A recurring Junkyard Builder theme is making something out of nothing. Transforming trash into treasure is the kind of magic we strive for. When it comes to Chrysler 8.75 inch differential centersections, swap-meet shoppers can mix and match cast-off parts and end up with a bulletproof third member for half the cost of a professionally built unit. In this story, we show you how to identify and combine parts in the quest for a “greatest hits” version of the venerable Chrysler 8.75 inch differential unit. Groovy Factoids *The documented existence of a 741 case Chrysler 8.75 inch rear axle under an all-original, unmolested ’70 Plymouth Duster 340 four-speed survivor disproves the general assumption that the 741 (and 742) cases were discontinued after the ’68 model year. Though the 489 case is most often seen after 1968, apparently Chrysler had stores of earlier centersections and “built them out” randomly for many years. We’ve also seen the “light-duty” 741 case under unmolested automatic-transmission 383 Road Runners. *Popular rear axle types from the muscle car era can be divided into two basic types: the Salisbury configuration uses a cast-iron centersection with pressed/welded axletubes and a removable differential inspection cover. The Dana 60, GM 10, 12-bolt, and later Ford 8.8 are classic examples. The Hotchkiss-type rear axle features a fabricated stamped-steel housing with integral axletubes and a dropout-style differential centersection, typically made of cast iron. The Ford 8- and 9-inch, pre-’65 Buick, Olds, Pontiac, and Chrysler 8.75 inch units are examples of the Hotchkiss-type rear axle. ➔Source Ratech; 513/742-2111; RatechMFG.com Chrysler used three cases between 1962 and 1974, with casting numbers ending in 741, 742, and 489. The finger points to the 489 unit found in most ’68–’74 applications. Its 1.875-inch-diameter tapered pinion shaft and 3.460-inch-diameter inner-pinion bearing are the largest and strongest of the group. Drawbacks include a non-rebuildable, cone-style Auburn Sure Grip unit and crush-style pinion support collar. The 741 case (bottom) is the wimp of the family and was used from 1964–1972. Its puny, 13⁄8-inch pinion shaft diameter and correspondingly small 3.050-inch-diameter inner support bearing become marginal behind a 383 four-speed Road Runner on slicks. Some 741 diffs even have a smaller, 81⁄4-inch-diameter ring gear in place of the typical 83⁄4-inch unit. Between these extremes is the 742 case (not shown). Found among ’62–’68 performance models and HD truck and wagon applications, the 742’s 13⁄4-inch-diameter pinion shaft lacks the heft of the 1.875-inch unit installed in 489 cases, but that’s about the only Chrysler used three cases between 1962 and 1974, with casting numbers ending in 741, 742, With the meet and greet out of the way, our story begins with how we bought this pair of unpopular diffs at the swap meet and our plan to merge their best parts into something we can use. The case on the left is a 489 packing 3.91:1 gears. As a Sure Grip unit, it’d be worth $500 to $700 all day long. But since it packs a street-unfriendly spool, demand plummets, and we scored it for $200. On the right is a weakling, small-pinion 741 case we got for $175. The object of desire is its Dana-Spicer Sure Grip unit. Most swap-meet buyers shun the 741 case; even in Sure Grip form, they fail to realize it shares the same Sure Grip unit as the rock-star 742 diff. By combining parts from these two orphans, we’ll come out ahead. With the meet and greet out of the way, our story begins with how we bought this pair of u Complete disassembly of both units reveals clean, undamaged subassemblies and big smiles knowing we paid only $375 for the lot. The 741 guts are in the foreground, and the 489 is spread out behind. In a nutshell, we’re going to scavenge the Sure Grip and driveshaft yokes from the 741 and swap them into the otherwise unchanged 489 unit. The leftover parts will be liquidated at the next swap meet to further soften the financial hit by another $150. The used spool can be installed in any 741, 742, or 489 case and is an easy $75. The uncommon 741-specific 3.91:1 gear set might get $50 (most 741 applications were grocery-getters with 2.94:1 or 3.23:1 gear ratios), and the empty 741 differential case ought to fetch $30. More on the yokes in a moment. Complete disassembly of both units reveals clean, undamaged subassemblies and big smiles k A sight to make rearend spotters drool, this is the Dana-Spicer clutch-type Sure Grip differential we yanked out of the ($175) 741 case. Chrysler stuffed these rugged units under Chrysler 300s, Max Wedge super stockers, altered-wheelbase A/FX cars, 426 Street Hemis (automatic transmission only; stick cars got the Dana 60), and half-ton trucks and vans through 1968. This unit can handle 500 hp without strain, and while clutch and plate rebuild kits are readily available, we know the car this one came out of and know that it’s healthy. In mid-1968, Chrysler switched to cone-style limited-slip diff units and used them through the end of 83⁄4-inch production in 1974. Technically capable of even greater load capacity, the cone-style unit is equally desirable, though rebuilding requires machine work. Luckily, both Sure Grip units share common side bearings and ring-gear bolt patterns, so they can invade any 741, 742, or 489 case to take the place of an existing spool or conventional (open) differential. A final che A sight to make rearend spotters drool, this is the Dana-Spicer clutch-type Sure Grip diff The design and size of the pinion-gear shaft is the major difference between 741, 742, and 489 differential centersections. Below, the pen points out the beefy, tapered 