. If the pin shows extreme wear, you may have to cut the pin in order to remove it. (I have had to resort to using a plasma cutter on a couple of occasions to cut the differential pin and housing into pieces to get them out. These were units that actually had a pinion gear fail, causing excessive wear on the pin.) This pin shows signs of typical wear and can safely be re-used. You can see the two oval-shaped witness marks near the center of the pin, which are typical from the axle shaft banging into the pin during cornering maneuvers. This is common and nothing to be concerned about.
Step-4: Remove Drum or Disc Brakes
The typical disc brake setup has the caliper still in place (left). In some instances the rotors or drums are rusted tight and cannot be easily removed. If this is what you fi nd, use a mallet to hit the drum or rotor between the wheel studs and drive it back on the shaft to break the rust loose. Be careful not to hit the wheel studs or you could damage the threads. This sometimes works. In a worst case scenario (right) you may need to resort to an oxyacetylene torch for help. Using a torch is a last resort, as this will surely damage any speed sensors that are behind the rotor. (I am not too worried about damaging the axle seals as I always replace them as a matter of practice.) If this doesn’t work, there are special pullers that can be used to apply pressure to remove the drum or disc. If all else fails, the drum may need to be cut in half with an angle grinder. This method is a messy, noisy, and time consuming process and care needs to be taken to not cut into any of the brake hardware that is underneath the drum.
Step-5: Remove Wheel Sensors
On most modern Mustang axles, the anti-lock brake system (ABS) tone wheel and wheel sensor is located on the axle shaft, and this needs to be removed. The ABS debuted on the SN95 model in the 1994 model year. Some of the earlier axles have it trapped between the ring gear and differential case, while pre-ABS units do not have them at all. The engineer in me hates this design because the speed sensor needs to be removed and is easily damaged during removal. To remove the sensor, push the axle shaft in far enough to gain access to the C-washer. An inverted Torx-headed bolt holds the sensor from the back side of the plating with perfect access. The correct socket with a 1/4-inch-drive ratchet is adequate to loosen and remove the bolt. Corrosion typically builds up on the sensor, and there is no easy way to apply load to it to remove the sensor. I usually end up with a 50/50 chance of damaging the sensors. Replacement sensors are readily available at a local parts store or Ford dealership.
Step-6: Remove C-Washer
Once everything is clear on the wheel end side and the differential pin has been removed, push the axle shafts inward with your hands to gain access to the c-washer that holds the axle shaft in place. The axle shaft should slide inward easily and not require excessive force. If excessive force is required, this is a sign that the axle shaft or housing may be bent.
Step-6: Remove C-Washer (Continued)
I have had to resort to a large hammer to drive a severely bent axle shaft inward but this is a rare and damaged unit. It is a gold-colored washer and slotted to allow it to match the groove in the end of the axle shaft to allow for assembly and disassembly of the unit. This washer can be a little tricky to get to and, with the gear oil and O-ring on the axle shaft, difficult to get loose. Use a small flathead screwdriver (page 34) to partially push it off the axle shaft and then use a magnetic pick-up tool to pull it out. Repeat this procedure for the other side of the axle shaft as well. The axle shaft groove and C-washer are shown at left.
Step-7: Remove Axle Shaft from Housing (Critical Inspection)
I always recommend replacing the wheel end seals; use two hands to guide the axle shaft out of the housing (left). Use a shop towel to help catch and strip off some of the gear oil as the shaft is being removed. Once the axle shaft has been removed, closely inspect the wheel end bearing surface as the axle shaft is the bearing inner race. This area often shows signs of wear, surface pitting, and frosting. This shaft and bearing probably made a growling noise in the vehicle that increased with speed (right). It needs to be replaced.
Step-8: Remove Wheel End Seal and Bearings
With a bearing puller tool (left) and slide hammer, use a half dozen or so hits with the slide hammer puller to remove the bearing and seal (right). This example has a pivoting foot at the end that goes in behind the bearing. Once the foot has been placed behind the bearing, reach in and align it, so it is perpendicular to the bearing.
Step-9: Remove Differential
Unbolt the bearing caps and remove them, making certain to keep track of their location. They will be re-installed in their original location and cannot be swapped from side to side or installed upside down. They are machined with the axle housing and are a matched set to the housing locations. Mark them and the axle housing immediately with two little center-punch marks on the seal surface at about 3 o’clock. Also mark the caps. The bearings and shims feature a factory interference fit on the differential assembly. The bearings should be a tight fit, but if there’s been a substantial amount of wear, they may be not be tight. To overcome this tight fit, place a large screwdriver on the housing and into the differential pin hole. Use a pry bar between the housing and one of the ring gear bolts. The differential with the ring gear installed is very heavy and you don’t want this to fall out, hit the shop floor, or hit any part of your body. If it does, serious injury to the differential or yourself could result. Also, never pry on the ring gear teeth as you will most certainly damage the tooth surface and ruin the gear. As the differential comes out of the axle housing, make certain to keep track of the cast-iron shims and which side that they came out of. You should bag and tag these parts to aid in reassembly. If for some reason one or both are cracked, just make certain to measure the thickness. I have used cracked shims in the past as they only act as spacers, but it depends on the amount of damage to the shim.
