We began our conversion by removing the stock 7.5-inch rearend housing out of our '67 Mustang so we could accurately measure the position of the spring mounts.
Car crafting is all about building a strong, reliable performance car from readily available and inexpensive parts. It's also about not spending the next year's worth of paychecks in the process. Right about here is where we're going to piss off a bunch of hard-core Ford guys. The standard Blue Oval rearend rule for the last 40 years has been "Use a 9-inch." While we will not dispute the strength of a nodular iron 9-inch, the more recent Ford 8.8 is a realistic alternative.
The only true way to discover if the 8.8 in question has a Traction-Lok limited slip is to remove the rear cover. If the differential has that S-shaped spring (arrow) in between the side gears, you've scored.
The focus of this buildup will be an 8.8-inch housing we pulled out of a wrecked '88 Mustang and rebuilt to spin the wheels on our '67 Mustang future road racer. With assistance from our buddy Tim Moore, we figured out a way to stuff this 8.8 under the early Mustang while spending less than half the cost of a typical $1,900 9-inch Ford. The end result is a rear axle assembly that's lighter and more compact and will be just as durable behind our small-block as any 9-inch-while not permanently maiming our wallet.
Boneyard Beginnings
It's no secret that the '80s Fox-body 5.0 Mustang can be found in virtually any boneyard in the country. All 5.0 'Stangs came with the 8.8 rearend. As we outlined in our original 8.8 buildup story in the June issue, "The Ford 8.8 Mustang Rearend," this third member can be purchased for much less than $100 and, if you're lucky enough to find it, a housing with decent gears and a Traction-Lok limited slip. The rearend is so designated because of its 8.8-inch ring-gear diameter, making it slightly larger than the much-vaunted GM 12-bolt. If there is a weak link in the 8.8 chain, it's that most Mustangs came with spindly 28-spline, four-lug axles. But there's an easy fix for that too.
We used the old 7.5-inch rearend out of our '67 Mustang as the template for the new 8.8. First we measured the distance between the centerlines of the original spring pads, which was 4231/44 inches.
Our automatic-equipped donor car was handicapped with a lame 2.73:1 gear ratio, but at least it came with a factoryTraction-Lok limited slip. Other vehicles equipped with the 8.8 rear can offer a wide range of ratio opportunities, so it's possible to find 3.45:1, 3.55:1, 3.73:1 or even 4.10:1 factory gears if you're lucky. Unlike the GM rear axlehousings, there is only one style of ring-gear carrier for all gear ratios, so if you find an 8.8-inch Traction-Lok, it will work regardless of the ratio all the way up to 4.56:1.
The quick fix to convert the Mustang 8.8 over to five-lug bolt pattern axles is to find a pair of left-side '90 and newer Ford Ranger 8.8 truck axles. The left-side axles are the same length as the Mustang pieces and will bolt right in. Of course you'll also need the drums, which will fit right over the existing Mustang backing plates and brakes. The smart move would be to just drop a few more bucks and buy new drums, which is what we did.
Moore used this Lamb Components digital angle-finder to determine the 4.5-degree pinion up-angle.
Converting To Leaf Springs
Perhaps the biggest part of swapping the 8.8 into our '67 Mustang is converting the housing from coil springs to early-style leaf springs. The first step is to remove all the coil-spring brackets off the housing. Ray Currie gave us a tip that the axletubes on these Mustang housings are especially thin, so never use a torch because the heat will permanently warp the tubes. The approved approach is to use a cutoff wheel and smooth everything with an angle grinder.
Disassembling the Ford 8.8 to remove the old four-bolt axles and brakes was much easier with the help of our Ingersoll-Rand cordless 31/48-inch impact.
Tim Moore recommends duplicating the position of the leaf-spring brackets to the pinion flange based on measurements from the stock '67 Mustang housing. Our new spring pads came from a small company called Ultimate Driveline; they were the only pads we found designed for the small Ford 211/42-inch tubes employed on the 8.8. We used a digital angle-finder to determine that the pinion angle was 4.5 degrees up with the leaf-spring brackets at 0 degrees or horizontal. Next, we measured the distance between the centerlines of the old leaf-spring brackets to get a figure of 4231/44 inches. Then we measured the overall width of our new housing from the outside edges of the housing flanges, which came out to 5411/48 inches. This produced a difference of 1131/48 (5411/48 - 4231/44 = 1131/48), which was then divided in half to come up with a spec of 5111/416 inches from the outside edge of the housing flange to the spring-pad centerlines. Once this was marked on the housing and the pads were positioned to create the proper pinion angle, Moore tacked each leaf-spring bracket in place on the housing and double-checked the positions of both brackets before welding.
