Early Bronco Rear End Basics

Let’s cover some basics. From 1966 to 1977, the Ford Bronco came with a Ford 9-inch rear end. The Ford 9" is well-known to be one of the most reliable rear ends that you can put in a passenger car or truck. This rear end is often used in racing applications for its strength, durability and simplicity to work on. When we mention the rear end, we are referring to the entire rear axle assembly. You can break the rear end down into four different sections:

• Brakes

• Axle Shafts

• Axle Housing

• Third Member

This guide will go into detail on each of these sections so you can better understand how to identify parts, determine compatibility, and consider upgrades for your Early Bronco.

Brakes

There are four different sets of rear axles that would have come stock in 1966-77 Broncos, and each used its own corresponding backing plates and brake drums. These are not interchangeable—with one exception.

For 1974-75 model years, Ford used large bearings on both the 10" and 11" brake drum options. The 10" version maintained the same drum as the 66-73 small bearing rear ends.

Identifying your Bronco's Brakes:

• First measure the inside diameter of the brake drum. This measurement should be either 10" or 11". Again, be sure you are measuring the inside diameter, not the outside diameter.

• For 10" ID Brake Drums: If your drum is 10" inside diameter, measure the outside diameter of the axle bearing.

  • If the bearing is under 3" OD, you have the 66-73 Bronco small bearing setup.

  • If it is larger than 3" OD, you have the 74-75 Bronco medium duty setup.

• For 11" ID Brake Drums: All 11-inch drum brake setups on 66-77 Broncos were large bearing. You will now need to identify the inside width of the brake drums to know if you have 66-75 Bronco or 76-77 Bronco brakes:

  • 66-75 Bronco Large Bearing brake drum measures 11" × 1 3/4" (Photo 2)

  • 76-77 Bronco Large Bearing brake drum measures 11" × 2 1/4" brakes (Photo 3)

Notice that on the 66-75 version, the backing plate appears wider compared to the later style.

WILD HORSES Early Bronco Rear Drum Brake Rebuild Kits: Once you have identified your rear brakes, head over to our listing for Early Bronco rear drum brake rebuild kits for a complete, bolt-on rebuild of your factory rear brakes.

WILD HORSES Early Bronco Rear Disc Brake Conversion Kit: For improved braking performance (at nearly the same price), consider upgrading to our Early Bronco rear disc brake kit. This kit includes parking brake style calipers and the brake cables are included.

Axle Shafts

The most common question we’re asked is: “How do I tell if I have a large or small bearing rear axle?”

Bearing Pocket Bulge Check:

The easiest way to identify if you have large or small bearing axles without removing the axle shafts and measuring, is to take a look at the end of the axle housing just before the drum brake backing plate. On large bearing applications, you will see a noticeable increase in diameter of the axle housing to accommodate the larger bearing size.

• Small Bearing Axle Housing (Photo 1): Look at the white arrow pointing to the end of the axle housing. At the point where the housing meets the backing plate, there is no noticeable size increase at the bearing pocket.

• Large Bearing Axle Housing (Photos 2 & 3): Also marked by white arrows, you can now see a distinct size increase at the bearing pocket. This increase in size allows the larger axle bearing to fit into the axle housing end.

Bearing Measurements:

If you have the axle shafts out, you can identify a large or small bearing axle by measuring the outside diameter of the axle bearing. The measurements you are looking for are as follows:

• Small Bearing Axle: Bearing measures approximately 2.835" (just under 3")

• Large Bearing Axle: Bearing measures approximately 3.150" (just over 3")

If you have loose axles, the outside diameter of the bearing is the simplest way to confirm the size of your axles.

Spline count:

When replacing axle shafts, it's important to know the spline count (See photo 4). All original early Bronco axles were 28-spline. However, it's not uncommon to find your Bronco has been upgraded to 31-spline or 35-spline axle shafts. Upgrading to a higher spline count increases the strength of the axle at one of its weakest points. To support this upgrade, you must change out the center section of your housing to accept the new axles. 

• Factory 66-77 Bronco Axle Shafts were always 28-spline: All original Early Bronco axle sets were 28 spline. However, do not assume your Bronco still has 28-spline axles. It's common to see upgraded axle shafts in these classic rides.

• 31-spline Axles are the most common upgrade: Upgrading to 31 spline axles is very common. These axle shafts are generally priced similar to that of replacement 28-spline shafts. However, to upgrade to 31-spline axles, you do need to install a compatible differential and should also purchase a third member rebuild kit for the conversion.

• 35 spline Axles add beef for heavy duty use: If you are looking for a heavy-duty option favored by extreme rock crawlers, converting to 35-spline axles may be the ticket. This upgrade also requires a compatible differential and rebuild kit. It's also typical that a new Third Member differential housing needs to be purchased as well to accommodate this upgrade.

WILD HORSES Black Forge™ Early Bronco Rear Axles: We offer 28-spline, 31-spline and 35-spline replacement axle shafts for your early Bronco. Our 31-spline and 35-spline axles are made using 1541H steel with black oxide coating, ensuring they are the strongest, most durable rear axles you can get for your classic Ford Bronco.

Axle Housing

The axle housing provides the foundation that holds the axle shafts, brakes, and third member together. The housing ends are also what distinguish small bearing from large bearing axles. As we discussed before, the best way to tell if you have a large bearing or small bearing housing is to look at the bearing pocket at the ends to see if they are enlarged to support the larger bearing.

• Small Bearing Housing (Photo 1): No size increase at the bearing pocket.

• Large Bearing Housing (Photos 2 & 3): Noticeable increase at the same spot.

Since axle housings determine what axles, bearings, and brakes you can run, identifying your housing style is critical when ordering parts or planning upgrades. 