Step-10: Remove Propshaft Flange
With the differential and bearings out, the pinion gear and bearings are the last items to remove. Newer Mustangs have a constantvelocity flange on the rear propshaft joint. With the independent type of flange, the spline on the pinion shaft has a slight helix, or twist, in the spline to remove any potential source of backlash or clonk from the interface. This is great but makes difficult work to remove and install the flange. Remove the pinion nut and use an impact gun to remove the gear. (See Chapter 4 for details about a flange-holding tool for reassembly.) A typical three-jaw puller works perfect to remove the flange. Once the flange has been removed, the pinion can be pushed out the back of the axle housing. The pinion comes out with the head bearing still pressed on, plus the collapsible spacer.
Step-11: Pry Out Pinion Seal (Professional Mechanic Tip)
The last parts still in the housing are the pinion seal, oil slinger, tail bearing, and races. The pinion seal just needs to be pried out with a seal remover tool; even a large screwdriver or pry bar suffices. Take special care to make sure you are prying on just the seal can and not the oil slinger behind it. The oil slinger needs to be re-used and you don’t want to bend or damage it.
Step-12: Identify Oil Slinger
Once the seal is out, you can see the oil slinger and tail bearing. The only part that is going to be re-used is the oil slinger.
Step-13: Remove Pinion Head Bearing
Use a typical bearing spreader tool (above) to remove the pinion head bearing (right). I am not going to re-use the bearing but want to gain access to the shim that is behind it. This will be important during the re-assembly process (see Chapter 4). Make certain that the tool is grabbing the back side of the bearing race and not the cage. If you remove the bearing by pulling on the cage, the bearing will be ruined. Even though I am going to replace the bearing, if the bearing is in decent shape, I can open up the inside diameter with a die grinder and make this a set-up bearing for the installation process. The picture on the right shows the pinion with the bearing removed and the shim on the shaft.
A clamshell-style tool can be used to remove the pinion head bearing. It’s a valuable tool because there is little chance of damaging the bearing. Of course, this tool is in the $400 range but can be used for the differential case bearings as well. If you are careful with a bearing spreader tool, you can do it without this tool.
Step-14: Clean Inside of Housing
The axle housing is completely empty now, except for the pinion bearing races. Drive them out with a steel punch. Place the end of the punch on the exposed lip of the bearing race and firmly tap and move the punch around the race. (This housing has two cutouts machined into the casting at the 3 and 9 o’clock positions to allow for access of the punch. Just keep alternating between the two sides to drive the race out.) Once the pinion bearing races have been removed from the housing, start cleaning the internal housing. Use cans of brake cleaner and fresh shop rags. In this picture you can see the pinion bearing feed port above the pinion bearing races and the return port below the races. A fair amount of metal debris and crud tends to pile up in this area, so make sure it is clean. Spray generous amounts of solvent down the axle tubes and push rags from the wheel end side to the inside of the axle housing until they come clean. The axle shaft or a broom handle works well as the pusher.
This is what the housing looks like with the pinion bearing races removed. Use plenty of brake cleaner to remove all of the debris. Always finish with a rinse of brake cleaner to remove any pieces of shop towel.
There are other cars that used the 8.8-inch axle as well, including Thunderbirds and Cougars with the 4.6- or the 3.9-liter engine with the 4R70W or manual transmission. The automatic overdrive (AOD) transmission cars typically received the smaller 7.5- inch rear axle. The Lincoln Mark VIII, Crown Victoria, Grand Marquis, and Town Car was equipped with the 8.8- inch axles.
With all of these different vehicles and engine/transmission combinations available, you are bound to run across many different axle variants. As with all Ford axles, the differentials and gear sets can be swapped among any of these axle housings, with the only exception being the four-wheel-drive reversecut front axles.
Traction-Lok Differential Disassembly
With all of the main axle housing parts disassembled, disassembly of the limited-slip differential is next. If an upgrade to a 31-tooth limitedslip or aftermarket differential is in the budget, taking the old factory unit apart can be skipped.
Depending on the mileage and wear on the axle, it may make more sense to just replace the entire unit. Excessive wear on the internal components, such as the pinion gear bores, differential pin, and even the bevel gears, may warrant a replacement of the entire differential.
