Walthers 70-ton 46' USRA Mill Gondola

In appearance, this is a very nice model, but to make it operate smoothly on my layout, there were a few changes needed.

The first problem was the coupler height.  According to my gage, the couplers were not only loose to move up and down, they were somewhere between a third and a half of a knuckle too high.  Stated differently: when coupled to another car with a properly positioned coupler, the upper half of one knuckle would only be in contact with the lower half of the other.

Why is this important?  On rough track, or (because the coupler was free to swing up and down excessively) when a train was under a lot of tension, it might be possible for this coupler to slip over the top of the one to which it was supposed to be mated.
It is important to make sure couplers on all your cars are very close to the right height, because two couplers at opposite extremes of too loose a tolerance will eventually find each other and on rough track or vertical curve (such as is found at the top of a grade) will part ways.
I found the second problem when I tried rolling the car over some of the industrial trackage on my layout.  Under certain conditions of sharp curves and a tilted car body, I could hear something rubbing underneath.
Why is that significant?  The trucks need to be able to move freely, not just to pivot, but also to rock back and forth and a little side to side, to keep all the wheels on the rail through undulations in the track.  If something's rubbing, it means the truck is being prevented from moving freely enough, and one or more wheels are about to leave the rail.
Laying the car on its back, I placed a piece of 18" radius sectional track on the wheels, and ran it back and forth while tilting it from side to side.  The problem was the flange of the wheel closest to the middle of the car was rubbing against one of the cross pieces of the frame.

Adding shim washers between the trucks and the body is one way to cure frame rubbing, but that would have made the coupler height worse.  The solution I chose was to remove the edge of the underframe that the wheel flange had found.  No one's going to see it anyway, unless there's a wreck.

I removed the trucks, and found there was already a 0.007" shim washer there, which I removed.  When correcting coupler height problems, every little bit helps.

With a sharp model knife, I beveled the edge of the cross piece until I reached the other edge, then investigated the coupler problem.

To correct the looseness in the coupler, and to lower its height relative to the body of the car, I removed the coupler and put in a rectangular shim.  I made the shim from a piece of 0.020" styrene sheet by punching a 1/8" hole in it and trimming it to fit into the coupler pocket.  I made the hole with a 1/8" paper punch I had purchased from Hobby Lobby and cut it to size with scissors.  I made it short enough so it could not be seen from the outside once the coupler was reinstalled..

When I reinstalled the couplers, and began to tighten the screw on the pocket cover, the couplers bound up and wouldn't pivot.  Replacing the cover with flat styrene solved that problem, as the original cover has a raised surface designed to keep it centered when the screw is put in place.  I just turned the original cover over and used it that way.  I also substituted a flathead screw for the original, after countersinking the hole in the cover a little with the point of a drill bit.  No great precision is needed here.

So as it turns out, the amount I removed from the frame was enough to prevent the rubbing, even without the 0.007" shim washers.  And while the coupler height still isn't perfect, I think it will do.

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