How Tos / Tech / Tech Articles

Step-by-Step: Upgrading a Ford 8.8 Rear Axle

 

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We decided to replace our '85 GT's weak 7.5-inch rear axle with a stronger 8.8-inch axle out of a 1989 Mustang. Since it would be upgraded with new stuff from Eaton, Superior Axle, and Ratech, we gutted the axle, had the housing sandblasted, then painted it black. Before we began reassembly, we took the 8.8 to the local car wash and cleaned it thoroughly to remove residual sand. The housing was dried with compressed air.

Before you even think of tackling a rear axle upgrade, you're going to need some special tools: a dial indicator set and a pinion gear depth setting kit, plus a standard torque wrench, inch-pound torque wrench, a micrometer (for measuring shim thickness), and a set of dial calipers (for measuring bearing races). Gee, another reason to go tool shopping.

Superior Axle supplied us with a 3.55-ratio ring and pinion gear set. Better known for fabulously tough axles and gears for Jeeps and other off-road vehicles, Superior also makes axles and gears for car applications. Superior Axle ring and pinion sets are CNC-machined from 8620 steel forgings, cut, and heat treated. Then, the gear teeth are given extra lapping to ensure an ideal contact pattern and quieter, smoother operation.

We could have rebuilt the stock Traction-Lok differential from the 8.8, but we wanted to upgrade to 31-spline axles. So, we opted to use an 31-spline Eaton Posi Performance differential. The Eaton is a clutch-style diff-it uses a set of spring-loaded, carbon fiber clutch plates to apply pressure to a pair of forged side gears. Under heavy load, the clutches apply more pressure to the gears, which in turn press harder into the axle case. That equalizes the load for better traction.

We've used Ratech ring and pinion installation kits forever, and have always been happy with them. Our Deluxe Installation Kit includes pinion and differential (carrier) bearings, pinion and carrier shims, a pinion crush sleeve, nut and seal, ring gear bolts, wheel bearings and seals, and a cover gasket.

We took the Eaton differential to the machine shop to get the bearings pressed on, then bolted on the ring gear. Each bolt got a dab of blue Loctite, and the bolts were tightened in a star pattern to pull the ring gear up evenly to the differential. The bolts were then torqued to 45 foot-pounds.

The pinion gear bearing races are tapped into the axle housing. We had the luxury of using aluminum drivers made for this purpose, but you can use old races, sanded undersize to fit inside the case, and a piece of thick-wall tubing to install the new races.

The Ratech kit comes with new axle bearings and seals. Just lube 'em up with high temperature axle grease and tap them into the axle tubes.

Superior marks its gears with the recommended pinion gear depth-in this case, 2.552 inches. By comparing this number with the actual pinion depth in the axle housing (we'll show you how to do that), you'll know how much to shim the pinion gear. The upside down number is the gear set ID, which is also marked on the ring gear. This indicates the gears are a matched set.

To make determining pinion gear depth easier and less expensive, we modified an old inner pinion bearing with a cartridge roll so it would sit snugly on the pinion gear, but could still be removed by hand. This way, we could add and remove shims as needed.

We used our Proform pinion setting tool to measure the distance between the main cap mounting surface and the deepest point of the differential bearing bore to determine the axle centerline. Then, we dropped in the pinion gear and inner pinion bearing to get a starting pinion depth measurement. Using these measurements, we found that the proper pinion shim thickness was .0028 inches.

Now it was time to set pinion gear preload. That's where this Ratech solid pinion spacer becomes so handy. Unlike a pinion crush sleeve, which can be used just once, the spacer can be used over and over. That makes setting preload tons easier. The spacer comes complete with a selection of shims (.010-, two .012-, .016-, and .020-inch thicknesses). It's a good idea to measure the shims and mark the thickness on each one.

The fully assembled pinion gear (oil bearings, pinion shim, spacer and preload shims) was installed in the axle housing and the pinion seal tapped in place. We slide the yoke over the pinion, and torqued the pinion nut to 125 foot-pounds. Pinion preload is determined by checking rotating torque with the inch-pound torque wrench. Rotating torque should be 18 inch-pounds with new bearings. If the reading is lower, preload needs to be increased by removing shims. If higher, add shims. We ended up with a .022-inch shim pack.

The Eaton differential/Superior ring gear is lowered into the axle housing with the original side bearing preload shims. The dial indicator is set up on the toothed side of the ring gear, and the gear is rocked back and forth several times to get a backlash reading. Backlash for an 8.8 ring gear should be .005 to .008 inches. Ours came out to .006 inches.

