I recently picked up a beautiful Sunset/3rd Rail AC-9 (2-rail) with original coal tender. But I was unhappy with the fact that the valve rods, combination levers, and radius rods flopped up and down so much while running, and that the valve rods hung down from the valve chests at a disconcerting angle even when the engine was not moving. I have had this problem with other models, too, but I've never tried to correct it. I haven't seen anyone else propose the particular fix that I came up with, so I thought I would post it here.
On a real locomotive, the valve rod is supported at its back end by a small crosshead that slides back and forth in guides that in turn are supported by brackets extending back from the valve chest. This small crosshead supports the linkage between valve stem, radius rod, and combination lever and allows the valve rod to move back and forth smoothly under their influence.
None of the the O-scale models I’ve ever seen provides support for the back end of the valve rods, so the linkage can just flop around, particularly if there is a lot of slop in the joints. This was less of a problem on older U.S. Hobbies models I have owned, I think because the valve rod usually was fairly large in diameter (not scale dimension), and was fitted into a little section of tubing in the valve chest, so that it had some support on the front end at least. On the AC-9, however, the valve rod is very delicate and tends to hang out of the valve chest at a weird angle, so the whole setup looks sloppy and does not operate smoothly when the engine is running. I had the same problem with my Rio Grande L-131.
After looking at the problem for awhile, I came up with the following solution, which seems to be working well so far. I put a very shallow V-shaped notch in a piece of 0.025” - diameter phosphor bronze wire, then bent a little tab on each side so that the wire could be slipped underneath the valve rod (at 90 degrees to its motion) with each tab resting on one of the brackets that support the valve rod crosshead guides, which are usually modeled as a small rectangular box at the very end of each bracket. I adjusted the depth of the "V" so that the phosphor bronze wire supports the valve rod exactly parallel to the piston rod below. I then secured the wire in place against the bottom front corners of the valve rod crosshead guides with ACC and trimmed it to length. Adding a drop of black paint made it essentially invisible.
I had intended to solder the carrier wire in place (just using ACC to test if the idea would work), but the ACC seems to be sufficient and makes installation cleaner and easier. The valve rod now rests on this little carrier wire, and can slide back and forth on it, while remaining parallel with the piston rod and not jumping around. A drop of oil on the carrier makes it a very smooth motion.
To my eye, the whole valve gear setup now looks and performs much better than before.
