GM 12 bolt rear end question

T

Todd Merrifield

I found a 12 bold rear end at a swap meet for my GMC today. Very similar to my 10 bolt, but it's a 12 bolt and has slightly better gears for the way I drive (3.40 vs. 3.73). When I took the cover off today I was presented with this:

rearend.jpg


My question is, what is the assembly on top of the regular gears, and what does it do? I'm sure it's quite common, but I have never seen it before. I'm talking about the parts mounted on rods going left to right in the picture. The bottom piece spins, and the top piece rocks back and forth. It is driven by an extra gear that goes around the planetary gear on the right.

Do I need it? It looks like some kind of sensor, but there are no wires going from it that I see. Everything else looks normal.

The problem I am having is that it has a big steel block behind it, and that seems to be preventing me from pushing the axles in to remove the C-clips. I need to remove the axles to see if they are tapered or not, as I'd like to get it narrowed slightly at some point. I removed the center shaft like you would normally do to push the axles in, but something is still preventing them from going in far enough to remove the C-clips.

If I need to remove the mystery assembly I need to know how to do it correctly. So far I'm not seeing it in any of the manuals I have.
 
get the rod out

Todd,
My son, Kevin works in a GM garage. He says you should find a bolt that goes through the center of the asembly. It is on the inside. Kevin says to turn the ring gear and watch for a 5/16 or 8mm bolt head on the left side. That will come out and then allow the axles to move enough to remove the C clips on the axles. Kevin says what you have is a limited slip gear set here. Not posi-traction. This one only supplies power to whichever wheel is getting the best traction. It is a completely mechanical item. Kevin works at the Coughlin Group on Rt 42 in London. He told me what you had here as soon as he saw the picture. Hope this is of some help. You may find a book at the library on this or a shop manual now that you know what you have.
Good luck,
Mike

ps: did the Mercury ever get any nibbles? I told my brother about it but it is newer than he wants.
 
Mike Stevens said:
Todd,
My son, Kevin works in a GM garage. He says you should find a bolt that goes through the center of the asembly. It is on the inside. Kevin says to turn the ring gear and watch for a 5/16 or 8mm bolt head on the left side. That will come out and then allow the axles to move enough to remove the C clips on the axles. Kevin says what you have is a limited slip gear set here. Not posi-traction. This one only supplies power to whichever wheel is getting the best traction. It is a completely mechanical item. Kevin works at the Coughlin Group on Rt 42 in London. He told me what you had here as soon as he saw the picture. Hope this is of some help. You may find a book at the library on this or a shop manual now that you know what you have.
Good luck,
Mike

ps: did the Mercury ever get any nibbles? I told my brother about it but it is newer than he wants.
Click to expand...

I got them out. Don't know what I did, but I jiggled some things and was able to get the C-clips out. I had already removed the bolt and pin your son mentioned, which is usually all it takes. I guess I just didn't try hard enough the first time.

Turns out they are tapered, so I can't shorten them. Now I am looking for shorter replacement axles. I was all set to order what I needed from Jegs for $99.00, until I realized what I was looking at was the price per axle. Now I'm going to look on ebay/Craigslist.

Still don't know what the extra assembly is. I think the other rear ends I have laying around are limited slip, and they don't have that stuff. Whatever it is it appears I can leave it alone at this point. At any rate, at least I will have a rear end I can work on and leave the other one on the vehicle until I am ready to make a swap, instead of having it on jackstands for days/weeks/months.

A few people are supposed to get in touch with me on the Merc. One guy wants to trade me a fire truck or two for it. Wasn't too thrilled until he mentioned he had a 65 Chevy pumper with a 409 in it. Lights, sirens, ready to go. I might be into that. :)

Did you go back and get those sirens? I didn't see them anywhere...
 
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