15 jan 2020
This information is intended to help assemble or diagnose Rambler trunnion front suspensions, and assumes you have a factory Technical Service Manual (TSM) or you're just winging it (heh heh) and that you have basic mechanical competence. This isn't a how-to for beginners. Though it's specifically for a 1963-1964 Ambassadors or Classics it applies in general to all of the "big chassis" cars (eg. 10, 80 series, not the Americans which are different) up through the 1970's when they switched to a ball joint on the upper arm.
While trunnions are really old fashioned, and pretty much all manufacturers had switched to ball joints by the 1950's, even, Rambler hung on to them for a long time; maybe they had a barn full of the damned things out back. Trunnions work well; even poorly maintained they last nearly forever (mine are 40 years old and still in fine condition) but are subtle and tricky to install right, and the information is simply not in the technical service manual.
(Though they do require some maintenance! They're not magic! Wayne Lamothe inherited a '63 Classic that someone had managed to, apparently, never grease. It takes a close look, in a picture, to see that the threads on the trunnion are worn down to where the cap simply slides off! (The driver was never in any danger as the arms with spacer holds the assembly together by itself, if necessary.)
PLEASE NOTE: You must have a copy of the factory TSM if you plan on doing anything more than the simplest maintenance on cars this old. This page assumes you have one.
All of the photos and procedures were worked out on my current car, a 1963 Classic wagon. It is dead-stock except as noted here.
The Rambler front suspension is actually quite a nifty thing. Technically it's very sweet; strong, very light, parts interchange over nearly a 20 year period, and you can bolt 1980's disk brakes to it with no modifications at all. Once you have them sorted out, they work great, last a long time and are very reliable. Mostly people whine about them because they don't understand them. That said, getting the damned things assembled right is tricky, but once done they will be trouble-free.
The two big gotchas with this suspension are:
I've worked out safe and repeatable solutions to these problems. Each has a section below.
The trunnion consists of a cast steel center cross with bronze bushing, two doubly-threaded cap nuts, a thrust bearing and assorted hardware; the steering knuckle is shown at the top of the photo. If lubricated at all, there is very little to wear out; recommended interval is 30,000 miles, and you need to remove the vent plug to grease it (RTFM).
The thrust bearing carries the downward suspension force and eventually wears, reasonably gently, and in a fail-safe manner. There were two outer-diameter bearings used in trunnion suspensions; the smaller diameter used on the early (63-64) cars are made of unobtanium, but allegedly you can simply install the larger one and let the dust-cap ding on the A-arm, the race is supposedly the correct size and only the dust cover is larger. I haven't tried this, but eventually I'll have to. My thrust bearings were still good at 190,000 miles (take that! late-model cars). They were in good shape when I disassembled for initial inspection when I first got the car, at 87,000 miles.
When you first look at the damned trunnion system, you may find the design to be somewhat baffling (I did). That's because there's actual subtlety in the design. It's rather clever in fact.
A trunnion is essentially a U-joint, and works the same as one in a drive shaft. The steering knuckle runs through the bronze bush, vertically, and allows the wheels to turn. That part is as simple as it appears to be.
It's the upper "A" arm assembly with the two doubly-threaded caps that is a bit of a puzzle, at first. The caps thread into each A-arm half with a weird shallow thread and simultaneously threads onto the trunnion itself. If all five parts (two arm halves, two caps, one trunnion) are not threaded together exactly correctly you'll stress the hell out of the parts and ruin them driving. The width of the wide end of the A-arm assembly (that fits into the chassis) depends on correct trunnion assembly too, and will ruin the press-fit rubber bushings if they rub on the chassis. The TSM is utterly silent on these little details.
(One of the subtleties is the weird double-threading. It's for safety, basically. The upper A-arm is held together not only by the obvious grade 8 bolt and spacer, but by the five threaded trunnion components. The trunnion would work just fine, in the normal case, if the caps simply threaded into each A-arm half and the trunnion had plain bearings, but would catastrophically disassemble should upper or lower arm components fail, such as in an accident (or worse, be pre-stressed into later unexpected failure by a an earlier survived accident). The puzzling thing is that as the suspension moves up and down, the steering knuckle actually moves fore and aft, due to the threads. It's just a small amount, weird-looking, but normal. Good engineers design for the worst case, not the best case!)
Thread a cap into each A-arm half. Tighten to a dozen inch-pounds by hand at this point just to take up slack.
Install the grease seal onto the threaded trunnion arms (wide end out) and thread the trunnion onto one of the arms. Just run it down, do not tighten.
now do the same with the other arm. it now begins to look like an A-arm.
the following steps take advantage of the fact that the trunnion threads into the caps. NOTE: this pic shows the arm assembled, with spacer and bolt tightened. the bolt isn't inserted until these steps are complete. i didn't take enough pictures, sorry. what i wanted to show is essentially the previous image but held as shown here.
STEP 1: place the A-arm on a flat table and set the spacer in place. it probably won't fit right the first try, likely too loose (a gap) or too tight (requires force, pushes the arms apart).
STEP 2: to adjust, hold one arm-half in one hand, rotate the other arm-half; counter-clockwise to increase the spacer's gap, or clockwise to close the gap. at this step, do not worry if the trunnion isn't centered between the arm ends. that's done later.
repeat until the spacer fits snugly but without any force or gap. when you have it correct, insert the bolt through the arms and spacer and tighten it. if you have it correct, with the spacer-bolt tight, the trunnion will turn freely in it's place between the arms. if it binds, you have it wrong, remove the bolt and repeat the three steps.
As a further check, measure the distance between the outer, wide part of the A-arm assembly. It should be 8.25" inside to inside. With the upper arm bushings pressed in, the A-arm assembly should *just fit* into the chassis. If you've assembled the trunnion wrong, it will either scrape on the inside of the chassis (trunnion caps too far apart) or jam onto the chassis inwards (trunnion caps too far apart). If you've got the spacing correct, the trunnion will pivot freely and the A-arm will fit nicely into place in the chassis. If it doesn't just fall into place it's not right!
at this point the A-arm looks like an A-arm; however the trunnion itself is probably not centered, closer to one arm or the other. now we fix that, again taking advantage of the threadded trunnion. the A-arm assembly is symmetrical; there isn't a top and bottom or left and right. simply turn the trunnion 180 degrees and check again. when centered, the side with the threads for the spring perch is "up". done!
Once you've got the A-arm assembled right, the rest is easy. The steering knuckle simply runs vertically through the thrust bearing and trunnion; don't forget the O-ring grease seals. The castellated nut (with washer) on top should just be run down with your fingers; there's no need for preload here at all. I finger assemble and move to the nearest hole for the fattest cotter pin I can find (a steel pin would be better). You can see the nut and pin in the image to the right, where it lives under the spring perch, not yet installed.
This is another one of those things left out of the TSM.
The upper A-arm pivot bushings, the ones that the caster and camber adjusting eccentrics run through, should have a dished washer placed or crimped over the inner sleeve, where it faces the chassis, on the inside of the upper A-arm (see first photo).
The problem is that most replacement press-fit bushings are not exactly correct; the rubber is brought out flush with the end of the inner sleeve. Unfortunately on the early cars this doesn't work right; the inner sleeve works its way into the chassis pivot hole and eats away at the end of the sleeve and elongates the hole. The solution is to cut away the rubber, as shown in the photo, and install the dished washer as it should be. The washer not only transmits the (small, but non-zero) side forces evenly, but the dished shape helps installation.