Rambler American front suspension 1958 through 1963

Lower control arm assembly, first method

Written by Tom Jennings 05-24-2022, edited by Frank Swygert 12-01-2025

This is my original, now deprecated, lower control arm assembly procedure. I think that the current method is superior but this may still contain useful hints.

Lower control arm assembly procedure

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 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. It is the least-change solution.

STEP 1: Trunnion cap and arm threaded end

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 has ruined many parts. I’m having approximately 50 % failure rate. I don’t yet have a solution to stripped or cross threaded arms except to find replacements. The following are unusably stripped. (Frank: I have had reasonably good luck with peening [hammering] the pressed through ends to tighten the hole. Care must be taken to not deform or tighten the hole to the point the bushing cap won’t start in the hole. Once the spacer/shock mount is installed and tightened the cap MUST thread into the hole when tightened on the stud though. Install one side, then the other. I ALWAYS put a tack weld near the outside end of the arm on the joint between the cap and arm. Just a good tack is all that’s required. This will prevent the cap from working loose. I’ve lost one on a trip — not fun! It came off a full two years after the suspension was rebuilt. The tack weld should be placed where it can easily be struck off with a chisel or ground off should the trunnion or arms ever need to be removed. The tack also makes inspection easier — if the tack has broken loose the cap is starting to bind and should be removed, cleaned, and greased then reinstalled after inspection to make sure it’s still usable.)

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 would not tighten this to more than 25 ft/lbs.

STEP 2: Install trunnion cap in one arm

Do this now or later: thread one cap into one arm, and leave the other out for now. If you have one arm that is less satisfctory than the other, then put that cap in now, tightened to XX ft/lbs. For this one cap, this will be its final assembly and torqueing.

STEP 3: Install pivot bushings

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

STEP 4: Trunnion assembly

This is the part of this process most affected by bend errors in the arms.

Your nemesis.

Add the grease seal and apply decent grease to the threads. The seal is not fussy; don’t bother with OEM, they’re all old and rotten. Fat O-rings work (3/4″ ID, .125″ or larger fatness) or U-cup hydraulic seals (surprisingly cheap and available from industrial supply stores). (Frank: I always use anti-seize compound topped with grease)

Push one arm half slightly out of the way, and thread the trunnion into the cap that you installed in the arm at the beginning. Bottom it, back it out until the big hole is parallel to the arm (“working position”) then unscrew it one turn further.

Add seal and grease to the remaining side. Now spread the arm halves apart and lift the arm end over the remaining trunnion threads. BIG HINT: Look at the threaded trunnion end poking out the hole in the arm, where you will soon install the trunnion cap. It should be centered in the hole. If it is not, the other arm, with the trunnion cap tightened in it, is bent.

This section requires care, and my recommended dimension may not be correct for all arms, but I had to start somewhere. I will update this as I get more experience, and if you have suggestions, please let me know.

Thread the remaining trunnion cap onto the arm until it just touches the arm, then back it off a half turn or so. Using a precise rule, hold the two arm halves apart — it will take a bit of force from one hand — exactly 2.500″ inches, measuring from the inside of the arm stamping directly in line with the trunnion. Holding it that distance, thread the cap on until you feel it catch the shallow threads in the arm.

If you are lucky and have a good arm, you should be able to thread it on by hand, or with a 3/8″ drive ratchet and 1-1/8″ socket. Be extremely careful not to cross-thread this, but it being threaded into the trunnion keeps it concentric with the arm hole.

The threads inside the cap, and those outside of the cap into the arm, are of different pitches. Threading the cap in moves the arm in.

If all goes well, the distance between the arm tips will reduce to 2-7/16″.

Ideally when the cap has bottomed on the arm, and is tightened to a moderate 20-25 ft/lbs, that the spacing between the stiffener mounting flats is precisely 1.500″. I suspect that this will not be the case…

At this time insert the stiffener/spacer and insert the bolt, leaving off the nut for now.

…if the spacer is loose between it’s flats, as the gap visible here shows, the gap will need to be shimmed to zero. Alternatively, you can install the nut on the stiffener bolt, and tighten it. If the trunnion turns freely in it’s caps, you’re done! That’s the goal. Probably it will bind, but if it doesn’t bind to the point you can’t easily rotate it you’re good.

If it does bind too much loosening the nut should allow the trunnion to turn freely. Shim that gap until it doesn’t bind — problem solved.

If the stiffener will not fit in the gap without force, and doing so binds the trunnion, then the spacer will have to be reduced in height or the arms straightened. If it doesn’t need much reduce the height of the spacer.

Check that the gap, if any, at the stiffener is parallel. If the gap is wedge-shaped, it means one or both arm halves are bent.

The critical end result is to have the spacer installed and tightened to 80 ft/lbs, while having the trunnion turn freely within it’s caps. I have done precisely one car where the trunnion turned very freely. My threshold of acceptability is that the trunnion turn when held with thumb and finger. If it is too tight, it will overheat from friction and run dry and fail. Torque applied to the caps, from the trunnion’s motion, will loosen the caps, leading to RUD.