Mopar 8 3/4 Rear-Gear Buildup - Gearing Up
Setting Up The Gears In An 8 3/4 Rear
From the August, 2003 issue of Mopar Muscle
By Steve Dulcich
Photography by Steve Dulcich
Although it's about as useful...
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 open differential just lying around. The gearset was pulled from one of the Mopars that passed through our hands over the years, and though the low ratio offered the potential for strip performance, the lack of a limited-slip differential negated the dig offered by the ratio.
One of the most appealing things about a Mopar 8 3⁄4 rear, besides its ruggedness, is the drop-out center-section. With this design, ratio changes can be made with relative ease and speed. And, setting up a fresh gearset can be done on the bench rather than under the car. It's not uncommon for Mopar fans to have a couple of extra sets of gears, packing away the hardware for the task at hand. Centersections for 8 3⁄4 rears are available ready to go from the many rebuilders and wholesalers, but with a little know-how, you can build your own from a junkyard core and the required parts. There is a certain amount of mystery in setting up a set of gears, but with a little time and care, a capable enthusiast can roll his or her own.
The first hurdle in tackling such a job at home is the seemingly endless list of special tools for the job. These items add up to an investment far greater than the average Joe can justify when putting a gearset or two together every other year. While lacking a boatload of professional tools 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-pound torque wrench to check the bearing preload, it can be done with just the basics. In fact, we've known guys with the magic touch who can set the lash and bearing preload by feeling it. Here's the basic rundown of what needs to be done.
Dismantling an 8 3⁄4 centersection is fairly easy to do. Begin by removing the adjuster-lock tabs at each bearing cap, loosening the main cap bolts and then backing off the adjusters at each side of the bearings with a suitable spanner wrench. We use a simple homemade spanner tool fashioned from a length of bar stock with a pair of hardened 1⁄4-inch bolts welded to the end to engage the drive slots in the adjuster.
The pinion gear rides on bearings placed at each end of its shaft and is tightened into the case by the pinion nut. Between the two bearings is a spacer that sets the minimum distance allowed between the bearings, acting as a stop to keep the pinion nut from crushing the bearings as it is tightened. Early 742 case rears used a solid spacer and shim arrangement, which is simple to understand. Tightening the nut increases the pressure applied between the bearings and the races until the bearings come to a positive stop against the spacer. By allowing the spacer length to adjust via added shims, the effective length of the spacer can be set to stop the bearings where they are preloaded correctly against the inner bearing races. This is the bearing preload. By placing an inch-pound torque wrench on the pinion nut-after it is tightened-and turning the nut, the torque wrench measures the resistance the bearing is subjected to. If the preload (inch-pound number) is too high, adding the appropriate shim will back them away from the races, lessening preload. If the pinion is too loose, removing some shim will allow them to bear more tightly into the races, increasing preload. To determine the required shim thickness, the pinion may need to be removed and installed a few times until it's correct, but the procedure is pretty simple.
The 489 case centersections that came later did away with the spacer and used a crush sleeve instead. Think of the crush sleeve as a variable spacer. As the pinion nut is torqued to spec, the crush sleeve collapses. Tightening the nut allows the crush sleeve to compress enough to let the bearings press against their races with the required preload, measured in the same way as the above spacer. The trick is to stop tightening when the required preload is reached. Problems arise if the crush sleeve is compressed too much, allowing too much preload. Backing the nut off will relieve pressure, but the crush sleeve is compressed and will not expand. A crush sleeve is designed for one-time use, and even removing and installing a new yoke can throw off the tension. What's worse is crush sleeves have been known to lose tension under extreme abuse, causing the pinion nut to lose its torque, which leads to the pinion flopping around like a fish out of water, trashing a set of gears. Fortunately, aftermarket shim-adjusted spacers are readily available for the 489 case-an upgrade we strongly recommend.
With the adjusters extracted,...
With the adjusters extracted, remove the main cap bolts and caps; the ring gear and differential assembly can then be lifted from the case.
The ring gear will need to...
The ring gear will need to come off the old open differential if the same gearset is being re-used. The attachment bolts are left-hand thread.
The ring gear is a light-to-tight...
The ring gear is a light-to-tight press fit over the register edge of the differential carrier, so it usually takes a little persuasion to get it off. A few taps around with a copper mallet had ours free.
For the 8 3⁄4, limited-slip...
For the 8 3⁄4, limited-slip differentials are available from a variety of sources and in a variety of styles, including original and replacement Sure-Grips, lockers, and even spools. We happened to have a decent looking cone-clutch Sure-Grip on our shelf, begging to be mated to our gears.
Cone-style Sure-Grips are...
