Ok trying to ID my rear end in the 71. Car was originally a 350/TH350 car. I believe the rearend to be original. I have looked everywhere on the axle tubes and can not find a rearend code stamped. Yes I used a wire wheel as well. The car is an Arlington plant built vehicle if that may help anything. It still retains an open rear probably a 2.56/3.08 as I have to redo the driveshaft spin to get a better accurate reading but it spun around about 1.5 times last night with one wheel locked. Based from what I read I believe it to be an olds O type 12 bolt unit by looking at the back of the backing plates mounting. I found a deal on a Auburn posi with 3.55 gears for 28 spline axles but not sure if it will fit mine. If I pull the cover what can I look at to positively identify it can I see the splines?

Thanks
Magna

 

Type O axles were last used in the 1970 model year, so if you really do have a Type O, it isn't original.

 

To find out what you have jack up the rear end,and put car in neutral,front wheels chocked.
Use some masking tape and mark the wheel well and tire at any position you like.
rotate the tire one full revolution and count the turns of the rear end pinion.
If it turns right under 3 turns-2.73,turns slightly more than 3,you probably have a 308,almost 4,3.73 etc.
If you rotate one tire and the opposite tire also turns forward,you have posi.
I found my axle code on the inside tube facing the engine,passenger side.

 

Your car should have an 8.5" 10-bolt.It will have a 10-bolt cover.Those rears will have the codes on the frontside of the passenger axle tube,between the pumpkin & the spring perch.If it is a 12-bolt O-axle,it will have a 12-bolt cover,and the codes will be on the backside of the passenger axle tube.

 

I believe there are 3 different carriers.
One for gearing up to 3
Another from 3-4.
Another 4.10 and up.
I believe you will need to upgrade the carrier if moving from one set to another..you cannot install 4:10 gearing in a housing originally equipped with a 2.43 for example.

 

Well its definately a 12 bolt and no ID code has been found on the tubes using a wire wheel. I had the car jacked last night with one wheel on the ground locking it. I marked the driveshaft and spun the tire around once and the drive shaft spun about 1.5times. I did this another night with both wheels off the ground and got the same result. Anyone have a picture of the tube id stamps or any other palce they could be? I thought I read that it should be a 10 bolt but looks like I got an upgrade! ;-)

 

Try spraying some PB blaster along the tube.
Although a wire wheel may have worn away the numbers.
I always try to use a scotch brite pad or brush rather than power tools.

 

I tried steel wool first and Im not seeing any hint of a stamping. But will try again tonight.

 

Well still no code stamp on the tube but did find the casting code 402227 and CFD5 which tells me its an a non W27 1970 rearend. So I assume that makes it a 28 spline?

 

The 402227 is the 12-bolt O-axle casting number,so we have that right.So,if there is still a code on it,it will be on the passenger side axle tube,facing the back.

 

It can have 31 splines in earlier versions and 28 splines in later versions.

 

The 31-spline version had a different casting number.The 402227 casting is 28-spline.However,you can convert the 28-spline rears to 31-spline,with a different carrier bearing set,and the axles.If it is a 1970 rear,then you would need to have axles made,due to the different bearing design,compared to the 68 style.