Noob pinion depth pattern: feedback please!

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Aug 6, 2021
Washington State
I'm rebuilding a diff for my '40... first timer. I have a new pinion and ring (Nitro) and ARB airlocker. I made up a set of setup shims and honed the outer bearing so it would slip on and off the pinion shaft easily. This makes the process go much more quickly than having to press or pound stuff in/out on/off between fittings.

Since I am replacing nearly everything inside the diff, including going to 3.73 from 4.11 gears, I didn't see much point in trying to use the previous shim setup as a starting point. So, to bracket it, we started with 0 shims and then went straight to 0.040". As we hoped, 0 shims was too shallow and 0.040" shims was too deep, so bracket achieved! We made three more iterations in our "binary search" for the proper fitting: 0.020" (too shallow, but close), 0.0215" (closer), and finally 0.023" which is where we stopped (patterns below).

The coast side looks primo to me (using ZUK's patterns as my main reference), but the drive side looks like it might be a hair deep. Also, I'd love some feedback on our process:

- Applied a thin coat of gear oil on (setup) bearings and assembled pinion into housing without any kind of sleeve.
- Applied just enough torque on the pinion nut to make the pinion stiff enough that it doesn't move when checking backlash, but easy to turn by hand. I figured that, as long as the inner race and shim(s) are firmly seated, the pinion nut doesn't need additional torque in order to get a valid, reproduceable pinion depth pattern... agree?
- Assembled the carrier and adjusted to the desired backlash. In my case, I'm shooting for the lower end of the spec range (0.006" - 0.007"). My reasoning here is that, since I will be taking my rig to the trail head on a flat bed, most of the miles will be slow, so the diff won't heat up as much as it would at higher speeds, say on the tarmac, so less backlash is better... agree?
- Measured the backlash, then tapped the carrier gently with a brass hammer and rotated the ring gear, and measured again on a different tooth. Up to this point, the cap bolts were only snugged enough to flatten the lock washers and the carrier adjustment rings have been adjusted with a spanner and punch and hammer (gently, by feel), so no measure of the carrier preload has been done. Again, my reasoning here is that carrier preload, as long as it is "snug", shouldn't affect pinion depth that much... agree?.
- At this point, we torqued the cap bolts to 50 lb-ft, rechecked the backlash (it didn't change at this stage on any of our iterations).
- Made sure the paint was spread thin and even (I ended up thinning it with some dub dee farty to make it spread better) and had my buddy put pressure on the ring gear while I turned the flange to get a pattern. One other note: I wiped all the paint off the pinion gear between iterations, which seemed to help with getting a clear pattern.

Below are the patterns I got, please let me know if you would run these, or suggest I try to optimize further:

Drive side:

Coast Side:
Correct me if I am wrong on your #s
.023 pinion shim
.006 BL?
Can you thin the marking compound with gear oil and apply as much load as you can so there is a more readable pattern please.
Also, there is nothing wrong with being on the tight end of BL if you can get away with it. Common misconception is deeper pinion depth for added strength but a good pattern with tighter BL numbers has been my go to for quiet strong setup.
You will want to hone out your small pinion bearing to make a setup bearing when you set pinion bearing preload with the SPS. Then replace with your new bearing when preload is right. It makes it way faster and once you get close .001 is like a mile so get ready to be down to the nats ass.
Thanks Justin! Those are indeed my numbers and I have honed out the small bearing, so I'm ready when the SPS arrives. They are backordered at Nitro, so I'm on the hunt for in-stock. I will get a new pattern for you ASAP (my helper works days) with thinned paint and as much load as he can muster!
FYI, I measured BL on all teeth today. It ranged between 0.005" and 0.007" with an average of 0.0056". Also, the runout on my ring gear is 0.003", which I gather is a tad high.
OK, second pass on paint pattern... thinned, with more load, and used a finer brush to get a more even initial coat. Definitely improved the pattern readability... thanks for that pointer Justin. Same same on BL and depth. Diff was disassembled and reassembled between the first and second paint pattern passes.

Initial coat:

Drive side pattern:

Coast Side:

Pinion gear:
Another question that has come up: I used the Nitro specs for the ring gear nut torques, which is 70 ft-lbs. This is lighter than the stock specs (76 - 86). I assume since the gear came from Nitro, that is the spec I should use, but if anyone differs, please let me know.
That looks good to go.
70#s and red locktite on the RG bolts.
FYI... got the pinion torqued (solid spacer with a 0.012" shim, 250 ft-lbs on the pinion nut, PPL in the low teens), and buttoned it up. Adjusted to 0.006" BL with a couple of clicks on the adjusters for carrier PL. Final preload in the high teens. Honestly, how do you precisely read a beam torque wrench in motion?

Below are the final patterns with everything torqued and BL triple checked at 0.006", including lots of tapping between adjustments with a brass hammer.

Next step is to get the ARB seal housing installed and the tube routed to the bulkhead.

Drive side:


Coast side:

Good , detailed work!
FYI, took it out for it's first test drive today. Diff cover temp (IR) was 121 deg F when we got back. Very quiet. Checked that the rear locker was working. All systems go so far! Will start on rebuilding the original rear diff to put in the front axle ASAP.

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