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1993 Big Block Ford Mustang - Brace Yourself - PHR Project CarSome Hard-Core Guts And A Few Custom Braces Are All You Need To Build A Budget Ford 8.8-Inch Rearend Strong Enough For The 7S. From the October, 2009 issue of Popular Hot Rodding By Stephen Kim Photography by Stephen Kim
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Even the Chevy guys can't deny the greatness of Ford's legendary 9-inch rearend, so boldly opting to install a lesser 8.8-inch rear in a Mustang requires a compelling argument. Fear not, for such a questionable antic is more than justified. Sure, the 9-inch's virtues of strength, affordability, and ease of serviceability-thanks to its slick drop-out center section-are irrefutable. However, its 8.8-inch little brother is lighter, cheaper, takes less power to turn, and can be built just as strong. Bearing an uncanny resemblance to the GM 12-bolt, which the 9-inch is often compared, the 8.8 has proven its mettle in the world's fastest Outlaw Mustang drag cars, where countless racers have pushed them well into the 7s. Since Fox-body Mustangs came... Since Fox-body Mustangs came equipped with 8.8-inch rearends from the factory, we didn't have to track down a core, however, considering that the 8.8 has been Ford's bread-and-butter rearend in its performance cars, fullsize sedans, trucks, and SUVs for the past 30 years, cores are plentiful and sell for less than $100. By simply welding on some leaf-spring brackets, you've got yourself a dirt-cheap rearend for your muscle car that's just as strong as a 12-bolt. The weak link in many 8.8s are their 28-spline axles, but those found on '94-98 SN95 Mustangs boast 31-spline axles and larger axle tubes. With such an impressive track record, a built 8.8 will laugh in the face of Project Fox's measly 775 hp. "People have run as fast as 7.50s in the quarter-mile with built 8.8 rearends, and I've had customers pull consistent 1.23-second 60-foot times with an 8.8 in nitrous cars," says Bill Buck of Bill Buck Race Cars in Austin, Texas. Bill keeps busy building race car chassis during the week, and serves as head suspension tuner on Mike Murillo's 6-second, 3,000hp, Outlaw 10.5 '93 Mustang on weekends. That said, we felt that he was the perfect candidate to not only get Project Fox caged and ready for battle, but assemble a bullet-proof 8.8 rearend for it as well. Somewhat surprisingly, it takes very little to beef up an 8.8-inch rearend far beyond the durability requirements of even the most serious street/strip machines. For this build, Strange Engineering set us up with 33-spline axles, C-clip eliminators, 3.90:1 gears, billet bearing caps, a spool, a differential cover, a 1350 yoke, and 1/2-inch wheel studs. Such components aren't any more exotic than what's typically used in a basic rearend build, a testament to the strength of Ford's design straight from the factory. Furthermore, the only additional fortifications necessary were some custom chromoly back braces, and welding the axle tubes to the differential housing. At the end of the day, parts and labor for our super-duty 8.8 rearend rang up at just under $1,800. That's about $500 cheaper than a comparable 9-inch equipped with goodies like a nodular iron case and Daytona pinion support. Plus, building an 8.8 makes for a much more interesting story than merely ripping open a wooden crate and stabbing a turnkey rearend assembly under a car. | WHERE THE MONEY WENT | | Item: | Part No: | Price: | | Strange axles, spool, studs, C-clip eliminator | P2000FM | $629 | | Strange differential cover | R5234 | $149 | | Strange billet aluminum main caps | H1124 | $88 | | Strange 3.90:1 ring-and-pinion | RSF888390 | $160 | | Strange 1350 yoke | U1630 | $89 | | Strange installation kit | R5231 | $94 | | custom chromoly bracing | N/A | $300 | | axle tube welding | N/A | $100 | | rearend assembly labor | N/A | $150 | | Grand Total: | $1,759 |  C-clip axles are prohibited...  C-clip axles are prohibited by the NHRA in cars running 10.99 or quicker, so the rearend was prepped for a Strange C-clip eliminator kit. Installing it requires cutting off a small portion of the axle tube outboard of the backing plate flange. Bill suggests leaving 1/2 inch of tubing in front of the flange to allow enough space for the backing plate to mount and self-center upon.  After disassembly, our 8.8...  After disassembly, our 8.8 was pressure washed and degreased inside and out. Just like when cleaning an engine block, removing the grime helps reveal cracks and imperfections. Since the axle tubes were to be welded in several locations, they were stripped down to bare metal.  Simple physics dictates that...  Simple physics dictates that any bracing will be most effective when extended as far outboard as possible, which in this case is the inner portion of the control arm bracket. Custom bent from 1.25x0.95-inch chromoly tubing, the lower brace hugs the bottom of the carrier housing while the front braces attach next to the area of the housing that surrounds the pinion yoke. Note the two 7/16-inch holes that are drilled in the carrier housing in preparation for the front braces.  Under severe loads, the axle...  Under severe loads, the axle tubes in an 8.8 can sometimes rotate inside the differential housing. Welding this weak point is an obvious solution, but the procedure is tricky since the differential housing is cast iron and the axle tubes are DOM steel. Using nickel rods, Bill MIG welded the tubes to the housing by carefully controlling temperature buildup to prevent cracking the metal.  While the ends of the back...  While the ends of the back braces can easily be welded onto the steel axle tubes, Bill prefers attaching them to the carrier housing with Grade 8 bolts. After drilling a 3/8-inch hole on each side of the carrier reinforcement ribs, steel locating tabs were bolted in to help position the lower brace. Next, the lower brace was welded to both the axle tubes and locating tabs.  Although 31-spline axles may...  Although 31-spline axles may have sufficed for our application, we decided to play it safe with 33-spline units instead. These top-notch Strange Pro Race axles are made from the company's proprietary Hy-Tuf steel, which is an ultra-strong, low-carbon, high-nickel, manganese alloy. Combined with through-hardened heat treating, these axles yield superb torsional strength and ductility.  