I picked up a 12 bolt posi carrier today with the seller indicating that it is a '68 3-series posi. I pick it up, it's got 12 holes, posi componentry, casting date of E 18 8, OK DEAL! Kid says he bought it for his truck and discovered that it is a car unit.

I get home and can't find the important casting numbers. I see 17-3 on each side, E 18 8 on one side, and a very small stamped F 28 l. But everything that my search returned in this forum says I should find EDB-30140 cast opposite the ring gear. I see nothing. What the heck is this thing?

http://www.gmperformance.org/users/tj/mystery%2012bolt%201.jpg

http://www.gmperformance.org/users/tj/mystery%2012bolt%202.jpg

http://www.gmperformance.org/users/tj/mystery%2012bolt%203.jpg

The bearing journals measure (by tape measure) 1 13/16"

Any help would be great.
TJ

I just went out and measured the bolt pattern center to center diameter to find that it's only about 6.5", so I was kind of bummed thinking that this is for something little and not a 8.875" 12 bolt. BUT I remembered that I have a set of gears, which have exactly the matching bolt pattern. So at least it's really for this 12-bolt application. I also happen to have some bearings around which appear to fit just right. So my only concerns are:

1. That ring gear isn't very snug on there. I remember taking a couple of these apart and having to tap the gear off of the carrier. This Motive Gear slips right on there (with an 1/8" to spare). Is that just how aftermarket gears fit?

2. Is it a 3-series? Does anyone have the measurements for the flange location for 2, 3, and 4 series carriers?

Thanks again,
TJ

It looks to me like the Eaton posi that was used in the big GM cars. It is not a 12 bolt Chevy posi, truck or car. Check the axle gears and I think that you will find 31 splines instead of the 30 splines that are in the 12 bolt Chevy.

I just counted the splines. 28. Double checked, both sides. Someone else had suggested that it could be an aftermarket piece since it doesn't have "GM" cast anywhere on it. I know that stockers were Eatons, but they had GM part numbers on them and usually "GM" too, right? Did anyone else aftermarket make a knock off of these? I'm finding that stockers (just browsing on ebay) have ribs cast on flange that extend pretty much the full diameter of the piece and are between every ring gear bolt. This one, as you can see above, only has 6 and they are partial length.

Here's a couple pics of one that certainly has the correct GM identification:
http://home.comcast.net/~mcederoth/100_9162.jpg http://home.comcast.net/~mcederoth/100_9166.JPG

It's not an after market unit. It is the one that GM used in the big cars, but it is made by Eaton. Nobody else made one like this. The reason that the ribs don't go all the way across is because it is a 2 series unit. The 3 and 4 series Chevy posi have the ribs all the way across. All of the Chevy 12 bolt rear ends have 30 spline axles. Go get your money back. You can't use this one.