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Author Topic: Rear Axles: Chrysler 7 1/4" Rear Axle  (Read 25861 times)

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Reggie

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Rear Axles: Chrysler 7 1/4" Rear Axle
« on: January 01, 2008, 16:33:19 »

Chrysler 7 1/4" Rear Axle

Chrysler Corporation's Spicer 7.25" rear axle was one of two differential types used in M-bodies, with the other being the stronger 8.25" unit. The measurement refers to the size of the ring gear. This gear, which is turned by the pinion gear, changes the direction of the power being transmitted to it. The pinion gear is turned by the driveshaft. The 7.25" axle first appeared in 1960 in the Plymouth Valiant, and it was produced in various forms through the 1990s. Built by the Dana Corporation, the housing is cast iron with tubular steel axle tubes pressed and welded in to form a complete carrier and tube assembly. The gear assembly is accessed by removing an inspection cover having either 9 or 10 bolts.  All 7.25" axles use the "7260" style U-joint

Although not known for its strength, the 7.25" axle has proven to be durable enough to have been used in millions of Slant 6 and V8 passenger cars and trucks over the years. It was even offered in a performance package back in 1967, appearing in 273 4-barrel automatic Formula S Barracudas with a 3.91 ratio and a sure-grip. In addition to being used in A-bodies and the F/J/M cars, it was reportedly used in some B-bodies (Coronet/Belvedere platform) and even some E-bodies (1970s Barracudas and Challengers). It will last a long time if it is not abused. Because the 7.25" axle is usually avoided for performance use, there is less aftermarket support for it than with other ring gear size axles, such as the 8.25" or 8.75" units.

There were at least three versions of the 7.25" rear used in cars:

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.

Factory axle dimensions (7.25" and 8.25" for all F/J/M cars:

  • Axle flange to flange: 54.34"
  • Perch center to perch center: 44.46"

Factory ratios offered include but are not limited to:

  • 2.26:1 - Used in many civilian M-bodies over the years
  • 2.76:1 - Used in 80s Dakotas
  • 2.93:1
  • 3.21:1 - Used in 90s Dakotas
  • 3.23:1 - Used in many Slant 6 cars from 1963-1969, and 80s Dakotas
  • 3.55:1 - Used in 80s Dakotas
  • 3.90:1 - Used in 90s Dakotas
  • 3.91:1 - Used in 67 Formula S Barracudas with power-lock
  • 4.11:1

Aftermarket gear ratios available include but are not limited to:

  • 2.26:1
  • 2.76:1
  • 2.94:1
  • 3.21:1
  • 3.23:1
  • 3.55:1
  • 3.90:1
  • 4.10:1
  • 4.11:1

Photos below:

  • A 9-bolt 7.25" rear (Contributor unknown)
  • Rear and side views of a 1980 Mirada 10-bolt 7.25" rear with 2.5" diameter axle tubes and large bolt pattern axles.  The other axle shown for comparison is an 8.25" axle with 3" axle tubes and large bolt pattern axles. (Courtesy of Moparts member 440newport)
  • Rear view of a 10-bolt 7.25" rear found in later M-bodies (Courtesy of Farley's member Woodvark)
  • Exploded view diagram of the 7.25" rear (Courtesy Chrysler RWD Factory Service Manual)

Sources:

« Last Edit: January 28, 2008, 23:16:27 by Reggie »
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