Alright guys.....it's winter (snow and salt) in Ohio and I've started doing minor things on my recently acquired 1971 Cutlass, 350ci (cam, headers, intake, holley carb) and a th350 trans. Right now I'm looking to do a rear end upgrade to a posi unit and obviously a different gear ratio.

After reading countless posts in the rear diff section of this forum I've gathered this much......I have an 8.5" 10 bolt with open 2.56 "grandma gears" based on cover style and the R2 code on the axle tube as seen in the pics below and in knowing the gears were never changed in this original rear end. (507Olds.....if i could only shake your hand haha). Also, I think I read somewhere that I should have the 28 spline axles right?





I've read that my best bet is to go with an Eaton Posi Unit and as this car won't see too much highway, and it's more of a weeknight/weekend cruiser/fun car, I'm going to go with either 3.73 or 4.10 gears.

So obviously since I want to upgrade to a posi unit, this takes care of needing to replace the 2-series carrier that I currently have and just need a 3-series Eaton unit and then gears to go with it.

The question I have, since the car has basically been sitting around for 20years or so it's probably a good idea to replace the normal wear internals, since it's going to be all apart anyways right? Just to be 100%, what all do I need, rough guess of prices that I'm looking at for my purposes? Sorry to be a pain.....new to this game.

As far as putting it all in, I have an Uncle that's going to help me, he usually tells me to just get the parts and we'll go from there.....

Thanks guys

 

I would go with the 3:73's.They will work great with your 350,and if the day comes that you swap to a 455,they will work with that too.The 4:10's are too steep in my opinion.
If you are going to do the posi & gear swap,this is the best time to rebuild the whole rear,especially if it has been sitting like your said,and it has never been serviced before.Change ALL of the bearing & seals in the whole rear.

Axle bearings - SET-9 or A-9,depending on brand
Axle seals - 712146
pinion bearing,inner - M802048
pinion race,inner - M802011
pinion bearing,outer - M88048
pinion race,outer - M88010
pinion seal - 2043

The carrier bearing are going to depend on your new carrier/posi unit.Most of the new units have a larger bearing hub,so the bearing that are needed are different than the originals.
The original bearings & races are LM501349 & LM501314
The newer/bigger hub bearings & races are LM102949 & LM102911
Do NOT confuse the LM102911 with the LM102910 that will be on the shelf.The LM102911 is slightly wider,which is what you need.The narrower LM102910 will allow the bearing cone to stick out the other side,which is where your pinion shims are,and will not work.
If you need help with this,just ask.

 

Thanks, I really appreciate it!

 

gears 150.00 ish
rebuild kit 70-100 depending on brand
axle bearings and seals 40-70 depending on brand
Posi 380-450 again depending on brand and place you get the parts

Hope that helps in that price area. Jim