With the limited street duty...  With the limited street duty Project Fox will see, we passed on the complexity, expense, and weight penalty of a limited-slip or locker differential and went with a spool instead. This Strange Pro Race steel piece weighs just 8.45 pounds, and is backed by a lifetime warranty.  While not quite as diverse...  While not quite as diverse as the 9-inch rearend's dizzying array of rear gear options, the 8.8 still offers plenty of ratios. With projected quarter-mile trap speed in the 140-145 mph range, our 8.8 was fitted with a 3.90:1 ring-and-pinion set, which will put our 532 big-block just past its 6,500-rpm power peak while crossing the traps in high gear on 28-inch tires. The Mustang rearend's oddball flat-flange pinion yoke was replaced with a more conventional 1350-series unit.  The aluminum differential...  The aluminum differential support cover not only looks trick, it also features two integrated load bolts that add preload to the main caps. This additional support reduces gear deflection, stabilizes backlash, and minimizes the potential for cap breakage. Speaking of main caps, these billet aluminum units are lighter and far stronger than the cast factory pieces.  Keeping the tech inspectors...  Keeping the tech inspectors happy at the track mandates 3-inch-long wheel studs. These 1/2-inch studs are plenty stout for our intended use, and are included as part of Strange's axle packages. With the axles clamped by a vise, Bill applied some red Loctite to the threads before cinching them down.  The C-clip eliminators-shown...  The C-clip eliminators-shown here after they have been pressed onto the axles-are essentially safety hub assemblies with integrated bearings that prevent the wheels from ejecting outward in the event of axle failure. The axles are locked into place by steel retaining rings.  While they can be installed...  While they can be installed on the axles using a hydraulic press, Bill prefers to first heat the retaining rings up with a torch before sliding them on. Since heated metal contracts beyond its original state as it cools, using this technique makes the retaining rings clamp more tightly to the axle than by simply pressing them on.  Before dropping the spool...  Before dropping the spool into the differential housing, new bearings were installed on a press. To prevent damaging the new bearings, Bill wedged an old bearing between it and the press piston. Since both bearings are identical in diameter, the likelihood of gouging or scuffing the new bearing is greatly reduced.  Whenever using brand-new components,...  Whenever using brand-new components, tolerances can be very tight. Instead of beating the ring gear onto the spool with a mallet, the preferred method is to slowly cinch down on the ring gear bolts. Once the ring gear was seated, Bill backed off on the bolts a few threads, applied some red Loctite, and torqued them down to 80 lb-ft.  The final step before setting...  The final step before setting up the rear gears was installing the pinion bearing race in the differential housing. The procedure isn't exactly rocket science, but using a proper bearing race installation tool is critical. Trying to tap the race into position with a hammer can easily ruin its mating surface.  After finalizing the pinion...  After finalizing the pinion depth, it was time to dial in the backlash, which simply determines how much the pinion gear will turn before engaging the ring gear. Moving the spool and ring gear closer to the driver side using carrier shims increases backlash, while moving it closer to the passenger side decreases backlash. Bill says that a range of 0.008 to 0.012 inch is acceptable, but he prefers a tighter 0.008-inch setting when using new gears so that backlash will still be within the ideal window once the ring-and-pinion assembly fully breaks in.  The crush sleeve, situated...  The crush sleeve, situated between the front and rear pinion bearings, prevents inadvertently over tightening the yoke on the bearings, however, it can take up to 400 lb-ft of torque to crush it, so an industrial-grade impact wrench is a must. Bill recommends cinching down on the pinion nut until all free play in the yoke is gone, then tightening it another 20 inch-pounds. Note how the front axle braces attach to the left and right sides of the differential housing.  Pinion depth is the distance...  Pinion depth is the distance between the centerline of the axles and the flat face of the pinion gear, and is altered by installing adjustment shims between the pinion gear and pinion bearing. With a 0.030-inch shim, the pinion wasn't engaging the ring gear deep enough. Swapping in a thicker 0.032-inch shim yielded an ideal gear contact pattern.  Bill's a fan of simply painting...  Bill's a fan of simply painting a rearend, since a thick powdercoat can hide stress fractures. As the saying goes, the finished product is almost too pretty to put underneath a car.  With the differential assembly...  With the differential assembly installed and the gear setup finalized, Bill slid the axles into the housing. This angle clearly illustrates how the C-clip eliminators firmly hold the axles to the backing plate flange in the event of failure. Welding on housing ends off of a 9-inch rearend works just as well as the C-clip eliminators, but requires upgrading to disc brakes. For this reason alone, we opted for C-clip eliminators, which enable retaining the stock rear drums to keeps costs to a minimum.  The aluminum differential...  The aluminum differential cover provided by Strange certainly adds some visual pop, but its primary benefits are reducing distortion of the differential housing and stabilizing the main bearing caps. This, in turn, helps maintain proper pinion depth and gear backlash. The cover's two load bolts are positioned to come in contact with the center of each bearing cap as they're cinched down, which provides cap preload and reduces the likelihood of distortion. After tightening the 10 cover bolts to 25 lb-ft, Bill torqued the load bolts to 10 lb-ft using an Allen wrench. Next, the jamb nuts were tightened to 10 lb-ft to lock everything in place.
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