I have a 12 bolt GM4 differential p/n 3859140N stamped CX 09 24 B with a cast date 1 10 4 it only has the posi unit installed no gears or axles.I was told only a 4 series gear will fit in this carrier.Can I install a 331,373.etc gear set in this housing.It came with the 1966 SS I am restoring and I would like to use it in this car.
Thanks Bob

You can install any gear that you want in the housing. You will only be able to install a 3.73 to 5.XX gear on the 4 series differential. Check the casting number on the posi case. If it's a EDB30174 it's 4 series. If it's a EDB30140 it's a 3 series.

Thanks for the reply;but I am not real clear on swapping.I have a GM 4 series carrier,could I install a 3:31 posi from a1968 chevelle in my 1965 series 4 carrier?I would only have to change the axles since I understand they are 1/2" longer.
Thanks Bob

http://www.chevelles.com/forums/showthread.php?t=260701

I think there is some confusion here on differential versus differential pumpkin which holds the differential. I believe GM4 is a casting that is not related what series carrier but what size pumpkin or maybe even plant casting location.

If you have a 12 bolt passenger rear axle, it should be able to take a 12 bolt passenger differential and passenger gears.

The housings are all the same. The only difference is what you put in them. The differential is the part that the ring gear bolts to. There are 2,3 and 4 series differentials. If you take all of the parts from a 12 bolt with 3.31 gears and put them into your housing then it will work fine, if they are set up correctly.
http://thumb7.webshots.net/t/59/659/3/15/26/2890315260041712107VGzqQV_th.jpg (http://rides.webshots.com/photo/2890315260041712107VGzqQV)
Here is a picture of the 2,3 and 4 series differential cases for the Chevy car 12 bolt. Notice the difference in the location of the ring gear flange. The case in front is a 2 series and the flange is closer to the end. The case in the back is a 4 series and the flange is closer to the center. This is the only difference in a 2,3 and 4 series differential case. Any of these cases can be installed in and Chevy car 12 bolt housing with the correct gears that go with them.