One of the most appealing things about a Mopar 8 3/4 rear, besides itsruggedness, is the drop-out center-section. With this design, ratiochanges can be made with relative ease and speed. And, setting up afresh gearset can be done on the bench rather than under the car. It'snot uncommon for Mopar fans to have a couple of extra sets of gears,packing away the hardware for the task at hand. Centersections for 83/4rears are available ready to go from the many rebuilders andwholesalers, but with a little know-how, you can build your own from ajunkyard core and the required parts. There is a certain amount ofmystery in setting up a set of gears, but with a little time and care, acapable enthusiast can roll his or her own.
The first hurdle in tackling such a job at home is the seemingly endlesslist of special tools for the job. These items add up to an investmentfar greater than the average Joe can justify when putting a gearset ortwo together every other year. While lacking a boatload of professionaltools won't stop a successful rear-gear build, it may slow you down.Other than a dial indicator to set the gear lash and an inch-poundtorque wrench to check the bearing preload, it can be done with just thebasics. In fact, we've known guys with the magic touch who can set thelash and bearing preload by feeling it. Here's the basic rundown of whatneeds to be done.
Getting Loaded
The pinion gear rides on bearings placed at each end of its shaft and istightened into the case by the pinion nut. Between the two bearings is aspacer that sets the minimum distance allowed between the bearings,acting as a stop to keep the pinion nut from crushing the bearings as itis tightened. Early 742 case rears used a solid spacer and shimarrangement, which is simple to understand. Tightening the nut increasesthe pressure applied between the bearings and the races until thebearings come to a positive stop against the spacer. By allowing thespacer length to adjust via added shims, the effective length of thespacer can be set to stop the bearings where they are preloadedcorrectly against the inner bearing races. This is the bearing preload.By placing an inch-pound torque wrench on the pinion nut--after it istightened--and turning the nut, the torque wrench measures theresistance the bearing is subjected to. If the preload (inch-poundnumber) is too high, adding the appropriate shim will back them awayfrom the races, lessening preload. If the pinion is too loose, removingsome shim will allow them to bear more tightly into the races,increasing preload. To determine the required shim thickness, the pinionmay need to be removed and installed a few times until it's correct, butthe procedure is pretty simple.
The 489 case centersections that came later did away with the spacer andused a crush sleeve instead. Think of the crush sleeve as a variablespacer. As the pinion nut is torqued to spec, the crush sleevecollapses. Tightening the nut allows the crush sleeve to compress enoughto let the bearings press against their races with the required preload,measured in the same way as the above spacer. The trick is to stoptightening when the required preload is reached. Problems arise if thecrush sleeve is compressed too much, allowing too much preload. Backingthe nut off will relieve pressure, but the crush sleeve is compressedand will not expand. A crush sleeve is designed for one-time use, andeven removing and installing a new yoke can throw off the tension.What's worse is crush sleeves have been known to lose tension underextreme abuse, causing the pinion nut to lose its torque, which leads tothe pinion flopping around like a fish out of water, trashing a set ofgears. Fortunately, aftermarket shim-adjusted spacers are readilyavailable for the 489 case--an upgrade we strongly recommend.
Pinion Depth
This is the most difficult portion of a do-it-yourself gear change.Gears are designed to mesh at a precise distance between the ring gearand pinion gear, and small production variations in gear-casedimensions, or the gears themselves, can throw the alignment off. Thering gear rides on the centerline of its bearings, so the position ofthe pinion in the case is designed to be adjustable. Behind the largebearing at the gear end of the pinion, a shim (or shims) sets the exactposition of the pinion gear in the case. More shim brings the piniongear closer to the ring gear, while less allows it to sit deeper in thecase. Only when it is at the right height will the gear mesh show thecorrect pattern (assuming the backlash is at spec--more on that later).To determine if the pinion height is correct, specialized pinion-settingtools are used. Unfortunately, not many of us can justify this equipmentto do an occasional rear project every few years. If the same gearset isgoing back into the same case with the same bearings, no problem, butthe more things are changed, the more likely the shim thickness willneed adjustment.
Although it's about as useful as a one-legged man in a kicking contest,we had this 4.10-geared, 489-case centersection with an opendifferential just lying around. The gearset was pulled from one of theMopars that passed through our hands over the years, and though the lowratio offered the potential for strip performance, the lack of alimited-slip differential negated the dig offered by the ratio. |
Dismantling an 8 3/4 centersection is fairly easy to do. Begin byremoving the adjuster-lock tabs at each bearing cap, loosening the maincap bolts and then backing off the adjusters at each side of thebearings with a suitable spanner wrench. We use a simple homemadespanner tool fashioned from a length of bar stock with a pair ofhardened 1/4-inch bolts welded to the end to engage the drive slots inthe adjuster. |
With the adjusters extracted, remove the main cap bolts and caps; thering gear and differential assembly can then be lifted from the case. |
The ring gear will need to come off the old open differential if thesame gearset is being re-used. The attachment bolts are left-handthread. |
The ring gear is a light-to-tight press fit over the register edge ofthe differential carrier, so it usually takes a little persuasion to getit off. A few taps around with a copper mallet had ours free. |
For the 8 3/4, limited-slip differentials are available from a varietyof sources and in a variety of styles, including original andreplacement Sure-Grips, lockers, and even spools. We happened to have adecent looking cone-clutch Sure-Grip on our shelf, begging to be matedto our gears. |
Cone-style Sure-Grips are worn out when the cone clutches bottom-out inthe case. To inspect this unit, the differential was dismantled and thenreassembled with some clay at the clutch-cone ends to gauge theremaining clearance. We found .060 inch, which indicated plenty ofservice life left. If the clearance is near or at zero, the clutcheswon't grab, and the limited-slip function will not work. Machining thecone ends and adding spacer shims behind the side gears can breathe newlife into a worn cone-clutch Sure-Grip. |
With the inspection passed, our used Sure-Grip was simply cleaned andreassembled with gear oil as assembly lube. |
We ordered a bearing and seal kit for our rear from Randy's Ring andPinion. The kit contains the required replacement bearings, races,seals, gasket, and crush sleeve, as well as a selection of shims andgear-marking compound needed to set up the gears. |
The bearings simply press onto the differential. We used a pressing toolthat bears on the inner race of the bearing. Pressing on the outerbasket will ruin the bearing. |