Cone-style Sure-Grips are worn out when the cone clutches bottom-out in the case. To inspect this unit, the differential was dismantled and then reassembled with some clay at the clutch-cone ends to gauge the remaining clearance. We found .060 inch, which indicated plenty of service life left. If the clearance is near or at zero, the clutches won't grab, and the limited-slip function will not work. Machining the cone ends and adding spacer shims behind the side gears can breathe new life into a worn cone-clutch Sure-Grip.
With the inspection passed,...
With the inspection passed, our used Sure-Grip was simply cleaned and reassembled with gear oil as assembly lube.
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 gear and pinion gear, and small production variations in gear-case dimensions, or the gears themselves, can throw the alignment off. The ring gear rides on the centerline of its bearings, so the position of the pinion in the case is designed to be adjustable. Behind the large bearing at the gear end of the pinion, a shim (or shims) sets the exact position of the pinion gear in the case. More shim brings the pinion gear closer to the ring gear, while less allows it to sit deeper in the case. Only when it is at the right height will the gear mesh show the correct pattern (assuming the backlash is at spec-more on that later). To determine if the pinion height is correct, specialized pinion-setting tools are used. Unfortunately, not many of us can justify this equipment to do an occasional rear project every few years. If the same gearset is going back into the same case with the same bearings, no problem, but the more things are changed, the more likely the shim thickness will need adjustment.
We ordered a bearing and seal...
We ordered a bearing and seal kit for our rear from Randy's Ring and Pinion. The kit contains the required replacement bearings, races, seals, gasket, and crush sleeve, as well as a selection of shims and gear-marking compound needed to set up the gears.
Like we said, only the correct pinion height will provide the correct gear pattern. We can set up the gears, look at the pattern, and see if it is right. If you get it right on the first try, great-it's your lucky day, and it's done. If not, the pattern tells us which way the pinion needs to move to get it right. The pattern shown on the ring gear will show which way the pinion needs to go, but not how much. Now it comes down to disassembling the gearset, making a shim change, putting it back together, and checking it again. It may take a few rounds of disassembly, but this is guaranteed to zero-in on the correct pinion height. 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 than if done with a setting tool. Taking this approach virtually requires that a crush sleeve isn't used during trial fitment since the rear may need to be assembled a few times before it's exactly right. The best place to start is a trial with the original amount of shim behind the pinion bearing, and then trial assembly and checking from there. If starting 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 preload are set at the same time and are related. The ring-gear carrier (differential) rides on bearings at both ends. Their races float in the housing bore, and threaded adjusters bear on the outside of the races at each side to set their position. Like the pinion bearings, the differential bearings need preload, which is set by the adjusters. Tighten them toward each other, and the bearings will be squeezed together against their races, giving preload. Turning the adjusters requires a spanner wrench-a reasonably priced tool.
Backlash is the play between the ring and pinion-the slop felt when the ring gear is rocked back and forth with the pinion held so it's not moveable. This free-play is required and must come in at correct specifications. Measuring backlash is simple: Rig a dial indicator against one of the teeth in the ring gear, and rock the ring to see how much free-play is recorded. Moving the ring closer to or farther from the pinion changes the backlash. The adjusters provide for the side-to-side travel, working the ring gear over to the correct backlash with a spanner wrench, while at the same time cranking up the preload good and tight. The photo captions go into more detail.
So that's it-four adjustments to nail down, and that 8 3⁄4 rear can be set up like a pro. Really, the only trick part is homing in on the correct pinion-depth setting. With the rest, it is pretty easy to tell when it's right while the adjustments are being made. Not such a daunting task once it's all broken down, is it?
The bearings simply press...
The bearings simply press onto the differential. We used a pressing tool that bears on the inner race of the bearing. Pressing on the outer basket will ruin the bearing.
Although we reused the same...
Although we reused the same gear set, the pinion gear was removed to change the pinion bearings and clean everything up. Step one after reassembly was to drive in the new bearing races using a socket as a driver.
Since our pinion was being...
Since our pinion was being reused in the same case, we didn't change the pinion shim, which sets the pinion depth, a critical spec for proper gear mesh. The shims slip between the pinion gear and the bearing before the bearing is pressed on. Required pinion depth is easily checked, but only if the elaborate set of special tools required is on hand. Without these tools, some trial and error is needed. It's best to just set up the gears with the same shim that came out and then check the gear-contact pattern after the rear is assembled. The contact pattern will reveal if the pinion depth is off and needs adjustment.
As stated, setting up gears...
