Early Bronco Ford 9-inch Rear Axle Housing

Early Bronco Rear Axle Housing Identification

All 1966-77 Ford Broncos came equipped with a Ford 9-inch rear end, but not all rear axle housings are the same. Over the Early Bronco’s 12-year production run, Ford used four distinct rear axle housing configurations, each with its own combination of bearings, backing plates, and brake drums.

Correctly identifying which rear axle housing you have is critical when ordering brakes, axle shafts, bearings, seals, or planning upgrades. This guide breaks down each factory configuration and explains the easiest ways to identify what’s under your Bronco.

Factory Early Bronco Rear Axle Housing Configurations

1966-75 Ford Bronco Small Bearing Axle

• Backing Plate ID Numbers: FF55264, FF55265

• Brake Drum: 10" x 2.5"

This is the earliest and lightest-duty rear axle housing used in Early Broncos. It features small axle bearings and 10-inch rear drum brakes.

1966-75 Ford Bronco Large Bearing Axle with 11" Brake Drums

• Backing Plate ID Numbers: 319130L, 317363L, 319130R, 317364R

• Brake Drum: 11" x 1.75"

This configuration upgraded the rear brakes to 11-inch drums while retaining a narrower shoe width than the later 76-77 setup.

1974-75 Ford Bronco Large Bearing Axle with 10" Brake Drums (Medium Duty)

• Backing Plate ID Numbers: 80174L, 80175R

• Brake Drum: 10" x 2.5"

This is a unique two-year-only setup. While it uses the same 10" x 2.5" drum as the earlier small bearing axle, it features large axle bearings and different backing plates.

1976-77 Ford Bronco Large Bearing Axle with 11" Brake Drums

• Backing Plate ID Numbers: 3203097L, 3203098R

• Brake Drum: 11" x 2.25"

This is the strongest factory Early Bronco rear axle housing. It uses large bearings, wider brake shoes, and a reinforced center section, making it the most desirable factory housing for off-road use.

Using Backing Plate Numbers for Identification

The backing plate ID numbers listed above are one of the most accurate ways to identify your specific axle housing. Each axle assembly includes a driver-side and passenger-side backing plate, and the numbers are stamped into the plates.

That said, these numbers can be extremely difficult to read after decades of dirt, rust, and gear oil buildup. If the numbers are unreadable, there are easier visual and measurement-based methods to determine what axle housing you have.

Identifying Small vs Large Bearing Axle Housings (Visual Check)

One of the quickest ways to identify your axle housing is to inspect the ends of the axle housing near the backing plates.

• Small Bearing Axle Housing:
  The axle tube maintains a consistent diameter all the way to the backing plate, with no noticeable size increase at the bearing pocket.

• Large Bearing Axle Housing:
  There is a clear increase in housing diameter just before the backing plate. This bulge exists to accommodate the larger axle bearing.

This visual check can usually be done without removing wheels, brakes, or axles.

Identifying Your Axle Housing by Brake Drum Measurements

Another reliable method is measuring the inside diameter (ID) of the rear brake drum.

1. Measure the inside of the drum (not the outside).

   • The drum ID will be either approximately 10" or 11".

2. If you have 10" brake drums:

   • Measure the axle bearing diameter:

     • 2-7/8" OD → Small bearing axle

     • 3-1/8" OD → Large bearing axle (74-75 medium duty)

3. If you have 11" brake drums:

   • Both versions use large bearings, but drum width differs:

     • 11" x 1-3/4" → 1966-75 Ford Bronco

     • 11" x 2-1/4" → 1976-77 Ford Bronco

The wider 76-77 drums use wider brake shoes and backing plates and are not interchangeable with the earlier 11" setup.

Interchangeability Notes

Each rear axle configuration uses specific driver-side and passenger-side backing plates, axle bearings, and brake components. These parts are not universally interchangeable across all years.

One important exception:

• The 1966-75 small bearing axle and the 1974-75 medium duty large bearing axle can use the same 10" x 2.5" brake drum, even though the bearings and backing plates are different.

Why Correct Identification Matters

The axle housing determines:

• Axle bearing size

• Brake backing plates and drums

• Axle shaft compatibility

• Upgrade paths for brakes, axles, and differentials

Identifying your rear axle housing correctly before ordering parts can save time, money, and frustration—and helps ensure everything bolts together the way it should.