Friday, September 18, 2020

Those pesky truck screws for brass models

Probably every modeler who has owned or worked on a brass freight car (and even some steam locomotive tenders) knows about the “standard” truck attachment used. It is a shouldered screw with a fairly long shank, and a coil spring around it. When the screw is tightened to where the bottom of the shoulder reaches the bolster of the model car body, the spring is compressed and holds the truck against the bolster.
     I show two examples below, along with one of those springs. The larger screw, on the left, has a 2.0 mm x 0.4 pitch thread, while the smaller one at right has a 1.7 mm x 0.35 thread. You can see that the spring is as long or longer than the shank, even without the thickness of a truck bolster in place, so it will be significantly compressed when the screw is tight. Rotation of the truck around the screw, for example as a model car enters a curve, then requires substantial force. And if you want to operate the model on other than straight track, that’s the problem.

     There are several solutioins to this situation. One is to clip off the spring to shorten it. I usually start by reducing it to about two-thirds of its height, then shorten further if needed. This can work, but you can see in the above photo that the original spring has a flattened turn at each end, keeping it from binding against or snagging either the screw head or truck bolster. Cutting off one end can lead to binding problems, but it’s a simple solution and may be worth a try.
     I’m sure modelers know right away that it can’t work to just omit the spring. We all learned to tighten truck screws down snug, then back off a quarter turn. But omitting the spring on a shouldered screw is like backing off 15 turns. Not okay.
     What I usually do instead is replace the screw. Metric screws with both these thread sizes are readily available for purchase on-line from a variety of hardware suppliers and (relatively expensively) from NorthWest Short Line (see them at: ). Problem is, they are small screws and don’t have wide heads like the screws shown above. So they can’t secure a truck with a bolster hole large enough for the shouldered screws.
     The answer is a washer, though I have not found any commercial washers the right size (doubtless they exist, but I haven’t found them). The easy answer is to make one from styrene. I just use a paper punch to punch out a disk from 0.030-inch styrene sheet. Paper punches are all different diameters; the one I use is just right for shouldered screws, 0.165 inches. Then I drill the disk with a #50 drill as a clearance hole for the 1.7-mm screws. That gives you a washer like what you see below, with its clearance hole for the metric screw at right.

     When installed, the replacement metric screw and washer work fine. The example below is a Precision Scale model car for which I abandoned hope of using the original brass trucks, and substituted Accurail solid-sideframe trucks and, as you see, old Kadee wheelsets. But the shouldered-screw attachment problem remained, and I used the system described above, in fact the same screw size, 1.7 mm. You can see the washer and screw head in the middle of the truck bolster. This finally permitted layout operation of this model.

     I should mention one other solution to this problem: just give up on the metric threads in the model bolster altogether, and drill and tap for 2-56 screws. As these screws are larger, it works fine to do this, and especially in stubborn cases where you don’t seem to be able to get the metric screw and washer method to work, it is a practical alternative. But it’s more work than what I show above.
     This substitution of a straight metric screw and styrene washer amounts to the best solution I’ve found for dealing with the unsatisfactory truck attachments of brass freight cars.
Tony Thompson

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