1.875-inch pinion shaft featured on all 489 gearsets (regardless of ratio). By contrast, the 13⁄8-inch pinion shaft used in all 741 cases (top) clearly lacks the heft of its big brother. The 13⁄4-inch pinion-shaft diameter found in 742 cases (not shown) is nearly as strong as the 489 unit but lacks the tapered configuration. The taper eliminates 90-degree surface intersections, which are common high-stress failure points. Also, note how the larger pinion shaft takes a larger inner-pinion bearing (seen just below the gear teeth). Keep in mind that when you see a car launching with the front tires off the ground, its entire mass is being supported momentarily by these components. The design and size of the pinion-gear shaft is the major difference between 741, 742, and The pens point out the differing bearing-seat diameters needed to match the varied inner-pinion bearing sizes. The seat pressed into the 741 case (left) is machined to accept its 3.050 bearing, while the 489 case’s seat (right) is sized to take a 3.460 bearing. The 742 case (not shown) uses yet another bearing size, which is why pinion gears for 741, 742, and 489 Chrysler diffs do not interchange. Forget about trying to machine the smaller cases to accept larger bearings; it’s unexplored territory, and these things are far too common to warrant the hassle. The pens point out the differing bearing-seat diameters needed to match the varied inner-p Chrysler used two driveshaft yokes: the small Spicer 7260 U-joint (left) was reserved for grocery-getters, while the larger Spicer 7290 U-joint (right) was used with factory 440 and Hemi muscle cars. Ironically, our weak-kneed 741-case Sure Grip centersection came with the big 7290 yoke, while the boy-racer 489-case spool came with the small 7260 yoke, so we swapped them. Generally speaking, Chrysler 741 and 742 pinion gears featured 10 splines (aka coarse spline), while 489 cases use a 29-spline yoke (aka fine spline). This is another way of saying most 489 cases came from Chrysler with a fine-spline yoke. Fear-mongers claim the stock strap-style U-joint retainers are too weak for race duty and should be replaced with U-bolts. Don’t tell that to Mr. Norm, whose blown, fuel, Hemi-powered Grand-Spaulding Dodge ’65 Coronet exhibition-match racer ran high 8s with the straps. Chrysler used two driveshaft yokes: the small Spicer 7260 U-joint (left) was reserved for The main complaint heard from 489 critics is a partly deserved reputation for durability issues stemming from the 489’s collapsible pinion-support collar (right). For mass-production efficiency, the collar is designed to crush a prescribed amount when the pinion nut is torqued to 300 lb-ft. The controlled deformation delivers the specified pinion preload. The problem is that shock load generated by repeated dragstrip launches with sticky tires can further compress the collar, especially with a manual transmission and a heavy car. This changes the preload and leads to breakage. Substituting the solid non-collapsible collar from Ratech (RatechMFG.com) cures the problem. Assembly reverts to the process of juggling select-fit shims (as employed on the 741 and 742 diffs), but the end result is worth it. Our ring gear is etched with setup markings that read “CD 2.769,” indicating that the compression distance is 2.769 inches. The Ratech collar is PN 4103 and sells for a mere $33. The main complaint heard from 489 critics is a partly deserved reputation for durability i The carrier-side bearings were in good shape and OK for reuse. Chrysler used the same side-bearing dimension on all 741, 742 and 489 cases (Sure Grip and open diffs alike), so swapping diffs between cases is a simple matter. The carrier-side bearings were in good shape and OK for reuse. Chrysler used the same side Many thanks to Tim Young (left) and Mike Robinson for sharing this money-saving trick and turning the wrenches. Mike’s 10-second, straight-six turbo Maverick was featured in the Aug. ’11 issue of Car Craft. Big Tim’s dragstrip pet is a small-block-motivated Fairmont pillar coupe. Many thanks to Tim Young (left) and Mike Robinson for sharing this money-saving trick and Despite the nearly 100 percent used-parts quotient, reassembly went smoothly, as evidenced by the ideal gear-tooth contact pattern and on-spec ring-gear backlash reading of 0.008. The only shock we discovered while disassembling the spool-equipped 489 unit was that the 12-ring gear bolts (which have lefthand threads) were bottomed out and not fully clamping the ring gear to the spool. It seems the aftermarket spool’s ring-gear flange is thinner than that of the stock Chrysler Sure Grip (0.389 vs. 0.401), creating a situation in which the stock Chrysler bolts become too long and bottom-out in the gear. Besides munching the threads, the bottomed-out bolts delivered a bogus torque reading and were not fully clamping the ring gear to the spool. It’s a recipe for disaster. The lesson: Check all blind holes for proper fastener engagement when parts are being mixed and matched. Despite the nearly 100 percent used-parts quotient, reassembly went smoothly, as evidenced Mounted under an altered-wheelbase ’63 Dart Match Basher, the completed “greatest-hits” centersection combines the 489 case’s beefy tapered pinion with the 742 case’s non-crush pinion-support collar architecture (again, thanks to the Ratech sleeve). The used 3.91 gears and Sure Grip differential complement the car’s 512-inch Max Wedge and leave even strips of rubber on the street, all for less than $400. End Mounted under an altered-wheelbase ’63 Dart Match Basher, the completed “greatest-hits” ce By Steve Magnante Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!