True Connections

Last month we discussed rear ends. In this article we will continue on to the trailing arms. The 1964-1972 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” 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 to 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 was welded to the arm, that boxed the arm for strength and also have 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 jack stands and support the rear end with a floor jack. The arm is unbolted using a ¾” boxed end wrench and a ¾” socket and ratchet. Raise or lower the rear end 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 2 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” 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” bit on a drill press. The mounting holes need to be located in the side of the arm on 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 rear suspension and make the car 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 4spd 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 1964-1967 style brace was formed into a U shaped channel and is available reproduction. The 1968-1972 style was much simpler looking like a piece of angle iron and is easily found used.

The upper trailing arms were again 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 rear end.

This style can also be fitted to the 1965-1967 arms which were the same dimensions but were not adjustable. The 1968 –1972 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 but these items can get very pricey and it all depends on how much road racing you actually do. Next month we move on to rear disc brakes.