Yoke Seal Shim?

[Fordum]

Folks, this one is not all that critical. Its just I have never seen anything like it, three other fairly experienced mechanics have not either, and it bugs me.
Guy brought a GM type truck into my friend's shop the other day and had rear end noise.
His differential was sounding bad too............... Sorry, something about what I had just written struck me as funny.
Turned out, he had a bad bearing. They priced a junkyard diff, and the cost to rebuild his, and decided to do a rebuild.
Bought a bearing and seal kit, and started in. Everything went fine until it came time to put the yoke on.
When they pulled the yoke off, a steel ring fell out. Looked like it was part of the shaft seal. When they put in the new seal, it fit the housing perfectly, but there was a large gap around the inside between the lips of the seal and the sealing surface of the yoke.
The yoke was perfect, no noticeable wear. They checked the seal part number, and it was the correct one listed for that rear end.
The parts book showed a shim, and the parts house said they could have one next day. It came in, and was basically a speedy sleeve used to repair the sealing surface of a worn yoke.
This thing is not a repair sleeve. Its almost 1/4 inch thick. Its a snug fit on the yoke, and is the same "length" as the original sealing surface, so it does not make the yoke longer, just thicker.
Rather than trying ordering a new yoke to see if its larger, they checked the original shim, and it looks good. No real wear at all.
They put a thin film of loctite on the yoke and slid the old shim on.
It fit the seal perfectly, and there were no leaks during the test drive.

The rear end is fixed, and all is well. So it doesn't really matter what this thing was, but I would like to know if anyone else has seen this.
It looked factory, and the owner of the truck did not remember any work being done on the rear end before. If its factory, my only thought is that its something that GM came up with to use up a bunch of yokes left over from a design change.
I figured that if anyone outside of the factory had ever seen it before, it would be some of this bunch.

Thanks

 

[obijuank5]

Yes it's just a sleeve and I remember in my catalogs somewhere a footnote mentioning it. No big deal.

 

[Fordum]

I agree its not a big deal, but it just puzzles me that I have never seen one before and neither has anyone else I have asked about it.
And its purpose eludes me. If I had to put the rear end back together without it, how would I do it? Is there a seal with a smaller ID that fits the yoke that is on it now? Or do they make a yoke with a larger seal surface?
The parts house had a listing for a shim or sleeve for that rear end, and they ordered it. But it was not anywhere near thick enough.
Of course, that rear end is fixed and out the door, but I still would like to know why the shim.
There are millions of GM rear axles running around where the pinion seal is running directly on the yoke sealing surface, so why the extra part?
I still think it a production change, and a new model yoke would have fit without it.

 

[dyeager535]

What rear end is it? What year truck? Only change I can imagine is the d44 to 10 bolt in front, and 12 bolt to 10 bolt in rear if half ton. I want to say the yokes at least 10 and 12 bolts were interchangeable. Maybe GM experimented with using different yokes on different axles to consolidate yoke production. Like 7.5" to 8.5"?

I havent had a ton of diffs apart, so I dont think my few experiences are meaningful if it was used for a short time. Is it something very obvious upon disassembly if you were just replacing the seal for instance?

This sounds interesting. All I've got at this point is speculation. Would be interesting to figure out what this is.

 

[obijuank5]

I think it was the aam changeover years.

 

[rksjd8420]

They are on all of the 2001 and newer AAM 2500HD and 3500 rear axles. It is just a different design. The 10.5 AAM, you can just remove both and install an old style seal. They may also be on the smaller rear axles. I hardly ever mess with anything smaller than a 10.5” or 14 bolt. The newer inner seal design doesn’t leak as bad between the two seals. They will usually leak between the yoke and the inner seal. I always replace the inner seal.