Last month we discussed rearends. In this article we will continue on to the trailing arms. The '64-72 A-Body lower trailing arms were all basically the same. They all used the same bushings, were the same dimensions, and are interchangeable, left and right. They were all constructed and mounted in an upside-down U-shaped channel, but if the 7/8-inch rear sway was added as an option, the lower control arm was boxed since the sway bar bolted to the arms. We get a lot of questions regarding why the lower trailing arms need to be boxed when adding a sway bar. If the arm was not boxed and you tried to install the sway bar with bolts on the inside of the arm, the steel of the arm is too thin to withstand the stress the sway bar would inflict on cornering and would eventually tear the arm. If you tried to install the sway bar with bolts going all the way through the arm, the arm would simply collapse as you tighten the bolts. To combat these problems GM devised an insert, which is welded to the arm, boxes the arm for strength, and has drop downs to prevent collapse. Fortunately, the insert is available through the aftermarket to convert any standard trailing arm to a boxed trailing arm.
The best way to convert the lower trailing arms is to remove them from the car first. Be sure to properly support the vehicle with jackstands and support the rearend with a floor jack. The arm is unbolted using a 3/4-inch boxed-end wrench and a 3/4-inch socket and ratchet. Raise or lower the rearend to relieve stress on the bolts so you can easily tap them out. Once removed, the arms need to be cleaned for welding. The best way is to bead-blast them, but a wire wheel on a grinder will suffice. Just make sure the two edges along the bottom of the arm are bare metal. Next, place the insert into the channel and center it between the two bushings. We like to MIG-weld the insert like GM did, with welds about 1 inch long, spaced evenly apart along each side. Once the arm has cooled, the sway bar mounting holes need to be drilled using a 7/16-inch bit on a drill press. The mounting holes need to be located in the side of the arm on the center of each of the two drop-downs of the insert. Once the center is located, center-punch your mark and drill all the way through the arm. Now is a good time to replace the bushings if necessary and repaint the arm to prevent corrosion. The arms can now be reinstalled with the sway bar mounting holes toward the rear of the car. Once everything is tightened up, the sway bar can be installed with the use of shims if necessary.
This will strengthen up your car's rear suspension and make it handle considerably better. Another factory high-performance item that is useful is the upper to lower trailing arm brace. This piece was mainly used in four-speed or big-block applications, and it spans the gap between the upper and lower arms, installing on the front mounting bolts of each trailing arm. The '64-67-style brace was formed into a U-shaped channel and is available in reproduction. The '68-72 style was much simpler, looking like a piece of angle iron, and is easily found used.
The upper trailing arms again formed a U-shaped channel, but they were never reinforced. In 1964 GM used an upper arm that had a tab welded to it where a special bolt and washer was inserted to adjust the pinion angle of the rearend. This style can also be fitted to the '65-67 arms which were the same dimensions but were not adjustable. The '68-72 upper trailing arms were not adjustable and were a few inches shorter than the other years.
There are plenty of aftermarket manufacturers of A-Body trailing arms, such as Global West and Hotchkis; for a price, these arms offer greater adjustability and added strength, but it all depends on what type of racing you actually do. Next month we move on to rear disc brakes.