With the backlash determined, it was time to set side bearing preload. With the old preload shims still in place and the dial indicator set to zero, the Eaton is pushed toward the pinion gear until they mesh. This reading, plus .004-inch for preload, is the amount of shim thickness required for the ring gear side of the differential. Using the shims from the Ratech kit, we put a .275-inch shim pack on the ring gear side and a .265-inch pack on the passenger side.

With the Eaton differential set up, the bearing caps were installed and torqued to 65 foot-pounds. There is a left and ride side-the caps have arrows on top that should point out toward the axle tubes. Don't mix the caps up--they're machined to fit their mounting locations in the axle housing, and won't mate properly if swapped. Mixing up the caps leads to bearing race distortion, and eventually bearing failure.

To verify proper ring and pinion gear tooth contact, we coated both sides of several ring gear teeth with gear marking compound. We rotated the pinion gear one complete revolution clockwise and counterclockwise to get a wear pattern. The pattern in the photo is what you want to see-nice and centered on each tooth.

This chart shows the range of ring and pinion gear tooth contact patterns and appropriate correction steps to take. The optimum tooth pattern is at the top.

Superior Axle earned its reputation making tough, dependable axles for the off-road racing and rockcrawling crowd. That experience is reflected in the company's Evolution Series axle kits for the 8.8. Forged, machined, and heat-treated in-house, our 31-spline axles have rolled splines that are 35 percent stronger than cut splines and a special corrosion-resistant finish. Superior even throws in Grade 8 wheel studs, axle bearings, and seals. The axles are also available with the stock 28-spline count.

The half-inch wheel studs are put in the axle before sliding the axle into the housing. We tapped the stud in from behind the axle flange, then threaded a lug nut onto the stud and used a socket to pull in and seat the stud. We found this handy half-inch shock spacer (arrow) to prevent the lug nut from galling the axle flange.

The 8.8 axle uses C-clips to retain the axles in the housing. The C-clip system is more than adequate for most street and bracket cars up to 400 horsepower. If you have an engine making monster amounts of power, a C-clip eliminator kit is recommended to prevent a broken axle. Strange has one, part number STR-A1092, for use with stock-type axles. With the axles in place, the differential pinion shaft is tapped into place.

With the axles in place, the differential pinion shaft is tapped into place. A locking screw holds the shaft in place. The screw should be torqued 23 to 30 foot-pounds.

The Summit aluminum differential cover is a heavy duty piece. It comes with bearing cap support studs to keep the ring and pinion from flexing under hard acceleration. Eaton does not recommend using a synthetic gear lube with its Posi-Performance unit because of the very high temperatures created by the friction discs. For heavy duty applications, Eaton recommends GM Limited Slip Lubricant Additive, available from your friendly GM dealer under part number 1052358.

Although the 8.8-inch rear axle found under zillions of 5.0L and 4.6L Ford Mustangs is a pretty good piece right from the factory, there are a couple of drawbacks: a dismal selection of factory gear ratios (ranging from 2.73 to 3.27) and somewhat weak 28-spline axles. We found out just how easy it is to improve upon these drawbacks when we recently purchased a 8.8-inch rear-end for a 1985 Mustang GT with a five-speed.

Our GT’s original 7.5-inch axle was marginal at best, even for the stock 210-horse 5.0L under the hood. The 2.73 gear and lack of a positraction unit made the situation even worse. We chucked the 8.8’s tired stock innards (Traction-Lok differential, 3.08 gears, and axles), sandblasted and painted the housing, then stuffed it with a brand new Eaton Posi Performance differential, Superior Axle 3.55 ring and pinion and 31-spline axles, and a deluxe rebuild kit from Ratech. Ratech also supplied one of its very trick solid pinion spacers, which we think is a must-have for a serious 8.8.

The fruit of our labor is a rear axle ideally suited for our streetbound Mustang. The 3.55 gears will vastly improve the GT’s acceleration without putting a serious hurt on highway cruising and gas mileage. The Eaton Posi and Superior 31-spline axles are more than capable of handling the power of our stock five-oh-and are capable of taking lots more if we decide to turn up the horsepower wick. Follow along and we’ll show you how to add some kick to an 8.8 axle.

SAG-F88355 Superior Axle Ring and Pinion Gear Set, 3.55 ratio
ETN-19588-010 Eaton Posi Performance Differential, 31-spline
RAT-305K Ratech Ring and Pinion Installation Kit
RAT-4104 Ratech Solid Pinion Bearing Spacer
SAG-EV81 Superior Axle Evolution Series Axles, 31-spline
SME-8510500 Summit Racing Aluminum Differential Cover
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2 Comments

  1. I have a problem and I don’t know resolve it I have a Mustang 1989 and it come with 7.5 on rear axle and i didn’t know that until I bought the 8.8 to put on and there my problem begin I need to now how it can be possible to modify it or what can I do some one help me please

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