As stated, setting up gears without the special tools for measuring pinion depth generally requires a full assembly and then a check of the gear-mesh pattern to determine if the pinion depth is correct. If the pattern shows the gear mesh is off, the gears will need to be removed to change the pinion shim. That is why the crush sleeve is not installed until final assembly. In final assembly of a 489 case like ours, a new crush sleeve is positioned on the pinion, and the pinion nut is torqued to collapse the crush sleeve until the preload on the bearings results in a turning torque within specs. Since the crush sleeve cannot be reused, if the pinion depth is off, installing a crush sleeve on initial assembly will ruin it. It is common practice to leave it out during the mock-assembly stage and simply tighten the pinion nut to reach the specified turning torque. Once the correct shim is confirmed by the gear-mesh pattern, tear the rear down to install the crush sleeve, and do the job all over again. To prevent the pinion from turning while torqueing the pinion nut, we used this homemade pinion flange holder that bolts to the yoke.
In lieu of the crush sleeve,...
In lieu of the crush sleeve, a solid spacer can be substituted into a 489 case, which is the better way to go for several reasons: The solid spacer holds its dimension better under high-load use; it can be reused multiple times; and it allows the drive yoke to be removed and replaced without the possibility of altering the gear setup. Most importantly, though, the solid spacer can be installed normally during the trial assembly, and if the mesh pattern checks out, you're done. The spacer (right) from Randy's fits in place of the stock crush sleeve (left), and uses shims to fine tune the distance between the inner and outer bearings. This provides the required preload on the bearings when the pinion is installed. A good baseline setting for the spacer can be estimated by measuring the length of the old, used crush sleeve and adding shims to the spacer to match. Install the pinion and tighten the nut to specs. The tuning torque should measure between 14 and 19 in-lb for new bearings, or between 6 and 10 in-lb if the old bearings are being reused, as measured with a beam-type inch-pound torque wrench. If it's too tight, add shim; if it's too loose, take some shim out.
With the pinion side of the...
With the pinion side of the equation in place, the differential and ring-gear installation is next. Place the differential into position with the bearing races in place, and loosely install the bearing caps. Thread in the adjusters, making sure the threads are lubed and they aren't cross-threaded, and run them in until they square up the bearing races. Make sure excessive play is taken out of the bearings, and rock the ring gear to make sure the ring isn't binding against the pinion gear.
| ||#741||#742||#489|| |
|Pinion BRG preload||(1)||13-15||15-25||14-19|
|Pinion BRG preload||(2)||6-8||7-10||6-9|
|Backlash (0.000”)|| ||.006-.010”||.006-.010”||.006-.010”|
|Ring gear bolt torque||55||55||55|| |
|Cap bolt torque|| ||90||90||90|
|Carrier to axle housing|| ||45||45||45|
|Pinion nut torque|| ||40+||240+||210*|
|New bearings ||(1)|| || || |
|Used bearings ||(2)|| || || |
*Minimum with crush sleeve
Fully torque one bearing-cap...
Fully torque one bearing-cap bolt on each cap, while snugging down the other. Turn the adjusters in until there is a small amount of preload. At the same time, keep a noticeable amount of backlash in the gears. You can feel the adjuster tighten and remove any bearing play. Starting with a large amount of backlash, turn the left adjuster (opposite side of the gear teeth) as tight as you can, pulling on a 12-inch long spanner wrench. Tighten the right-side adjuster to make sure it's tight, and then measure the backlash. Randy's Ring and Pinion recommends .006-.010 inch of final backlash, easily checked with a dial indicator. Backlash should be checked at four different positions by rotating the ring and pinion. If the backlash is too wide, back the right (ring-tooth side) adjuster off a notch, and then bring the left back in to full preload. Again, wrench the right adjuster to make sure it's tight, and recheck. Work the ring gear toward the pinion in this manner until the backlash is within specs. Once the backlash is set, torque the remaining main-cap bolt on each cap.
Use gear-marking compound...
Use gear-marking compound to check the meshing pattern of the gears (see chart on page 37). This will establish a visual reference of the gear-mating pattern. If the backlash and bearing preload are correct, the only thing that can cause a poor pattern is an improper pinion-depth setting. This means a full teardown to remove the pinion and change the shim. If you get a good pattern, throw it in the housing.
Swapping centersections in...
Swapping centersections in an 8 3⁄4-equipped Mopar is a quick and easy process. Start by removing the axleshaft retainer nuts.
The axleshafts should slide...
The axleshafts should slide out of the housing with little trouble. If they stick, loosely bolt a reversed brake drum onto the wheel studs to act as a slide hammer.
At this point, the centersection...
At this point, the centersection can be unbolted and removed. Have a drain pan in position to catch a serious load of oil once the seal to the housing is broken. If it sticks, a floor jack under the yoke will usually break it free.
We used a bead of silicone...
We used a bead of silicone to provide the seal, balanced the case on a floor jack, and shoved our "fresh" 4.10-geared Sure Grip center into place in our Duster. All that was left was to replace the axles, fill it with lube, and enjoy the punch of gear reduction laying torque to both wheels.