[1966geeto]

Just bought a 1972 GP rear nd is a 12 bolt wit 3.08 gears looked for the id stamp but no luck . How do I tell if this is an 8.5 or 8.75 rear end want to build a posi for it.

['ol Pinion head]

To confirm, look at the lower webbing of the center housing, the casting number should end in 278. The rear cover will have the inverted V, splash lip, stamped into it. that's what you will see on a c-clip axle 12 bolt out of a '70-72 GP. The '65-72 c-clip axle car 12 bolts used an 8.875 diam chevy 12 bolt ring gear, & 30 spline carrier. An Eaton 3 series or Earon clone posi carrier would be my choice to set up with the 3.07's.

[1966geeto]

Originally Posted by 'ol Pinion head

To confirm, look at the lower webbing of the center housing, the casting number should end in 278. The rear cover will have the inverted V, splash lip, stamped into it. that's what you will see on a c-clip axle 12 bolt out of a '70-72 GP. The '65-72 c-clip axle car 12 bolts used an 8.875 diam chevy 12 bolt ring gear, & 30 spline carrier. An Eaton 3 series or Earon clone posi carrier would be my choice to set up with the 3.07's.

So what you are saying is I have an 8.875 not an 8.5 ?

['ol Pinion head]

Yea, your original '72 GP 12 bolt uses the common 12 bolt GM car 8.875" ring gear. The rear itself was built by McKinnion Industries in Canada and originally had a 3R pinion flange and used the non Chevrolet style backing plates (no spotwelded on flanges). Few other minute differences too. The carrier needed will be the 3 series posi used with 3.07-3.31-3.42-3.55-3.73 ring and pinion.

The '67-70 type "O" Olds rear is the bolt in axle GM A-body rear that oftens gets confused (or is passed off) as a true 12 bolt rear. Unfortunately, there are many sources that continue to refer to the '67-70 "Type O" Olds rear aa a 12 bolt, though it is not. While 12 small bolts attach its smooth cover, the "Type O" center housing contains a 10 bolt carrier which a near 8.5" diam ring gear bolts to. Type "O" Olds 10 bolt rear, all the way. The Olds Type O is also one of the more expensive late '60's GM A-body rears to convert to posi and to change gear ratio.
to change ratios &

[1966geeto]

Originally Posted by 'ol Pinion head

Yea, your original '72 GP 12 bolt uses the common 12 bolt GM car 8.875" ring gear. The rear itself was built by McKinnion Industries in Canada and originally had a 3R pinion flange and used the non Chevrolet style backing plates (no spotwelded on flanges). Few other minute differences too. The carrier needed will be the 3 series posi used with 3.07-3.31-3.42-3.55-3.73 ring and pinion.

The '67-70 type "O" Olds rear is the bolt in axle GM A-body rear that oftens gets confused (or is passed off) as a true 12 bolt rear. Unfortunately, there are many sources that continue to refer to the '67-70 "Type O" Olds rear aa a 12 bolt, though it is not. While 12 small bolts attach its smooth cover, the "Type O" center housing contains a 10 bolt carrier which a near 8.5" diam ring gear bolts to. Type "O" Olds 10 bolt rear, all the way. The Olds Type O is also one of the more expensive late '60's GM A-body rears to convert to posi and to change gear ratio.
to change ratios &

Thanks so much great info.