When making a noticeable increase in tire diameter to the drive wheels of the rearend, it’s always best to also change the ring-and-pinion gear set. Increasing or decreasing tire size on your drive wheels will affect the final drive gear ratio and engine rpm.
Garth Wise owns a bitchin’ ’31 Ford five-window coupe that is powered by a Buick V-6 with an automatic transmission. He’s been driving it with 29-inch-diameter BF Goodrich radials in the rear. The Ford 8-inch rearend was set up with a 2.80 ring-and-pinion gear set. He decided to make a tire change from the normal 29-inch BFGs in the rear, to a pair of cheater slicks to create a more aggressive look. Because he was going from a smaller 29-inch-diameter tire to a larger 31-inch-diameter tire, the change would affect the coupe’s acceleration. To maintain a compatible gear ratio, he decided to replace the 2.80 gears with a Motive Gear 3.55 ring-and-pinion gear set. The gear change would allow the Buick V-6 to cruise at 2,400-2,600 rpms on the highway.
We approached Currie Enterprises in Corona, California, to assist with the appropriate gear change. Follow along as Armando Nila, a Currie installation technician specialist, disassembles the Ford 8-inch third member and installs the Detroit Truetrac carrier, Motive Gears 3.55 ring-and-pinion gear set and a Currie third member setup kit with bearings and seals.
After pulling the third member from the Ford 8-inch rearend, disassembly of the third member begins with the removal of the ring gear carrier main caps and adjusters.
The ring gear carrier, pinion gear and pinion support housing are removed from the third member housing.
After everything is removed and disassembled from the third member, the parts are laid out on the bench for inspection.
The new Ford 8-inch third member components installed are a Motive Gear Performance 3.55 ring-and-pinion gear set and a stronger Currie webbed cast iron third member case. To eliminate the previous peg-legged rearend, a Detroit Truetrac carrier is installed.
A Currie pinion support housing is used along with a Currie third member setup kit.
A hydraulic press is used to properly press on the new Detroit Truetrac carrier bearings. The Truetrac unit will allow the independent axles to interface and become one with equal traction output. This allows the rear wheels/tires to rotate at independent rpms while cornering.
Armando uses a torque wrench to tighten the ring gear carrier bolts to the proper torque specs at 60 lb-ft on the ring gear.
The hydraulic press is used to press the bearings onto the pinion gear shaft.
After the pinion support bearing seal is pressed into the housing, the pinion gear and support are assembled.
The pinion gear yoke slides over the pinion shaft splines and is secured by tightening the gland nut on the shaft.
Armando installs the pinion support housing onto the third member housing and torques the support housing bolts to 60 lb-ft.
The ring gear carrier assembly and bearings are lowered onto the bearing saddles of the third member housing.
A 5/8-inch socket is used to secure the third member main cap’s bearing bolts.
A 5/8-inch socket and torque wrench are used to secure the main caps and adjusters to 60 lb-ft.
It’s very important to set the correct ring-and-pinion backlash between the ring-and-pinion gears. The backlash is set using a dial indicator mounted firmly to the third member housing. The indicator is mounted in such a way that the plunger intersects the ring gear surface. The dial indicator plunger is set against the ring gear’s surface. A spanner wrench is used to rotate the ring gear that indicates the amount of backlash. The dial indicator reading should be approximately .008-.012 inch. To make adjustments, shims are placed behind the rear pinion bearing.
Armando applies gear-marking compound to the ring gear to check the pinion gear depth and ring-and-pinion gear mesh pattern. After rotating the ring gear back and forth, an impression appears where the ring-and-pinion intersects. The sweet spot is in the center of the ring gear teeth. Adjustments are made by adding or removing shims from the pinion gear.
The complete Ford 8-inch third member.
382 North Smith
Corona, CA 92880
Text and Photos by Bob Ryder