Next, Moore determined that the spring pads needed to be 5111/416 inch inboard from the outer edge of the housing flanges to center the 8.8 housing in the car.
One point worth mentioning with rearend swaps like this is a dimension called pinion offset. This spec references the lateral position of the pinion gear centerline relative to the exact true centerline of the housing. All rearend housings have a pinion-offset dimension, with each housing offering a different placement based on its design. The difference in pinion offset from the stock 711/42-inch housing and the new 8.8 Ford places the pinion slightly more toward the passenger side by 11/44 inch, but according to our sources this should not present driveline vibration issues.
FORD 8.8 REHAB SPECS Procedure Spec Pinion bearing preload 14-19 in-lb (new) 6-8 in-lb (used) Backlash 0.011-0.016 inch Ring gear torque 60 lb-ft Carrier bearing cap torque 60 lb-ft Pinion nut torque 140 lb-ft (min.) for crush sleeve
We went with a complete 3.55 Motive ring-and-pinion gear set from Strange along with an installation kit with all-new carrier and pinion bearings, crush sleeve, seals, and a new pinion nut.
Installing The Gears
Once we had the leaf-spring pads in place, all that was left to do was install the new gears. We're going to skip over the details of installing the Strange 3.55:1 gears and new bearings in our 8.8 housing, since that information has been covered in the past and the installation is exactly the same as with any other integral housing such as a GM 10- or 12-bolt, or even a Dana 60. Along with the new gears, we also included a Strange installation kit that included all the new bearings as well as pinion seal and pinion nut. The key to setting up any ring-and-pinion is proper pinion depth combined with the right backlash figure to ensure a good pattern. Tim Moore set up our 8.8 Ford rearend using tools from T&D Machine.
Once the spring pads were accurately placed and Moore had double-checked the position and the angle, he tacked the pads in place and then carefully welded the pads with 11/42-inch-long bead welds to minimize heat concentration.
It's All In The Splines
Axle strength can be directly tied to the number of splines on the shaft, when correlated with axleshaft diameter. The weakest link in the stock Ford 8.8 is the 28-spline axles used in the '80s and early '90s Mustangs. We used new 28-spline axles for this rebuild because we decided to retain the production 8.8 Ford Traction-Lok limited slip. But if you're willing to trade a little more coin for more strength, the best move is a set of 31-spline axles. Ford even realized this chink in the 8.8's armor, upgrading to 31-spline axles with the '05 and later Mustangs. Using the stock 28-spline axles also means using the factory C-clip for axle retention. The problem with C-clip axles is that should the axle break anywhere inside the housing, it's free to slide out, doing all sorts of evil things to the sheetmetal.
Randy's Ring & Pinion supplied a 1330-styleU-joint pinion flange to replace the flat flange style used on '80s 5.0 Mustangs. With the engine swap we have planned, now we can have a custom driveshaft made with stronger U-joints to increase the strength.
The best solution is to secure the axle at the end of the housing flange just as the 9-inch axles are retained. It is possible to have a shop remove the stock axlehousing ends and weld on new 9-inch-style ends that not only increase the axle bearing size, but also eliminate the C-clip at the same time. Strange and most other driveline companies offer housing ends, the appropriate-length axles, and bearings to make this work, which also allow you to retain the stock 8.8 drum-brakes backing plate (with a little modification). This is a far better solution than a C-clip eliminator kit, especially if there are 31-spline axles in your future.
Budget 8.8 Build Up
It's pretty easy to rack up over $1,000 in rearend parts if purchasing everything new. So we thought we'd try the low-ball route by pulling as much stuff as possible from the used parts pile. This outline doesn't include brakes, but if you are frugal, you can actually assemble this rearend for less than $400. Of course, we're assuming that the used axles and gears are in decent shape and will survive for a while. Just the price of axles and a good limited slip will bump the cost by $700 or more. With these parts, you have a very durable rearend that will live behind even a big-block, especially with 31-spline axles.
