66 cuda project begins

Sure looks like a 7 1/4 rear to me.

Original Version:

Introduced in 1960, these units had 9-bolt covers with 2.5" diameter axle tubes from the axle flanges to the differential housing. These were originally small bolt pattern (5 X 4" bolt circle) axles used in A-bodies (Dodge Lancers, Darts and Plymouth valiants). From 1973 through 1976, they were also offered with a large bolt pattern (5 X 4.5" bolt circle) on cars with front disc brakes. Large bolt pattern versions were even used in some Slant 6 E-body Barracudas and Challengers. The small bolt pattern was discontinued around 1975 or 1976. They continued to be produced with the large bolt pattern as late as 1980, showing up in F/J/M platform cars. 1980 appears to have been a transition year. Per Randy's Ring & Pinion website, the 9 bolt rear uses a different case and spider assembly which is not compatible with later units.

Intermediate Version:

These units had 10-bolt covers with 2.5" diameter axle tubes from the axle flanges to the differential housing. As far as it is known, these were all large bolt pattern axles. This was likely an intermediate version of the 10-bolt rear before the all Chrysler axle tube ends were standardized to 3" at the axle flanges. The change to this version seems to have taken place around 1980, with both 9-bolt and 10-bolt versions appearing that year. A 10-bolt 7.25" rear with a 2.5" diameter axle has been verified in a 1980 Mirada.

Late Version:
These units had 10-bolt hexagon (stop sign) shaped covers with 3" diameter tubes at the axle flanges that stepped down to 2.5" at the differential housing. This appears to have been a bean counter change to standardize the size of the spring fastening and shock plate hardware between the 7.25" and 8.25" axle, which has 3" diameter axle tubes. As far as it is known, these were all large bolt pattern. These units appeared in all light duty (civilian duty - not police or taxi) M-body cars (Diplomats, Caravelles, Gran Furys and Fifth Avenues) from the early 1980s through the end of the production run in 1989.

Many people mistake the 10-bolt 7.25" rear for the beefier 8.25" rear due to the bolt count being the same. There are two ways to tell the two units apart:


The diameter of the 10-bolt 7.25" axle tubes will either be 2.5" entirely, or they will reduce to 2.5" as they get closer to the differential housing. 8.25" units will have 3" diameter axle tubes all the way from the axle flanges to the differential housing.
The 10-bolt 7.25" axle rear access cover will look more like a stop sign. The 8.25" unit will have a cover that appears to be rounded or oval.

Even later versions of the 7.25" axle were also used in 80s and 90s Dakotas. The axle tube diameter has not been verified, but it is likely that they had 3" diameter tubes at the axle flanges that stepped down to 2.5" at the differential housing like the ones used in 80s M-bodies.



John B.