I have an open 3.08 O type rear end. I also have a 2.78 posi O type rear end. Can I make a 3.08 posi out of these two by switching either the gears or the posi unit.

 

you can use the 2:78 posi carrier up to 3:23 gears as long as they are both type O. I would replace all the bearings and races.

 

All references I have read indicate that the posi carrier for the 3.08 (casting 671) is not the same as for the 2.78 gears (casting 673).
Check with Jim (MONZAZ) or Brian Trick.

 

You will need to use a ring gear spacer to use the #673 posi unit with the 3:08 gears.I have made them before,but don't have any stocked right now.

 

No. This is wrong. The 2.78 carrier can be used with 2.56 gears but another carrier is needed for the 3.08 and 3.23 gears.

 

Do you still have the posi carrier and/or 3.08 gears?

 

I have NEW O type posi units Eaton type
or a few reconditioned 8.8 ford units machined to 'o' type olds rear a little cheaper.

As Brian stated we can make you a spacer plate custom to fit the 2.56-2.78 posi unit you have to the 3.08 3.23 carrier gears.

Jim
JD
PM if interested in anything