Upgraded 1976-77 Bronco Housing: It's worth noting that for the last two years of the Early Bronco, Ford upgraded the axle housing by adding a more beefed up center section. This housing style is the most desirable factory housing among hard-core off-roaders. Our Super 9 Ford 9-inch Axle Housing is modeled after the 76-77 Bronco upgraded housing - but made even stronger.

Third Member (Differential Housing)

The third member, also known as the hog’s head, chunk, pumpkin, or differential housing, bolts into the center of the axle housing (Photo 5). This component houses the core of your Ford 9" rear end including the differential, ring and pinion gears, and bearings. This drop-in design makes the Ford 9-inch extremely popular in high-demand situations. Racers are able to build a backup third member that simply drops right in if the first one has any issues.

Ford 9-inch Third Member Drop-Out Components

• Differential Carrier: Select from factory open or limited slip, or upgrade to a Yukon or ARB Locker.

• Ring and Pinion Gears: Make sure to use quality gears to ensure smooth and quiet operation.

• Bearings & Seals: We offer full rebuild kits that include all the bearings and seals you need to build your third member.

• Pinion Support: Common option is upgrade to Daytona Pinion Support for improved load capacity and bearing longevity.

• Gear Case: Use 3.062 Bearing Journal Case for 28-spline & 31-spline. Use 3.250 Bearing Journal Case for 35-spline.

Stock Gear Ratios:

The most common gear ratio in a 66-77 Ford Bronco rear end was 3.50:1. For some setups, or as custom order, the early Bronco could also come with 4.11 or 4.56 gears. The 4.11 gear ratio was the factory default when a Ford 170 or 200 L6 engine was included in the Bronco. It was also an optional upgrade for V8 models where the customer wanted more torque. The 4.56 gear ratio option was typically offered as an upgrade for those running an inline 6-cylinder engine.

Differential Types

There were two types of differential offered from the factory on early Broncos. These options included an open differential or a limited slip ("posi") differential. There are also several upgraded aftermarket differential options available for your 66-77 Bronco. We'll discuss the most popular options below.

Fatory Open Differential

An open differential is the stock setup most Early Broncos rolled off the line with. It allows each wheel on the axle to spin independently. That’s great for pavement and daily driving because it reduces tire wear and makes cornering smooth. But off-road, an open diff sends power to the wheel with the least traction. If one tire is on slick mud or lifted off the ground, it’ll spin freely while the other tire with traction gets almost no power.

Pros:

• Smooth, predictable on-road handling

• Lower cost and simpler design

• Less stress on axles and drivetrain

• Minimal tire wear during turns

• More controlled in ice and other slippery road conditions

Cons:

• Poor traction off-road when one wheel loses grip

• Limited capability in mud, rocks, or snow

Fatory Limited Slip Differential (LSD) - Clutch Driven

A limited slip differential uses clutches, gears, or cones inside the carrier to keep power going to both wheels, even if one starts slipping. It’s a middle ground between open and fully locked differentials. Early Broncos could be ordered with Ford’s Trac-Lok limited slip option in the Ford 9-inch rear.

Fluid note: Clutch-driven Limited Slips require a friction modifier to prevent chatter.

Pros:

• Much better traction than open diff

• Automatically transfers power to the wheel with more grip

• Good compromise for mixed street and off-road use

• Doesn’t bind up like a full locker in turns

Cons:

• Still not 100% locked—if one tire has zero traction, the other may still struggle

• Clutch-style LSDs wear over time and may need rebuilding

• More expensive than open diffs

If you’re keeping a mostly stock tire size, do light trails/forest roads, and want a period-correct feel, a factory limited slip with the right oil/additive works and preserves factory manners, but ultimate traction is limited vs lockers.

Yukon Grizzly Locker - Mechanical Automatic Locker

The Yukon Grizzly Locker is a mechanical auto-locker that delivers true 100% lock under load and allows differential action when coasting through its ratcheting mechanism; built with 8620 internals and a patented design to reduce common failure modes. 

Pros:

• Provides true 100% lock when under load

• Very strong full-carrier design built for durability

• Works automatically, no switches or air lines required

• Smoother and quieter than older style mechanical lockers

• Generally more affordable than selectable locker systems

Cons:

• Can produce clicking or ratcheting noises in turns

• Increases steering effort if used in the front axle

• Less street-friendly since it cannot be turned off

• Some users report occasional reliability concerns

• Harsher on-road behavior compared to LSD or selectable lockers

The Yukon Grizzly Locker offers predictable on-road behavior for many drivers. Howver, there can be a noticeable ratcheting/clicking in tight turns and more push/understeer vs LSD. That’s normal mechanical-locker behavior. If your Bronco will be spending more time on the trail than the street, or you just want increased off-road performance and are willing to accept the tire chirps as a means to the end, this locker is a great choice.

ARB Air Locker - Selectable Air Locker

The ARB Air Locker is a selectable locker: open when off, 100% locked at the flip of a dash switch using compressed air. It’s the traction you want off-road with open-diff road manners the rest of the time. If money is no object, and you want the best of all worlds, it's hard to beat an ARB Air Locker.

Pros:

• Lets you switch between open and locked modes at the flick of a switch

• Offers full 100 % traction when locked

• Keeps smooth, normal driving behavior when unlocked

• Engages very quickly—usually less than 1/10 of a second

• Doubles axle strength compared to a stock open diff

• On-board air compressor can also be used for tire inflation and other needs

Cons:

• More complex system (compressor, air lines, wiring) that requires careful installation

• Higher cost upfront and more components that could potentially fail

• Air lines can leak or get damaged if not well protected

• Requires maintenance—air lines, seals, and oil need occasional attention

• Adds system complexity vs simpler mechanical lockers

Differential Options Breakdown