I just rebuilt my 12 bolt Eaton posi differential with new clutches and pinion gear thrust washers.

I am installing new 3.08 gears with new pinion bearings and new differential side bearings.

Pinion bearings preload 20 in-lb, diff caps bolts torqued to 55 ft-lb.

Differential installed tight enough that I cannot pull it out by hand and the super shim packs need to be tapped (not pounded) in. I am able to turn the ring gear by hand by jerking on the gear.

Ring gear backlash 0.0045".

Please take a look at the ring gear patterns and let me know how I'm doing. The first two pics are from one ring gear location and the last two pics are from another location. Thanks in advance. Wayne

You've got too much pinion shim in there and not enough backlash. If you take about .004-005 out of the pinion, then the backlash will grow a bit and you might be pretty close.

Yea, the pinion is too deep and the backlash should be .007 to .009. What brand of gear is this?

what brand gears? most gears take more than 4-5 thou bl. like bryan said, pull some shim out of the pinion. i'd do 3 thou. you need to find out the proper bl setting. tom

Many thanks for the responses. I would never have attempted this rebuild and gear change without this resource.

Now I know what to do next - I'll take out .003" on the pinion, set backlash to .008" and repost photos of the patterns.

The number on the ring gear looks like a US Gear part number, G01 888308 40-13 USA (symbol - G in a shield) A6. Thanks, Wayne.

if it is us gear , i would set the bl to 6-8. tom

I hoped it was US Gear and not Richmond. The Richmond 3.08 is cut bad and almost impossible to get set up correctly. US Gear should be fine. Set as recommended above and let's see the pictures.

OK, I replaced the original .036" pinion shim with a .033".

Set pinion bearings preload to 20 in-lb, torqued diff caps to 55 ft-lb.

Backlash was .0055" so I swapped a .003" shim from the passenger side to the ring gear side and got .009" backlash. This seems like a lot of backlash change for such a small shim change. I am wondering about the differential side bearing preload being low because although I need to tap the super shim packs in, I was able to jerk the differnential off its seat by hand.

Then I took the following patterns. Like before the first two pics are from one location on the ring gear and the last two pics from another.

What do you think? Thanks, Wayne.

Looks a lot better. I'd try to get the shims a little tighter so that the bearings have more preload, but the pattern looks pretty good. Get the shims tighter without changing the backlash.

you may need about 3-5 more shim on the carrier bearings. the pattern looks good. get the BL closer to 6. it will slightly open up after breaking in. tom

Great thanks Bryan, Freddie, and Tom! I will work on getting the BL to 6. I noticed that the surfaces of the casings for the shim packs were kind of rough out of the package, not smooth like the new pinion shims. Would it be worth smoothing the surface of the casings with emery cloth? Have you ever heard of this before? I am thinking the rough surfaces may be why I need to tap in the shim packs even though the diff side bearing preload is light. Thanks, Wayne.

After shuffling shims back and forth, I ended up adding .005" to get the diff in nice and tight and finished with a .0075" backlash. I am hoping to install the crush sleeve, new axles, and get the rear back under the car to start breaking it in tomorrow.

BTW, the gears I took out were 5.57's (39-7) so I am looking forward to running at highway speed for the first time since I started restoring the car in 1991. This rebuild is the last major item for the car. Thanks again to all, Wayne.

:thumbsup:

Add another success story for Team Chevelle. I rebuilt my 12 bolt (new clutches, all new bearings including axles, new spider gear thrust washers, and changed gears from 5.57 to 3.08 with parts from BGH and advice from those that responded to this post.

Took the car out on the road for the first time today to test the newly rebuilt rear end. For the first run, I just drove around a few blocks a few times for about 5 minutes mostly to test the brakes. The rear was absolutely quiet and tight - no leaks. Let the rear cool 30 minutes.

For the second run, I drove around town on 45 mph streets for about 10 minutes with a stop every half mile or so. The rear was absolutely quiet and tight. Let the rear cool for 30 minutes - no leaks.

For the third run, I took the car out onto the highway for 10 minutes, gently varying between 50 and 60 mph. The rear was absolutely quiet and tight - no leaks and no popping at all going around corners on the way back to the house. I'll take it easy on the rear for the next 100 miles or so, but I think it is broken in just right.

After putzing around with those 5.57's for this past year after breaking in the cam and finishing the interior, the car is really a joy to drive now.

Many thanks to all who helped. You guys make this site great. Wayne