Why Was This Rear End Called A 9 Inch?
The rear end got its moniker for the distance of its outside ring gear, which measured nine inches. The rear end supported the weight of the axles and looked a bit like a banjo, and was offered with both 28 and 31 splines (the latter proved to be an instant hit among hotrodders and racing enthusiasts for years).
What Fords Had the 9-Inch Rear End?
Many of the mid-sized and full-sized sedans that Ford manufactured from the late 50s were equipped with this read-end. The rear end would find its way into the F-100 and F-150 from the moment it was introduced in 1957. (Ford would keep it until replacing it with an 8.8 in 1986). The Ford Bronco would enjoy it from ‘66 - 83, and the Ford Thunderbird from 1957 - ‘71. The rear differential even found its way into models like the ‘64 - 73 Mustang, ‘68 - 76 Torino, and the Ford Fairlane (‘57 - 71).
When Ford developed the 9-inch in the fall of 1956 for the Edsel, Thunderbird, and Ranchero, they knew that the larger V8 engines they were developing would require a stronger, more significant rear-end differential. While Ford sought to build the units in-house at their Sterling Axle plant (a plant they had just opened a few months before), it wasn’t long before they were slapping this rear-end into every truck and full-size sedan Ford produced.
What Is So Special About The Ford 9-Inch Rear End?
One of the reasons that the Ford 9-inch rear was so loved was that it was a semi-floating unit with a drop-out axle. Most rear ends of the day were serviced through the rear cover, which made them the bane of a technician’s work day. The Ford 9-inch allowed access to the internal differential gears through the front cover (after dropping the driveshaft). The differential gears came out as part of the front cover, and this innovative design helped technicians take the self-contained unit out and perform repairs on their workbenches. The design saved Ford technicians headaches and made them more efficient because they didn’t have to deal with repairing a gear housing inside a rear end still hanging from the car. (They also found that performing repairs on new disc brakes being used in full-sized Fords was much easier with the 9-inch rear)
The front pinion shaft sub-housing had a 5-bolt pattern rather than the 10-bolt sequence used on GM products. (Previous Salisbury/Spicer designs had been used since the twenties). It wasn’t long before racing teams found that this simpler center section allowed mechanics to get access and made gear adjustments much faster. Racing teams could unbolt axle housings and found that they could make quick adjustments to the gear ratio, ring and pinion sets, and axle tubes with this simple Ford rear end.
One of the features of the Ford 9-inch rear was that the pinion depth could be adjusted easily.
Ford used a 3.0:1 to 5.0:1 gear ratio for its nine-inch rear along with increasing the tooth contact of the gears by lowering the pinion gear on the ring gear. The application helped to make the rear end more efficient and stronger, but it also sacrificed some efficiency.
The new design didn’t just rely on the ease of access to be special, the whole design is a work of art in its simplicity. The axle shafts were secured with reinforced interior retainer plates rather than flimsy C-clips that tended to break when stressed. A large ring gear allowed for more contact between the gears and this increased surface helped the differential to withstand excessive torque loads.
Many truck owners were crying for pickups that could haul heavy loads and were ecstatic when Ford put this rear-end on its trucks. Both performance and power numbers increased, and as the towing and load numbers went up, Ford trucks showed a significant difference compared to GM and Dodge. Suddenly, Ford was gaining attention in the truck market again.
The rear end was as durable as it was sleek, and it could literally run forever. They were placed on many of the F-series trucks and E-series vans, in addition to heavy full-sized sedans and SUVs like the Ford Bronco. Given the strains that off-road warriors, farmers, and ranchers put their trucks through in the middle of pastures, Ford had to have a rear differential that would last no matter what kind of punishment it endured.
Over the years, Ford made several variations of the 9-inch rear end, experimenting with increasing the thickness of the housing, and trying to combat the increased friction that the larger ring gears were producing. The increased heat often meant decreased fuel economy, which Americans began to realize in the 70s after going through two oil embargos and watching the price of gasoline wreak havoc on their budgets. Eventually, Ford sought to modify the rear differential to produce better torque and not use so much fuel while doing so.
Today the 9-inch and its aftermarket renditions have ruled the drag racing and hot rod worlds. This is where the 9-inch really made its mark. Due to the ease with which the third member could be dropped and replaced, the racing teams often had different third members tweaked at different gear ratios to that they could be tested and swapped with ease. Even though Ford pulled out of NASCAR in 1966, their cars continued to perform on the track and in truck events and the 9-inch crossed the finish line on numerous occasions. The rear end was so well loved that NASCAR racing teams adopted it for more models than just Fords.
Despite the fact that Ford hasn’t made the rear end for almost forty years, any number of aftermarket companies both manufacture and sell every component of the 9-inch differential.
It isn’t hard for customers to find parts for installation, as the supply is probably stronger than it was when Ford was actually sending units off of the production line.