Global West Suspension manufactures a 1958 Impala rear upper A-arm for differential.
The rear upper A-arm controls two important aspects of the rear suspension. First, it controls the lateral location of the rear end in the chassis. Second, it controls the pinion angle.
There were several design flaws in the original suspension because the stock pinion angle was adjusted by a rubber offset bushing. As time progressed, the rubber bushing would loosen when the driver went over bumps, thus distorting the pinion angle. Even with repairs, the angle’s position would eventually shift again.
The design was only used for one year (1958). The following year (1959), the Impala went to an upper trailing arm for adjusting the pinion angle and then used a panhard rod for controlling the lateral location of the rear end. Technically the 1958 upper arm had better lateral control but terrible pinion angle control.
Global West part number TBC-58 upgrades the 1958 upper A-arm to a tubular design and substitutes the rubber bushing to a spherical bearing; the new rear end mount, which houses the bearing, replaces the factory mount with a spider-like bracket. The bracket/mount permanently sets the pinion angle with shims, and the bearing allows the differential to pivot properly without deflection.
The kit can be used on a 9 inch Ford or a stock Chevy differential. Installation requires welding to the differential housing.