HOTROD | How-To - Transmission

How to Set Up Gears in a Mopar Rear End

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.

Steve Dulcichphotographer

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.

Like we said, only the correct pinion height will provide the correctgear pattern. We can set up the gears, look at the pattern, and see ifit is right. If you get it right on the first try, great--it's yourlucky day, and it's done. If not, the pattern tells us which way thepinion needs to move to get it right. The pattern shown on the ring gearwill show which way the pinion needs to go, but not how much. Now itcomes down to disassembling the gearset, making a shim change, puttingit back together, and checking it again. It may take a few rounds ofdisassembly, but this is guaranteed to zero-in on the correct pinionheight. It's a trial-and-error process that takes time, but in the end,a perfect pattern shows that the gear setup is correct, no less so thanif done with a setting tool. Taking this approach virtually requiresthat a crush sleeve isn't used during trial fitment since the rear mayneed to be assembled a few times before it's exactly right. The bestplace to start is a trial with the original amount of shim behind thepinion bearing, and then trial assembly and checking from there. Ifstarting from a bare case, .030 inch is an average amount of shim.

Ring Gear Backlash and Preload

Although it seems we're dealing with two topics, backlash and preloadare set at the same time and are related. The ring-gear carrier(differential) rides on bearings at both ends. Their races float in thehousing bore, and threaded adjusters bear on the outside of the races ateach side to set their position. Like the pinion bearings, thedifferential bearings need preload, which is set by the adjusters.Tighten them toward each other, and the bearings will be squeezedtogether against their races, giving preload. Turning the adjustersrequires a spanner wrench--a reasonably priced tool.

Backlash is the play between the ring and pinion--the slop felt when thering gear is rocked back and forth with the pinion held so it's notmoveable. This free-play is required and must come in at correctspecifications. Measuring backlash is simple: Rig a dial indicatoragainst one of the teeth in the ring gear, and rock the ring to see howmuch free-play is recorded. Moving the ring closer to or farther fromthe pinion changes the backlash. The adjusters provide for theside-to-side travel, working the ring gear over to the correct backlashwith a spanner wrench, while at the same time cranking up the preloadgood and tight. The photo captions go into more detail.

So that's it--four adjustments to nail down, and that 83/4 rear can beset up like a pro. Really, the only trick part is homing in on thecorrect pinion-depth setting. With the rest, it is pretty easy to tellwhen it's right while the adjustments are being made. Not such adaunting task once it's all broken down, is it?

Although we reused the same gear set, the pinion gear was removed tochange the pinion bearings and clean everything up. Step one afterreassembly was to drive in the new bearing races using a socket as adriver.