Differential Swapping



Gears are available for most every differential. But buying and installing a bunch of parts can get expensive - fast. It would probably be a bunch cheaper in the long run to find the rear with the gears and anti-spin that you want.


To upgrade an open differential to a posi, you must change the carrier to a posi type. That is, just the "guts" inside the rear end. You can even keep your existing gear set. A new or rebuilt unit should be about the same in durability. If you have a regular "C" type rear end then parts for this are very easy to find. However, if you have a "O" type rear end used in 1967 to 1970 models then a posi carrier is very difficult to find. Best bet is to get a rear end specialist to do the work.

An entire rear end from another GM product is a bolt-in fit as long as you're doing it in the same body style. Minor differences such as brake lines and U-joints are the only trouble spots. Buick, Olds, and Pontiac rear ends are interchangable if they are the same frame size (an "A" body GTO rear will fit in a Cutlass).

In regards to swapping 1964 to 1967 A-body 10 bolt rear axles, with 1968 to 1972 A-body axles, the entire housings will swap. The 1964 to 1967 axle is ¼" narrower. But at 1/8" per side means, for all intent and purposes, they are the same.

As a general rule, most of the axle shafts will swap as long as they have the same spline count. This excludes 10 bolt "C" axle shafts (as in Ch*vy) which are unique in that they have an internal "C" clip to retain the axle in the housing and use a roller bearing in the tube end. This is as opposed to a bearing which is pressed onto the axle and then bolted to the tube end as most other sane manufacturers do.

Most GM diffs carriers carry ring and pinion sets up to 3:42 ratio, from 3:73 and up you have to change the carrier. The reason is that as the pinion gets smaller (with the lower gear ratio), the deeper the crown gear has to be. Instead of making extemely thick crown gears, GM made two types of carriers instead. Some after market suppliers have a spacer which allows you to put a 3:73 gear set on a 3:23 carrier. I have done this before on several cars than run both on the street and the strip, and don't want to change the carrier.

As for gear combo, say if you had just a posi carrier with say a 2.73 gear, you could put a 4:10 gear or even 4:33. Consult the chassis manual or the gear manufacturers catalog for compatible gearing. The only thing you will need to do is buy a shim kit because when you put higher gears on the carrier, the carrier shifts relative to the pinion. These shims go between the carrier bearings and the start of the axle tubes in the housing.

The service manuals from 1967 and 1969, and also the illustration section of part listings from that era, have pictures and diagrams of the center section castings (using the ribs to ID the rear) and rear covers (Chevy rears have that goofy wrinkle).


If you have an old bearing handy, it has the # on the race that can be used to get a replacement. This # is a standard that can be found in any bearing parts shop or even at the usual auto parts place that you frequent.

[ Thanks to Bill Culp for this information ]

Differential Internals

The big question for swapping the internals is of course, just what did the new internals come out of. Many cars being very close in model year used different internals, even though the cover on the differential has the same number of bolts. Identify what you have first.

You need a few tools to do the swap correctly:

  • Impact gun, for loosening and tightening the pinion nut
  • Dial indicator, for setting gear backlash
  • An inch/pound torque wrench
  • Pinion depth guage (no, you won't find that one at Pep Boys or AutoZone)

If you don't have these tools, you can buy a set of ring & pinion installation tools for $150-$200, and an impact gun would run you around $90-$120 for a cheap one (of course, that is assuming you have an air compressor).

Now if you have or can borrow all the tools, the critical things to measure are the pinion depth, pinion bearing preload, carrier side clearance, and gear backlash. These are all outlined in the shop manual (which you should refer to when doing this job).

[ Thanks to Bob Barry for this information ]