8.8 Diff Build ContinuedSince I was re-using the pinion bearings, that was one less shim to deal with up front and pinion depth would already be set (that shim lives under the bigger bearing which presses onto the pinion). That basically meant I still need to move ring gear over to my new diff, add bearings, rough in the shims to check backlash, check tooth pattern, and set preload.
Getting the ring gear off isn’t terrible. It basically takes clamping the whole thing hard in a vise and using a drift to drive the ring gear down by banging on the bottom of each the now empty bolt holes in a circle. I wrapped a bunch of newspaper and plastic wrap around the body to keep the ring gear from falling when it came loose.
My new diff is an Eaton/Detriot TrueTrac. I’ve always liked the helical gear differentials behavior wise and for road course duty it made sense to me.
I’ve been told you can cook the ring gear at 300 for an hour to warm and at the same time throw your diff in the freezer aid with reinstallation. However, I didn’t find that necessary. I did find that getting the hole patterned aligned is critical and not very forgiving.
Once you get it close you want to start getting the hardware threaded and pull it the rest of the way in with fasteners. I used old hardware to pull it down. Removed and grabbed the new bolts, add Locktite 242 (blue) and 77 lf-lbs in a star pattern follow to lock it in.
Pressing bearings on was mostly straight forward with one exception. To get the new inner bearing race to bottom out you have to press it a bit further than what would be flush to the outside surface. After rooting around for a piece of tubing or a socket that would work—striking out in both cases—I caved and decided to just pull a race from the old hammered diff.
I was seriously convinced my puller was going to break trying to get this off. I finally decided I was going to torque the puller till something lets loose. Thankfully it was the race but I did bend my handle pretty good.
You can see how ugly the bearings were in the other diff.
One you have an inner race separated that becomes the perfect tool to finish the press of the good bearing.
Now comes the tricky part. I took quite a few measurements of the old carrier vs the new and it was close but I was finding numbers with a few thousandths variation. The solid shims that came with my diff were 0.264” and 0.280” so I started by using the shim stack that came with my rebuilt kit to mock in a shims at 0.259” and 0.275” (0.005” under on each side, but potentially leaving a bit of preload since I had to use the spreader to get them in).
That actually worked out pretty well. My runout on the assembly was only about 0.0015” on 0.003” allowed.
Note: backlash, run out, and tooth patterns all have to be checked with the carrier caps torqued. Another spec at 77 ft-lbs.
Backlash was between 0.009 and 0.011 depending where I checked vs. runout. The allowable range is 0.008” to 0.015” backlash with a preferred target of 0.010”
It was close enough I went ahead and checked tooth pattern.
Paint on the goo (also included in the rebuild kit) and give it one rotation pass of the pinion. You effectively just crank over the pinion a few time while holding firm pressure on the ring gear in the opposite direction to ensure it gets some pressure at the gear contact point.
Pattern looked pretty good—a nice confirmation of my pinion depth as well—on both drive (vertical) and coast (angled) sides of the teeth.
That meant now I was ready to set preload. To keep my backlash and gear pattern I needed to be adding and subtracting the same amounts on each side vs the 0.259 and 0.275 I’d just checked. For the aluminum housing you want a total of 0.014” preload or +0.007” on each side vs. whatever shims you can just slide in by hand (With an iron housing the correct spec is 0.012” total preload or +0.006” each side). Problem is that my new bearing races were just a hair’s breadth larger than semi-circular bores I was dropping this into.
That made shifting the races sideways to get them snug horizontally in order to slide in new shims a royal PITA. I finally found that the only method that worked was to be sliding shims in with the races in place but not quite seated all the way down in the half bores of the housing where it would bind.
Unfortunately I’d used all but one of all of my shim stacks to make the 0.259” and 0.275” I was trying to adjust up or down from. That was already 0.010” less than what came in the diff so I pulled one 0.009 shim and tried it 0.019” under my originals. That was just a bit loose, and I figured I maybe need a few more thousandths.
So then I used the solid 0.264” shim with a stack up measure 0.266” on the opposite side. Backlash would be wrong but this would work out to exactly 0.014” under my original solid shims if it fit well. At the end of the day this setup it was a little snug but I could still move the full diff assembly in and out by hand relative to the shims with no spread to the case.
I called that pay dirt, used my case spreader and installed my original 0.264 and 0.280 carrier shim. Final backlash was 0.011” so just a hair loose of target (but good for road racing where things may get hot).
Rechecked tooth pattern and results weren’t quite as perfect; I found some minor shifts in and out between teeth. However, it varies based upon which teeth I check and I’m sure I’m being more anal retentive than a shop doing this day in and day out so I called it good. Two areas shown for comparison.
One last final twist. The TrueTrac diff comes with an access port intended to allow install of one of two provided spacers which in turn keep the axles with C-clips (IE those used with a solid rear axle) from sliding inward.
The problem is that when using this diff with IRS stub axles and circ clips the spacer is just free to fall out either axle bore and I won’t be able to easily remove the diff cover once in the car to install the spacer after the fact.
While some guys on the net report running their IRS TrueTrac’s without the spacer I was a bit concerned that my stubs could still try to slide inward putting unnecessary wear on my shaft seals. Like the stock diff, the circ clips pop out past splines into a void space (it’s not actually a dedicated circ clip groove).
I ended up calling Eaton to chat it up. They recommend I install the spacer with a dab of heavy wheel bearing grease to keep it in position and just be careful to keep it horizontal during install. The little bit of grease shouldn’t compromise the gear oil so I’m going to give it go and see what happens.
