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Part 4: Working on the Rear DiffMaking an 8-3/4 a perfect 10 by closing an open differential with the help of National Drivetrain From the September, 2002 issue of Mopar Muscle By Randy Bolig Photography by Randy Bolig
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Step One for any rearend work... Step One for any rearend work is to drain the gear lube. Building a heavy-duty rearend for your Mopar used to mean either buying a Sure Grip 83/4 or going for the famed Dana 60. Often, due to cost or availability, neither is realistic. Fortunately, there is a low-buck solution to maximizing rearend traction. We contacted National Drivetrain and uncovered all the hardware needed to turn our open differential into a genuine locker without breaking the bank. We pulled the brakes so we... We pulled the brakes so we could clean, sandblast, and paint all theparts and replace the shoes. A locker? That's right. National Drivetrain has the parts you need to give your red light brawler all the traction it craves. Our axle assembly featured a 741 casing (which we upgraded to a 742 during the course of this buildup) with an open differential and 2.76 gears--definitely a weak link for anything other than casual highway cruising. To fix that little problem, we received from National Drivetrain a 3.91 ring-and-pinion set. So much for the torque-multiplication dilemma. To transfer that torque equally to both axle shafts, we selected a Powertrax Lock-Right unit. The Lock-Right replaces your existing spider gears with an automatic locking gear set. This is a great upgrade for do-it-yourselfers who want to keep their street car...well...streetable, but are looking for a little straightline advantage. Here are the five nuts (arrows)... Here are the five nuts (arrows) you need to remove in order to pull theaxles. A hole in the axle flange provides access to loosen the nuts witha socket. With the nuts removed, simply pull the axles out of thehousing. It works something like this: During typical driving conditions, a Lock-Right-enhanced differential allows your axles to turn at different speeds (or differentiate), much like an open or limited-slip differential gear set. It's a bit clicky-sounding during sharp, low-speed cornering (when the meshed teeth overrun each other), but this is perfectly natural. Since the unit is spring-loaded, under straightline operation (both axles turning in unison) the axles are continually locked together. That's the key to full power transfer to the rear wheels and a solid hole shot. Installing a Lock-Right is a fairly straightforward job, and the best part is that it doesn't require you to replace the differential or go through the tedious gear setup process. All you need to do is replace the existing spider gears with the Lock-Right. If, however, you need to change gears like we did, a complete gear setup sequence will be necessary. With the rearend pointing... With the rearend pointing straight up, loosen the nuts that secure thecenter section to the housing and remove them. We secured our housing tothe lift so it wouldn't move while lifting the center section out. The Lock-Right installation instructions, included with the kit, are easy to follow, but you may want to have a service manual for the rearend you are working on. With the exception of a dial indicator and magnetic base (if you're changing gears), the required tools to install the Lock-Right are probably sitting in your garage. This particular rearend housing came with the engine we bought for our Valiant project. It is an A-Body rear, complete with big bolt pattern axles, although the open differential and highway gears definitely needed help.  Take a small punch and index...  Take a small punch and index one bearing cap to the housing so you canreturn the caps to their original position during installation. NOTE: Ifyou are changing the gears, remove the pinion nut with an impact wrenchbefore you remove the differential from the casing. The nut is torquedto 240 lb-ft, so without the differential in place you cannot hold thepinion gear in order to loosen the pinion nut.  Now you can lift out the differential....  Now you can lift out the differential. Unlike the 81/4 rear, with the83/4 you must remove the ring gear in order to remove the thrust blockshaft. If you are changing gears, knock the pinion gear out with arubber mallet.  When you remove the ring gear,...  When you remove the ring gear, remember that the bolts are left-handthread.  In order to remove the spider...  In order to remove the spider gears you must first remove the roll pin that holds the thrust block shaft in place. Do this by tappingit out with a small drift pin.  With the roll pin removed,...  With the roll pin removed, you can now tap the thrust block shaft out ofthe spider gears.  With the center section empty,...  With the center section empty, score the bearing race with a cut-offwheel. Next, break it off with a chisel blow to relieve the tension, andtap the race off. NOTE: Be careful not to cut into the center sectionwith the cut-off wheel when scoring the race.  With the races and the spiders...  