1964-1976 Cutlass / 4-4-2 Differential Identification Information

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For phone inquiries regarding technical assistance and / or identification questions, call our tech line at 503-641-1442.

10 Bolt B or P Series Axles.

The 10 Bolt B or P differentials were used under most 1964-1966 Cutlass, 4-4-2 and Vista Cruisers. And were also used on some 1967 models. From the exterior, B or P differentials can be identified by this cover shape:

From the side, a B or P differential can be distinguished by the ribs in the center section:

Internally, B or P 10 Bolts utilize an 8.2" ring gear. All use 28 spline bolt in axle shafts - not c-clips for axle retention. All have a 1.438" diameter, 27 spline pinion stem.

When equipped with Anti-Spin ("posi"), most were the cone (not clutch) style carriers. The cone style carriers are really not rebuildable and were replaced as an assembly by the dealers. Most P cone style carriers use six bolts to join the carrier case halves together and utilize two pinion gears. When used under 1966 and 1967 W-30's, the P carrier featured eight bolts to join the carrier together and used four pinion gears.



12 Bolt O Axle

The Oldsmobile 12 bolt rear end (O axle) was used under most 1967-1970 Cutlass, 4-4-2, Hurst/Olds and Vista Cruisers. The housing cover can be identified from the pictures below. Internally, the Olds 12 bolt O axle rear end features an 8.5" ring gear. Which is attached to the carrier by 10 bolts (this 8.5" 10 bolt ring gear is not the same as the 1971 and newer 10 bolts). All use 28 spline axle shafts except the 1967-1968 models originally equipped with 3.42 thru 5.00 gears. These use 31 spline axles. The 31 spline models also use larger carrier bearings than the 28 spline models. Consequently, the 31 spline carriers cannot be used in a housing originally equipped with 28 spline axles, and vise-versa without the use of special bearings. 1967-1969 axle bearings are a sealed ball bearing design. 1970's axle bearings are an open roller design lubricated by the differential oil. All O axle shafts are retained by a four bolt flange (not internal C-clips). Three different carriers were used depending on ratio: 2.56-2.78, 3.08-3.23 and 3.42 thru 5.00. These have different pinion centerline to ring gear mounting flange offsets (The 28 spline and 31 spline carriers feature the same mounting flange offset). All O axles use a 1.626" diameter, 27 spline pinion stem. All Anti-Spin models (Oldsmospeak for "posi") are clutch type differentials. Which utilize an "S" shaped spring to provide pressure against the clutch packs.


Factory carrier casting number location (673 = 2.56-2.78)

12 Bolt O Axle



Casting Number







28 spline axles




28 spline axles




28 spline axles




31 spline axles


Casting code is located on both left and right sides of most factory carriers.



All differentials have a stamping code indicating gear ratio.  This code is usually located on the right side axle tube near where the steel brake line is located.  However, we have also seen the codes stamped on the front, top or bottom of the tube – as well as on the left side. We have also seen the codes stamped upside down.  Please note Oldsmobile often used the same ratio codes for many years as well as on different differentials and therefore cannot be used to determine the specific year of a housing.   Ratio code stampings can be found in the factory chassis manuals as well as in our book “4-4-2 By The Numbers”.



As noted earlier, there are two different axle bearing and seal designs used.  All 1967, 1968 and 1969 models use a sealed ball bearing with the axle oil seal located inside the axle tube. 


1967, 1968 and 1969 axle, bearing and seal identification.


1970 axle, bearing and seal identification.

In 1970, the axle bearing was changed to an open bearing design which receives its lubrication from the differential oil.  Consequently, the axle oil seal was relocated onto the axle shaft itself - between the axle bearing and the four bolt axle retaining flange as shown above.

Please note, over the years we have encountered some very early production 1970 rear ends that utilize the 1967-1969 axle bearings and seals (as well as of course the early style axle shafts and axle tubes).  Also note most aftermarket replacement 1970 axle bearings use a lock ring similar to the 1967-1969 style and a smooth outer race on the axle bearing.  For positive identification of which bearings and seals to use, always go by the seal location – in housing (early design), on axle shaft (late design).  



There are three basic 12 bolt "O" Axle housings. The 1967 through 1969 housing was used for all models except wagons.  The original part number for this housing is 230727 and will have the casting number 39413 or 394139.  This same housing was also used for the 1967-1968 models with 3.42 through 5.00 gears, but because these models used larger carrier bearings, the housings were machined accordingly and therefore required a different part number.  The part number for the 1967-1968 3.42-5.00 housings is 320575. 

 In 1970, due to the axle bearing design change, the housing part number 231000 was used for all models except wagons and of course the W-27 aluminum housing. The W-27 aluminum housing used the part number 231042.  The casting number for the non W-27 housing is 402227.  The casting number for the W-27 housing is 406784.

Housing casting number location



For external identification of a 12 bolt O axle vs the 12 bolt C axle, note the difference in the covers. Both feature two parallel ribs cast in the side of the differential housing. Internally, 12 bolt C axles use an 8.875" ring gear that is attached to the carrier by 12 bolts. C axles can be further identified by the use of internal c-clips for axle retention.


1971-1976 10 Bolt

Used under most 1971 thru 1976 Cutlass, 4-4-2, Hurst/Olds and Vista Cruisers. Externally, they feature a smooth (10 bolt) cover much like the 12 bolt O axle as seen above of this shape:

From the side, the center section has two parallel ribs cast into the housing unlike the single or converging ribs on the B - P 10 bolts. Internally, the 71- up 10 bolt utilizes an 8.5" ring gear. The pinion gear stem is 1.625" diameter and uses 30 splines. Axle shafts are 28 spline and are the bolt-in design - not c-clipped and feature open (lubed by differential oil) press on bearings. Anti-spin ("posi") is the "S" spring, clutch style.



 For phone technical and / or identification questions, call our tech line at 503-641-1442.





COPYRIGHT © 2005 Greg Rollin/Supercars Unlimited. Any reproduction of the text and/or images strictly prohibited. All rights reserved.


Oldsmobile 12 bolt posi rear end, Olds 12 bolt posi