Truetrac Mopar 8 3/4 Rear End Rebuild

The article featured on this page is from the June 2011 issue of Auto Enthusiast Magazine.

Bringing back the power potential in an 8¾-inch differential

Story Dave Verna

Mopar ring and pinion overhaul kits

Chrysler’s 8¾-inch rearend was in production from the late ’50s until 1974 in the passenger car line. It was used under literally millions of Mopars over the years.

In a street car, with its removable center section, it’s a tough rear to beat. Gear changes can be done in under an hour and you are back on the road. While not bulletproof with slicks and high horsepower, the rear is plenty strong for a weekend warrior that will see the track on occasion.

The rear we are working on has had a hard life under this 1970 Dodge Challenger. It’s still driven (and driven hard) by its original owner. The 383 four-speed car has seen over 114,000 miles with the original drivetrain. The engine and transmission were finally serviced at the 112,000 mile mark and the new-found power, combined with the 26x9-inch slicks, took the 489 cased, cone-type diff to the end of its line. The telltale signs start at the burnout. One wheel would not spin all the time and then the 1-2 shift would not get traction to both tires, causing the car to turn at the shift point. Not very good with concrete barriers on either side!

In 1970, the only sure grip you could get was the cone type. While it did hold up to 41 years of flat shifting and the occasional trip down the strip with slicks, it finally gave up on us. There are a few tricks to get a few more miles of use by machining off a few thousandths and shimming them to get the cone to engage again, but it’s just not the answer. The correct way to repair a cone-type sure grip differential is to replace the unit. We did some research and Eaton has one of the nicest diffs around for our application: the Detroit Truetrac. There is no maintenance on these, just traction! They don’t even require limited slip diff lube. The choice was easy after talking to Eaton, since the helical gear design eliminates the need for wearable parts and the unit has four pinions in the 8¾-inch setup compared to other housing designs. The effective design seemed like a home run in our book!


1 The entire 8¾-inch lineup: 742, 489 and 741. The 741, with its 13/8-inch pinion is the smallest of the bunch. Both pinion depth and preload are set with shims. The 742 has a 1¾-inch pinion and still maintains a straight stem like the 741 version. Both pinion depth and preload are set with shims. The 489 has a 17/8-inch pinion and tapered stem. Pinion depth is set with a shim but the preload was originally set with a crush sleeve.

2 The Detroit Truetrac differential by Eaton comes with four helical gears under the attached plate. If you still want to maintain the adjustable tapered axle bearings, they supply a spacer to put in-between the axle shafts to maintain an accurate axle end play.

3 The original crush sleeve and the new crush eliminator. The new collar eliminates the crush setup and replaces it with shims. The factory setup worked great on the assembly line; all you did was set the pinion depth and crank down on the pinion nut until the pinion had enough preload on it and away it went. This process will take longer, but will be stronger. This rear suffered from a pinion seal leak. The pinion wants to be pushed out of the case and is pushed against the crush sleeve its whole life. Over time, this can change the preload, and once that preload is gone, the seal will not do its job.

4 We split the original sure grip case and the cone on the passenger side showed no excessive signs of wear. The coiled area on the left creates friction on the cover to the right. The springs are what give away the cone- type differential. They press the cones into the case and supply traction to both sides.

5 This is the driver’s side cone. The wear on its face is the sign that the cone side has worn down so far that it basically bottoms out (shown). Once this happens, the show is over. As we mentioned before, you can cut the face and shim the back, but it will not last.

6 The pinion depth is set on all 8¾-inch rears with a shim under the bottom bearing. The stock depth was retained in this setup which was .031-inch.

7 The set it and forget it process with a crush collar is faster, but the eliminator will keep the preload accurate longer and will seal the pinion from leaks better in the long run. Neal is laying the shims out (a standard kit comes with .020-, .016-, .014-, .012- and .010-inch). The shims go on top of the crush sleeve and under the top bearing. This is what now makes up your preload on the bearings.

8 The first time we set it up there was no preload at all. Upon further review the crush collar inner diameter was interfering with it seating all the way on the pinion stem. A little cleanup and we were back in action!

9 With the collar not interfering with the preload anymore the .014-inch shim was the right number for us. The required torque to spin the pinion is 20 to 25 inch-pounds for the new bearing. You want to have this all set up and then install the seal. The last thing we had to do was replace the locknut, since they really are a one-time use item.

10 Don’t forget to stamp the caps and the housing to put them back in the proper location. You can wind up in a mess real fast by swapping them.

11 The instructions are clear: you need to make sure there are no burrs or anything on the surface on the back of the ring gear. We got ours nice and flat with a piece of glass and some 400-grit paper.

12 Neal took our old ring gear bolts and ground the heads off and added a few slots, so he could locate the ring gear on the Truetrac unit. The ring gear is heated to 150 degrees for one hour and will grow just enough to slide right on. This is the correct way to install a set of gears. With his oven mitts in hand, Chef Neal proceeds to use the old bolts as guides and slides the ring gear home; with a nice clunk things are set and he waits for it to cool down.

13 With the ring gear bolts installed, the bearing can now be pressed on. The bolts holding the ring gear on are left-handed threads, so be sure to keep that in mind.

14 The Truetrac unit can now be placed in the housing. The caps are loose so the adjusters can be threaded in without too much hassle. The adjusters will eventually set your backlash. For now, everything is loose and will be assembled first to get things close.

15 The caps will be torqued after the backlash is close and checked. It might take several times to get things just right but the time invested is worth a quiet operating rear that will give you miles of trouble-free driving!

16 In order to check the backlash you need to set up a dial indicator and check at least three spots along the ring gear. The ring and pinion are a matched set. They were cut, heat treated, lapped and tested together, so having an incorrect backlash setting will cause lubrication issues as well as a short bearing life. An engine stand can be attached to the 8¾-inch housing for this procedure. Be sure to check the backlash several times around the ring gear. There should be no more then .002-inch variation from all measurements. Remember when we heated the ring gear? That pays off as we had +/- .001-inch variation.

17 &18 Your last step is to run the pattern. The supplied marking compound is applied to the ring and the pinion runs through it, creating the pattern you see here. The teeth should create a pattern centered in the teeth from peak to valley of the ring gear. It should be an even, wide pattern that is centered in the gear mesh. This is where all your adjustments will show if you are within specifications.

Don't forget your Mopar ring and pinion overhaul kits.