With the races and the spiders removed, this is all that is left. Cleanit up and start rebuilding.  According to the Powertrax...  According to the Powertrax instructions, place the two tapered outersections of the Lock-Right unit into the carrier, then slide in themiddle sections by lining up the machined grooves with the two outersections.  When all the Lock-Right sections...  When all the Lock-Right sections are in the differential carrier,install the four sets of double springs through the holes in the middlesections (arrows). Also, the gap between the two middle sections shouldmeasure approximately 5/32 inches. You now have a "closed" open rear.  If you are replacing the ring...  If you are replacing the ring and pinion, file any burrs on the face(flat side) of the ring gear that may keep it from resting flat againstthe differential. Also, lightly chamfer the inner edge of the ring gearwith the file (just enough to remove the sharp edge). Using the suppliedbolts, we bolted the ring gear to the carrier (remember, the bolts areleft-hand thread.) Apply LockTite to the threads and torque to 55 lb-ft.  To press the bearing onto...  To press the bearing onto the carrier, we used a piece of thin-wall pipe(ours was an old round-style jackstand with a removable center thathappened to be the same size as the inner bearing race). When installingthese or any bearings, make sure you apply pressure to the bearing race,not the basket or cage.  With our casing cleaned and...  With our casing cleaned and painted, we installed the pinion races. Ifyou do not have a bearing and a race installer, use a brass punch toseat the race into the pinion bore without damaging it.  We used the same piece of...  We used the same piece of jackstand to install the larger bearing ontothe pinion gear. If your old pinion had any spacer washers, replace thembefore installing the bearing. Hopefully, this will get you close to theright depth setting on the gear. Ours had no spacers, so we hoped forthe best.  Since we don't often do this,...  Since we don't often do this, and we didn't have a pinion depth gauge,we'll show you how to set up the gears without one. Put the pinion gearin the housing, then install the front (small) bearing (be sure to oilthe bearings before you install them). When the bearing is far enough onthe pinion shaft to start the yoke nut, tighten the nut to 240 lb-ft. Ifyou have the proper bearing preload, you should have some resistancewhile turning the yoke with a torque wrench. A new bearing should be setat 20-30 lb-in, while reused bearings are set to 0-15 lb-in. If it istoo tight, add shims. If too loose, remove some.  Install the carrier into the...  Install the carrier into the housing to adjust the backlash. With thecenter section in the housing, install both bearing caps in theiroriginal location (you did mark the caps, right?). Snug the cap boltsfinger tight only. Put the bearing adjusters (arrow) on each side andhand tighten to remove bearing endplay. You may need to tap the bearingcaps with a small hammer to align the threads for the bearing adjusters.Now torque one bolt on each cap to 85-90 lb-ft. Mount the dial indicatorand magnetic base to the gasket face of the housing and let theindicator contact the ring gear directly on any tooth face. Tighten bothbearing adjusters equally until .0005-. 0015-inch backlash* is achieved.Next, install the bearing adjuster lock on the bearing cap on the sideopposite the teeth of the ring gear. Now, turn the gear side adjusterone hole at a time until backlash* measures between .006 and .008inches. Tighten the other two bearing cap bolts to the same torquespecs.*Backlash is the distance measured when you move the ring gear back andforth until it touches a tooth on the pinion gear without moving thepinion gear.  After applying marking compound...  After applying marking compound to the ring gear, you can see that thepattern is not where it should be. This pattern means we needed to addpinion shims. This is solved by tearing it all apart and adding theshims between the big bearing and the pinion gear. When adding spacersunder the big bearing, include the same thickness spacer washer(s) underthe small bearing to maintain proper bearing preload. When you get thispattern correct, you are ready to install the casing into the rearhousing. Next, replace the axles and the brake assemblies and installthe proper lubricant to complete the job.  Since our rearend had been...  Since our rearend had been sitting idle for a good while, the brakeassemblies rusted, so we contacted Performance Suspension Technologiesand ordered new brake drums, shoes, and wheel cylinders. PST also hasall the springs and retainers needed. Remember, when you install brakeshoes, the longer shoe goes on the rear of the assembly. Also, make surethe brake adjuster is in position so you can turn the star wheel downfrom the outside of the backing plate if you need to.  We didn't want to reinstall...  We didn't want to reinstall rusty brake lines, so we contacted The RightStuff Detailing in Columbus, Ohio. The lines fit great with minimal"creative maneuvering" required.
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