GM-12 Bolt Rear End ?'s

[mjgord51]

I am looking to change out my 3.73 one legged 3rd member and put in a 3.42 posi unit someday. I ran across this in my Chevy/GMC Truck catalog from Classic Industries and was wondering what they mean by "medium case or large case" I haven't gotten real involved in the change over's specs/info yet but will have to start somewhere. thanks in advance. Marvin

[Fast68Chevy]

some axles have different pinion gear placement due to larger or smaller diameter pinion head. taller ratios like 2.21 2.56 2.73 3.40 have larger pinion head so sometimes have to be further away from ring gear in the case,


but theyre all 8.875" and 1.438"

AFAIK all the different series in the truck 12 bolts have the same case.



and i dunno why 3.08 when it was 3.07

and there were 2.21 an 2.56 and 2.73 and 3.40 and some other oddball ones too


i really have never sene option availability for anything more than 4.11 in the truck 12 bolts


not sure from where or how thye get their axle info. doesnt seem accurate correct.

[GASoline71]

The "carrier series" in a 12 bolt truck rear end changed from a 3 series carrier to a 4 series carrier at 3.73 or lower (numerically higher). So for 3.42's you would need a 3 series carrier. Might be what the mean by "medium" and "large".

Gary

[mjgord51]

Originally Posted by GASoline71

The "carrier series" in a 12 bolt truck rear end changed from a 3 series carrier to a 4 series carrier at 3.73 or lower (numerically higher). So for 3.42's you would need a 3 series carrier. Might be what the mean by "medium" and "large".

Gary

Gary here is what I have found from "Mark Williams Enterprises". 3 series are 3:08 to 3:73, and 4 series are 3:90 to 6:14. Also see pics below for more info to tell which from each other.

[mjgord51]

Originally Posted by Fast68Chevy

some axles have different pinion gear placement due to larger or smaller diameter pinion head. taller ratios like 2.21 2.56 2.73 3.40 have larger pinion head so sometimes have to be further away from ring gear in the case,


but theyre all 8.875" and 1.438"

AFAIK all the different series in the truck 12 bolts have the same case.



and i dunno why 3.08 when it was 3.07

and there were 2.21 an 2.56 and 2.73 and 3.40 and some other oddball ones too


i really have never sene option availability for anything more than 4.11 in the truck 12 bolts


not sure from where or how thye get their axle info. doesnt seem accurate correct.

Randy I understand the 8.875" but how does the 1.438" come into it?