Rambler American Front Suspension 1958-63

Lower control Arm Assembly, Preferred Method

Written by Tom Jennings 08-01-2023, edited by Frank Swygert 12-01-2025

new 24 may 2022

This is the current lower control arm assembly procedure. The previous deprecated method may still be useful or contain useful photos. The initial steps are the same for both and there are other overlaps.

Given the complete lack of any direction from AMC, I have worked out the following procedure which I believe should be repeatable for new or used parts in moderate condition. I have added steps to deal with bent parts. This is a straight-up how-to but there is some discussion of the reasoning and logic behind my decisions. If you have corrections or better ideas please let me know.

Reality is that not only do we not have any new parts available, we do not even know what the correct dimensions are for them. Therefore I am shimming the stiffener (described in procedures below) to adjust the critical spacing necessary for the trunnion to not bind. I’ve determined experimentally that 0.020″ of error in the arms will cause some binding in the trunnion cups. Moderate binding will increase wear rapidly, and reduce the effectiveness of greasing them. Heavy binding will grind metal, cause heating and rapid degradation of scarce parts, and could lead to RUD (Rapid Unscheduled Disassembly). What’s written here is my personal experience, I am not responsible for work you do on your own car, this is not professional advice. Refund available on your way out the door.

Shimming (so far, well under .100″) has no effect on strength or geometry. This is the least-change solution.

BEFORE YOU START: Obtain a valve spring

A spring is required as an assembly tool. I used a used AMC 232/258 spring from my junk box. Almost any auto valve spring or equivalent will work, it should have moderate (20 – 50) pounds of tension when compressed to 2″ to 2.5″. The large-ish diameter id is required, and a major feature of this tool is that it holds the arm halves parallel and in tension. You’ll need a 3″ bolt, 1/2″ or 9/16″ diameter, to clamp it together. See photos below.

BEFORE YOU START: Examine critical parts

Before starting assembly ensure that a trunnion cap can be reliably threaded into each arm end. This is a critical step, as this is where decades of bad shop work and driver neglect has ruined many parts. I’m finding approximately 50% of the used parts I get have failed or are unusable. I don’t yet have a solution to stripped or cross threaded arms except to find replacements.

Here is a 1964 Ambassador upper arm for comparison. This is what the threaded arm end should look like. The “big car” upper trunnion is different from the “small car” lower trunnion, but the trunnion cap is identical. The big car trunnions are a good design, easy to assemble and have very little load so the arms are rarely damaged.

The cap has a flat area (remove the Zerk) and the threaded boss in the arm is flat; (try to) start threading the cap in gently and check it for parallel in two planes 90 degrees apart before wrenching it in. In other words, don’t you be the person who cross-threads a scarce part. Note that in the second photo here the two rules are not parallel; this arm is badly cross-threaded and I was unable to get a cap into it without further damage.

The arm end itself must be square — the self-tapped boss face parallel to the stiffener mounting boss.

Straight edge shows correct alignment in this plane….

…but not in this plane. This arm turned out to be unusable.

This is the correct fitment of the cap in the arm. This is not a high-torque fastener; there is no torque specification but I tighten these something like 40 ft/lbs. Do not assemble these yet, follow the steps below.

STEP 1: Install pivot bushings

I apologize for not taking enough photos for this. This step has no photographs on this page.

Install the stock-replacement type inner pivot bushings (control arm bushings) onto the stamped arms first, and loosely assemble the arms onto the pivot bar. Which should be in a big bench vise for assembling the lower arm. This work cannot be done with the pivot bar in the car.

You will not be able to remove the arm halves from the pivot bar after assembly so make sure you have the correct cupped washers installed. I generally tighten the nuts enough so that the arms stand up on their own.

I use fabricated soft polyurethane bushings instead of OEM type bushings. Whatever you use they should be installed by now.

STEP 2: Position components for assembly

For this and all subsequent steps the two control arms are installed onto the pivot bar in their final position and the pivot bar is clamped firmly in a bench vise, arms pointing up. Once the trunnion is installed the arms cannot be removed from the pivot bar. Make sure you have the inner cup washers and all necessary parts in place before proceeding.

At this step you’ll want to tighten the nuts on the pivot bar so the arms stand up in the vise.

At the outer (trunnion) end, insert the trunnion central casting in the big holes. It will just rattle around in the big holes. Then insert the valve spring and through-bolt.

Using a precision rule — emphasis precision — tighten the spring through-bolt until the distance between the inside of the control arm tips is exactly 2.50″ apart.

STEP 3: Initial installation of trunnion caps

This is the step I really regret not taking adequate photos.

Looking straight into the big hole at the end of the arm, you’ll see the trunnion’s threaded end there. At this point the trunnion can be positioned in the space between the arm ends easily, since it’s just rattling around in there now. Push the trunnion so it’s threaded end pokes out. Lubricate a cap with chassis grease and thread it a few turns onto the end of the trunnion.

Thread a cap onto the other end of the trunnion, poking out the hole in the other arm.

As you do this the cap will touch the big hole in the end of the arm. Lubricate the “threads” on the outside of the cap lightly and begin to thread a cap carefully (extremely carefully) into the arm end, making absolutely sure the cap is parallel in both planes as you go.

The trunnion cap is of course threading onto the trunnion on the inside, and the outside of the cap is also threading into the hole in the arm. The spring will compress further as you do this, while it holds the two arms parallel.

I go back and forth, advancing one then the other, and the photo shows this, with both caps partially threaded into both arms and trunnion.

STEP 4: Bottom trunnion caps onto arms

The caps should bottom when the space between the stiffener flats (where the spring is mounted) is 1.50″. Don’t torque the caps just yet, just bottom them onto the arm ends.

Since I neglected to take adequate notes it is possible that the resulting space will not be 1.50″ (probably larger). The solution is to determine the error (say 1/4″ too far apart) and adjust the initial spring setting from 2.50″ to 2.25″ and try again.

The pair most recently done by me ended up at 1.55″ and 1.60″. I bought shim washers from McMaster-Carr in various thicknesses, shimmed my homemade spacers and torqued the 9/16″-18 grade 8 bolt.

(Frank: I always put a tack weld on the joint between the trunnion cap and arm near the outside so that the tack weld can be struck off with a chisel or ground off should the cap need to come out again. This prevents the cap from working loose, especially needed if the arms were in questionable shape or needed the hole in the arms tightened with a hammer. It makes inspection easier also — if the tack is broken the cap is starting to bind on the trunnion stud. I have lost a cap on a trip a full two years after the suspension was rebuilt — not fun! If you can’t find replacement lower arms the caps can be fully welded to the arms, but I consider this a desperate measure. If a lower arm or the trunnion ever needs work again both will have to be replaced. If welding make sure the caps are pulled out near the outer end of the arms — they usually wear to the inside. )