Changing differential, gear pattern issue


New Member
Jun 20, 2022
Hello all, Been reading some info on rear end setup and I'm starting to spin in circles. I've had this pinion and carrier out probably 30 times. I am replacing all bearings in this, as well as the axles. This is my first attempt at this so be easy on me.

Its an 8.5" 10 bolt with factory 3.73 gears. I recently removed the Auburn posi and bought an Eaton Detroit trutrac. The backlash was measured at .009" and the pinion gear had a .035" shim behind it. I didn't take a gear mesh pattern off of it before disassembly, wishing I would have now. I did a depth measurement from the face of the housing to the top of the pinion gear with a depth mic. For me to obtain the original depth I would have to remove .014" from the pinion shim. I have tried everything from .019" to .035" pinion shim with .009" backlash and nothing seems to be what you want on the coast side, as these are used gears and the drive side is hard to tell.

In my first photos I have a pattern with a .022" pinion shim and .009" backlash
picture: 10bolt22thou9thou and 10bolt22thou9setup

In the 2nd set of pictures I have .022" pinion shim with .012" backlash
picture: 10bolt22thou12thou and 10bolt22thou12setup

The .030" pinion shim was way too deep in the tooth and I scrapped that idea.

I know there are a a ton of people on here that know way more than me when it comes to this stuff, so that's why I am asking for help here

What is my next step to try and get that oval wear spot centered on the tooth?


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New Member
Jun 20, 2022
Yes, I am trying to be very consistent with the preload, its anywhere from 18-20 in/lbs. I have one of those crush sleeve eliminator kits too, so that will make it easier to obtain the number in the final assembly without going over.

The pictures today have a piece of paper in them with the specs on them so its easier to see what's what. I didn't get as much done as I wanted too today, but there is always tomorrow to try a few more depths.

The marking on the coast side seems to be pretty well centered, but still way too high on the tooth.

The heel toe dimension is controlled by the backlash and the root crown dimension is controlled by the pinion depth?


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Veteran Member
Aug 27, 2003
The Netherlands
Thicker shim (0.025) is moving the pattern in the right direction for sure. Still looks a bit on the shallow side. I think that the original shim thickness is about where it should be. I would move to a 0.028 shim first.

I recently built a rear end and used a solid shim instead of the crush sleeve as well. I start the build by adjusting the pinion preload using the solid shim. It's a process you'll have to go through anyway. Once you've got it right, you don't have to worry about it during the rest of the build/trials.


New Member
Jun 20, 2022
I had to take a day off from the rear end as it was way too hot in my garage to even think. I went at it tonight and did a few more runs. I am almost to the point where I am going to buy a new set of gears and throw these ones as far as I can. I am making these changes in pinion depth and backlash and I'm seeing very little change. Here goes what happened tonight. I am currently using an oil based paint marker (yellow) for my markings. It was that warm in there tonight that it was drying before I could spin it around. I switched to a white one and added some gear lube to it to keep it from drying out, let me what you think.

Am I wrong in thinking that I need to run the same backlash as before to maintain a quiet gear set, or can that be changed with little effect on the noise? As you'll see in the one trial I went really low to 4.5 thou just to see what the pattern would do, pretty sure spec for these is .006-.010".


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Veteran Member
Aug 27, 2003
The Netherlands
Man, you're giving us a hard time here.... I would say 0.028/0.008 gives the best results, but it's hard to tell....

Overall I can tell you to start with adjusting the BL first before checking the pattern after shim change.
I would stick with the BL you measured before disassembly as it is within the specs.
Last thing, I see you make a lot changes to the BL, don't do this. Most important reading is from crown to root and is adjusted by pinion depth. Correct BL is needed for proper oiling of both surfaces and has an slight effect on heel to toe orientation of the pattern.

Hope this helps.


New Member
Jun 20, 2022
I'm sorry if I came across harsh, I'm not giving you guys on here a hard time, I'm just losing patience with myself and the rear end. I guess I was anticipating a job that would take me a couple days and it's snowballed into a couple weeks. I am sure that with your guys help, you'll walk me through this and I'll have it together soon.

Yes, thank you, this helps greatly. I've had this thing apart likely 30 times and each time it's a pull the bearing off, warm it up, slide it on, wait til it cools, load it in, check rolling torque etc. Very time consuming. I'm super stubborn and hate paying someone to do something I'm capable of (or so I thought).

Should I try let's say, .026 and .027, or does that small of a difference not really matter? All my patterns from now will be measured with .009" backlash.


Veteran Member
Lifetime Gold Member
Jul 10, 2002
Accord,New York
Biggearhead is very knowledgeable and a great guy.
I was having issues during my build and he was very patient trying to help.
I lost confidence in my ability to get it correct and shipped it to him.
He found what I was doing incorrectly, assembled it and shipped it back.

I haven't seen him on here in awhile.
You can PM him.

Hoping all is well with him

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