I have decided that I am going to disassemble the differential, thoroughly clean the parts, and inspect them for wear. One caveat: Never clean the clutch plates in any type of solvent-based cleaner as this may damage the resins that bond the friction material to the plates. This solvent may also soak into the friction material and change its properties to the point of the material no longer functioning correctly.
Always replace the clutch pack unless the friction plates look great. After all, the main reason for servicing the axle is to replace bearings and seals. For any performance build, carbon friction material is recommended (see Chapter 4 for more detail).
Step-1: Remove Ring Gear (Critical Inspection)
During the rebuilding process, be sure to perform a thorough inspection. Always wear safety glasses during. You need to take the differential apart and clean and inspect the pieces. First, remove the ring gear, which is press-fi t onto the differential case. Stand the differential on end on a block of wood and remove all of the ring gear bolts. Once these bolts have been removed, drive the gear off by using a punch in the bottom of the ring gear bolt holes. Alternate in a crisscross pattern to evenly remove the gear. Never hit the gear back face or teeth directly with a hammer because damage will occur, which could cause runout on the ring gear. Also, don’t hit partially engaged bolts, as you will damage the threads in the gear. I have seen many ring gears with dents on the back face and bolt threads that are damaged from these tactics.
Step-2: Remove Preload Spring
Use a soft punch and a hammer to drive the preload spring partially out of the differential. A soft punch helps avoid any nicks or damage to the spring. Just make sure that the punch is squarely on the spring and alternate the taps until it starts to come out.
Step-3: Drive S-Spring from Housing (Professional Mechanic Tip)
Use a hammer and punch to drive the S-spring out of the housing, but keep in mind that the spring is under pressure. As it is driven out of the housing, it follows the side gear angled profile. Once it clears the housing, the spring releases its latent energy and can become a flying projectile. Be careful. To prevent the spring from flying out of place, put a shop rag on it. With the shop rag in place, you can easily continue to tap it out.
Step-4: Remove Side Gears
If the clutch pack is worn, as most used units are, you can easily rotate the differential pinion gears by hand so they spin out of the differential case. This can be achieved with your bare hands and no special tools if the unit is worn (left). In some cases, the clutch pack is still tight and requires the use of tools for removal. I made such a tool (middle) using a piece of an old shaft welded to a chunk of steel to hold it vertical in my bench vise. Now I just set the differential over the spline, engage it with the side gear spline, and twist the differential housing. This makes quick work of spinning the pinion gears out of the differential. If the correct spline is not part of this homemade tool, I just use the original axle shaft, bolt it to a wheel, and stand it on end (right).
Step-5: Remove Pinions from Housing
As the pinions rotate in the differential case, they align with the open windows and can be easily removed. Hold the top side gear in place as you removed the pinions. You want to avoid the side gear just falling out. Also, double-check to make sure that you have removed the pinion thrust washer. At times, it tends to stick in the differential case from a thin layer of gear oil. If this is the case, just reach in and pull out the washer.
Step-6: Remove Entire Clutch Pack and Side Gears (Documentation Required)
With the pinions out of the way, nothing is holding the side gears and clutch pack in place. As with pinions, always double-check to make sure that you have the entire clutch pack, including the shim, out of the differential case. If you are going to re-use the clutch pack (which I rarely do), keep them in the order and on which side they were removed from: ring gear flange side or passenger side. Wipe down the plates with a rag but never use any type of solvent or cleaning solution on the plates as it is absorbed into the clutch material and contaminates it. These clutch packs are prone to wear out and I always replace them, unless in a very rare case they are within specifics for wear. (See Chapter 4 for the correct order of friction and reaction plates.)
All of these parts have been removed from a Traction-Lok differential. For visual identification, the clutch plates are together and the pinion washers are still loosely in place. If during the axle disassembly process, the differential pin shows any signs of wear, chances are the pinion bores are worn and everything should be replaced. A new replacement carbon fiber, 31-tooth-spline Traction-Lok (PN M-4204- F318C), can be purchased from any Ford dealership or motorsports distributor for about $280. Ford previously carried a paper-plate version (PN M-4204-F318) for the 31-tooth spline for about $260. Although it is has been discontinued, I have found some dealers still have stock. The 28-tooth version (PN M-4204-F288) has also been discontinued, although some dealers still have stock. The clutch pack kit for paper (PN M-4700-B) and for carbon fiber plates (PN M-4700-C) are available. You also need two bottles of friction modifier; one for soaking the new plates and one for filling the axle. If the parts are in good condition, just clean the gears in solvent, check the pinion washers for wear (they should be .030 to .032 inch thick and not have any signs of grooves or wear marks), and clean up the differential case. Always inspect the differential case after cleaning for any signs of grooves in the machined pockets where the gear washers locate or in the differential pin bores.
Written by Joe Palazzolo and Republished with Permission of CarTech Inc