Early Bronco Ford 9-inch Axle Shafts

Early Bronco Rear Axle Tech Info

Understanding the components of the rear axle assembly is essential for any classic Bronco enthusiast. Below, we will cover the basics of early Bronco rear axles. Please feel free to reach out to our knowledgable sales team should you have additional questions.

What are the components of an early Bronco rear axle assembly?
The early Bronco rear axle assembly, commonly referred to as the rear end is comprised of the following key components:

• Axle Housing
• Axle Shafts
• Third Member
• Brakes

Let's dive into the frequently discussed topics on each of these components.

Early Bronco Axle Housing Identification.
All 66-77 Ford Broncos came with a Ford 9-inch rear end. However, there were four different axle housings offered throughout the early Bronco's 12 model-year run. 

• 66-75 Ford Bronco Small Bearing Axle
  • Backing Plate ID Numbers: FF55264, FF55265
  • Brake Drum: 10" x 2.5"
    
    
• 66-75 Ford Bronco Large Bearing Axle with 11" Brake Drums
  • Backing Plate ID Numbers: 319130L, 317363L, 319130R, 317364R
  • Brake Drum: 11" x 1.75"
    
    
• 74-75 Ford Bronco Large Bearing Axle with 10" Brake Drums
  • Backing Plate ID Numbers: 80174L, 80175R
  • Brake Drum: 10" x 2.5"

• 76-77 Ford Bronco Large Bearing Axle with 11" Brake Drums
  • Backing Plate ID Numbers: 3203097L, 3203098R
  • Brake Drum: 11" x 2.25"

The backing plate numbers listed above are a great way to identify your specific axle housing. However, these numbers can be extremely difficult to read through the decades of grime that builds up. To more easily distinguish between large and small bearing rear axles you can inspect the ends of the axle housing. A small bearing axle shows no significant increase in size at the axle bearing pocket near the backing plate, whereas a large bearing axle displays a noticeable size increase at this junction.

Each configuration includes a driver and a passenger side axle, with corresponding backing plates and brake drums specific to each set. It is important to note that these components are not universally interchangeable. However, the 1966-1975 small bearing and the 1974-1975 medium duty large bearing can use the same brake drum.

You can also identify which axle housing you have by measuring the inside of the Bronco rear brake drum. The ID of the brake drum will be either approximately 10" or 11". If you have 10" brakes, measure the axle bearing diameter. It should either be 2-7/8" (small bearing) or 3-1/8" (large bearing). Of the two 11" brake drum options, the 66-75 Ford Bronco rear axle version features a narrower drum and brake shoes, marked as 11 x 1-3/4". This is compared to the 76-77 Bronco version which uses an 11 x 2-1/4" drum and shoe.

Early Bronco Rear Axle Shafts:
All original early Bronco rear axle sets were constructed with 28 splines. A common upgrade in the early Bronco rear end is to replace the stock axles with new 31-spline rear axle shafts. This in conjunction with the use of higher quality heat treated steel has been estimated to increases the strength of the rear axles by up to 60%. In order to upgrade your early Bronco rear axle shafts to 31-spline axles, you will simply need to replace the axles and install a 31-spline differential to accept them. We also highly recommend a new third member rebuild kit when performing this upgrade on your classic Ford Bronco.

For those engaging in extreme rock crawling, upgrading to 35-spline early Bronco rear axle shafts will maximize the strength and durability of your early Bronco rear end. This upgrade includes new axles and differential, but also often requires a new differential housing.

Early Bronco Third Member:
The Ford 9" rear end found on early Broncos utilizes a drop-out third member, also known as the differential housing, hogshead, or chunk. The third member houses the differential, ring and pinion gears, and bearings. This component is integral to the axle housing.

Most early Broncos came with a 3.50 stock gear ratio. Heavy duty packages were also offered which could include 4.11 or 4.56 gears. There are numerous other ratios available as well. A higher gear ratio (3.50 for example) will result in improved gas mileage and keeps the engine RPM's lower at high speeds. The downfall to higher gears is that they are more sluggish at initial acceleration. Low gears (4.11 or 4.56 gears for instance) are ideal for off-road use and when looking for more acceleration and higher torque. When upgrading to large tires, it is highly recommended to install lower gears as a high gear ratio will result in extremely sluggish performance off the line.

The differential itself came stock either as an open or a limited slip "posi" type, with several upgrade possibilities including Detroit Locker, ARB air locker, and electric lockers.