View Full Version : 71 rear

02-15-2002, 10:05 PM
Guys I have a 71 cutlass and will be looking into upgrading to a posi and new gears. The car is a 71 culass , 350 nonposi 10bolt with 308 gears.I hear that GM went with a corporate rear right around 71. Could someone tell me what came on this car in 71. I believe it's 8.5. What is an O type Differential? I want to order a posi and gears but not sure what exactly to ask for. Will 3.73 work with this diff.Any suggestion on a setup are appriciated. Thanks

[This message has been edited by 71greenrocket (edited 02-15-2002).]

02-16-2002, 06:56 PM
The rear end in your 1971 Cutlass is the corporate 8.5" 10 bolt & is the same used by other divisions 1971 & later. If it has 2.73 or numerically higher gear set then it can hold the 3.73 GM gears. The 2.56 carrier is a stand alone unit & only useable for the 2.56 gear set. Most 71s got 2.73 gears so be thankful that your's has 3.08s. That rear end is plenty strong for a street car & will live at the track too as long as you aren't running slicks & a big block.

The Olds 12 bolt was used late 1967-1970 & is an 8.875". Although it uses a 12 bolt cover it has 10 bolt guts. No parts interchange with the 1971 & later 10 bolt. There are 3 carriers; 1 for 2.56 & 2.73; 1 for 3.08 & 3.23s; & the performance carrier which holds 3.42,3.91,4.33;4.66; & a wild 5.00 gear set (4.33 & steeper were dealer installed). There are spline differences on the axles as well depending on what years & gear set. Parts are expensive & it is NOT any stronger than the 8.5" 10 bolt.

If you want a cheap source of a posi for your 8.5" 10 bolt find a mid 80s full size GM sedan or wagon with the factory trailer towing package (4 core rad is a giveaway). These cars have a 3.08 posi standard. Pirate the guts from the carrier & install them in your housing. If you want you can run 2.73, 3.08, 3.23, 3.42 or 3.73 GM gear sets & most are still available right at your local dealrship. My advice is to stick with the 3.42 gears which are the best all-around choice for performance econonmy.

Last 1972 442 W-30 4spd
12 Second '72 455 Cutlass
1965 442 455 Nash 5spd
1969 Delta Royale W-33
1986 & 1987 442

02-17-2002, 10:51 AM
Thanks for the detailed response.It deffentely answered a question that iv'e been getting some conflicting reports on.

02-17-2002, 04:54 PM
Your '71 should also have the "O" axles and not the chevy axles with the "C" clips. The "O" axles are pressed in.

02-20-2002, 06:32 PM
That is, the bearings are pressed onto the axle shafts and a retainer is sandwiched between the bearing and the wheel flange. When the axle shaft is inserted into the tube, four bolts are used to bolt the retainer to the end of the tube.

03-03-2002, 08:05 PM
I found the numbers on the rear axal tube. The numbers were R2 B 3641. According to my 1971 assembly manual. This would be R2= 2.56 ratio , B= BUICK and not sure about the 3641.Now I'm confused. IS there a buick axal in my 71 cutlass. Also I was told by the person that redid the car in 1999 that he put 3.08's in.If there is 2.56's in that would mean I"d have to change the whole carrer to put in 3.42"s. also found these numbers stamped on the actual Diffential housing, 1235197 and a big A on the right side and a big Z on the drivers side of the housing, not sure what that means. Can anyone shed light on this. Thanks

03-05-2002, 06:57 PM
Guys any ideas on the codes I found on the axal.

03-06-2002, 07:58 AM
According to my book, which is usually flawless, your numbers don't make much sense. R2 is not listed, RZ is. Here is a breakdown of axle codes. the first 2 letters are the axle code. 3rd letter is manufacturer identity. next three numbers are date code, next letter is day shift or night shift, last letter is locking axle identification. This is '71 and up axle I.D. for General Motors. They say the code is stamped on the rear of the left axle tube about 3" from the carrier. If my book is incorrect and somebody has better info, please let me know. Your rear may also be pre-'71 which would make my ramblings meaningless.

[This message has been edited by 442E-Dan (edited 03-06-2002).]

03-06-2002, 01:23 PM
I can say for certain that the R2 code is a 2.56 non-posi since I actually have a couple of those. The code number is the same for the 10 bolt and the 12 bolt models (I have one of each). I have used the 2.56 posi carrier in an axle with 3.08 gears, but I had to shim the carrier over about .125 inch to achieve proper gear backlash. I have never tried with lower gears, though.

The only way I see to make sure of what gears you have is to pull the rear cover and look on the gears themselves. The ring gear will be stamped on the rim with something like "41 A 16". The first number is the number of teeth on the ring gear and the second is the number of teeth on the pinion gear, so 41 divided by 16 equals 2.56. These numbers are stamped on the face of the pinion gear as well, but aren't as easy to see inside the housing.

03-06-2002, 03:46 PM
So My 71 has that funky R2 code too. if I wanted to go to a say a 3.42 posi I would need to change the carrier to a 3.42 ..Right? and do they make a 10 bolt posi 3.42 carrier

03-07-2002, 01:00 PM
Yes, you need a posi carrier as well as new gears. The carrier isn't gear specific, it will work with a wide range of ring and pinion gears, so one from, say, 2.73 up to 3.73 or so would work with 3.42 gears. 3.08s were really popular on a lot of vehicles, and that would work OK. New posi carriers are available from Auburn for about $400, just look in Summit or Jegs. The other option is to go junque yarde jaunting and locate a used 8.5 inch posi carrier.