Ford 9” Rear Axle Information
The 9” Ford rear axle, so called because of a 9” ring gear diameter, is the choice of many circle track racers because of its strength, availability and ease of maintenance. Produced from 1957 to 1987 by the Ford Motor Company, it was used in full size Ford, Mercury and Lincoln passenger cars, ½ ton pickups and vans and even in some intermediates.
The Ford 9” was produced with 28 and 31 spline axles that even in their smallest form, are larger in diameter than those in the 10-bolt Chevrolet rear axle. The gears and the carrier have proven to be significantly stronger than the 10 and 12-bolt Chevrolet, which use smaller 8-1/2” and 8-7/8” diameter ring gears, respectively. The housings are also noted for sturdiness with material thicknesses ranging from .150” to .250” thick.
The 9” rear axle was manufactured in seven different housing widths, from 57-1/4” to 69-1/4” wide, any of which may be ideal for a particular chassis. Determine housing width by measuring the distance between the axle flanges located on the outward end of each axle tube. Some axles are more desirable than others.
Favorites among circle track racers include the rear axles from in ‘66-’70 full size Fords and Mercurys, ‘71-’72 Lincolns and ‘68-’72 F100 Ford trucks. This axle is outfitted with 31- spline axles, large ball-type axle bearings and is 61” wide. Many of these units also have larger 3-1/4” axle tubes. The only drawback is that these axles have a 9-3/8” diameter ring gear, however, a traditional 9” differential assembly can be easily interchanged.
Favorites of leaf spring-equipped Camaro or Nova racers are the ‘79-’81 Lincoln Versailles and ‘77-’80 Ford Granada rear axles. These axles have extra thick wall housings and are fitted with factory disc brakes, 28 spline axles, large ball type axle bearings, leaf spring mounting pads and a 58-1/2” axle width.
All 9” rear axles share another benefit: a removable center section which allows the ring and pinion gears to be set-up on a bench, rather than in the housing. Gear ratios can be quickly changed by removing the axles and driveshaft, unbolting the center section, exchanging the ring and pinion and reassembling. This feature allows racers to carry extra center sections that are fitted with different final drive ratios. The most desirable center sections are made of sturdy nodular iron and can be easily identified by a large “N” cast in the outside of the casting. Heavy duty, aftermarket nodular iron and aluminum replacements are also readily available from Ford Racing and 9+.
The popularity of the 9” Ford has caused the aftermarket to produce a wealth of heavy duty parts engineered primarily for racing applications. Complete housings, carriers, spools, mini-spools, axles, housing ends, bearing and shim kits, pinion supports and yokes are all available from manufacturers such as Allstar Performance, Ford Racing, 9+, Ratech and Richmond. Spools and yokes are available in traditional steel as well as in lightweight aluminum and equally lightweight, but costly, titanium versions for racers looking to quicken their lap times. Additionally, ring and pinion manufacturers including Richmond and Motive Gear offer a large selection of gear ratios for racers looking for the perfect ratio.
All circle track race cars should use a spool, which directly replaces the stock carrier. A spool delivers the same amount of power to both wheels, unlike positive traction units that can be inconsistent. A spool is also lighter, simpler and more durable because parts prone to breakage, like the posi-traction unit, cross pin, spider and axle gears are eliminated.
Budget minded circle track racers can use a mini-spool, a special unit that inserts inside the existing carrier to replace the spider and axle gears, effectively “locking” the rear end like a traditional spool. A mini-spool is also a much safer and more reliable way to “lock” the rear end than the age old method of welding the spider gears. If a spool is out of the question, another option is the “Detroit Locker”, a very heavy duty, ratchet type differential.
There are several helpful hints that will extend the life of the Ford 9”. Most replacement 9” ring gears interfere with the area of the case where the pinion pilot bearing is located. Rather than machining the housing, the ring gear should be carefully chamfered using a small die grinder to provide adequate clearance. Rears used in oval track applications should have either a custom baffle or internal housing seal to prevent gear oil from being built-up in the right axle tube.
If the rear axle being used has a stock Ford pinion support, make sure it has the #HM89443 rear cone. A heavy duty pinion support is strongly recommended for any 9” rear. Heavy duty “Daytona” style supports are available for standard 28-spline pinions, use a larger bearing, and are popular with oval track racers. Regardless of application, all 9” rears should be fitted with a rear end filler bung and cap for easy filling. Most experts agree that you should always use top quality racing gear oil for the best results and reliability.