All of us who deal with brass models of railroad equipment (mine are HO scale), eventually deal with the metric screws that they have. Sometimes this is a hassle, sometimes not. Last year (partly at the request of an email inquiry), I wrote about modifying models to use or avoid the truck screws on these models. (that post is here: https://modelingthesp.blogspot.com/2020/09/those-pesky-truck-screws-for-brass.html ).
(And incidentally, in the post just cited, I showed how I have made washers when replacing brass model truck screws of the shouldered variety, with straight screws. In fact, NorthWest Short Line offers quite a nice thrust washer which will work with most brass trucks, 2.0 mm ID and 4.2 mm OD, and they cost $1.29.)
Today I want to turn to coupler screws. Usually brass models have these screws in the box, but I have purchased second-hand brass that does not have any screws. And in at least one case, I carefully put the coupler screws for a brass model in a “safe place,” which was indeed very safe, because I haven’t found it yet. Either situation requires knowing what size these screws are so that you can replace them.
I thought to myself, “Well, this is a common problem, so it must be well covered on the Internet, one of the many times that ‘Google is your friend,’ no doubt, and I can just look it up.” Wrong. I could only find a couple of mentions of specific models, nothing in general. That’s why I wrote this post.
Let’s begin with what you see. What you hope to find when you look at the coupler pad of a brass model is a combination of screw holes, both a pair of holes for the side shoulders on the Kadee coupler box, and a center hole. An example is shown below (a Challenger brass GS gondola). There is also a hole at the back of the coupler pad, which I’m told is for a European coupler box, but that’s hearsay.
Personally, I always prefer the center-hole mounting, and usually cut off the Kadee side-mount protrusions. That center screw is usually a 2.0 x 4 mm screw, preferably pan head. In the photo below, you see a Kadee coupler box, with “ears” cut off, installed with a 2.0 x 4 mm screw. The coupler is a #58.
I should mention that I have encountered locomotive tenders that did not have this 2.0 x 4 mm-size screw, but most brass models in my experience have this size center screw hole in the coupler pad.
Unless, of course, they don’t have a center hole, but just the two side holes (and that rear hole). An example of this is shown below. It happens to be a Precision Scale tank car, but I should hasten to add that many PSC brass models do have center coupler-mount holes. This particular one didn’t.
The screws for these “side” holes are usually 1.7 x 4 mm or 1.7 x 5 mm screws. The drawback now is that this coupler pad requires use of the Kadee coupler box side mounts, and on some model cars, they interfere with truck swing. Here is my installation, again a #58 coupler.
One solution, of course, is to drill and tap the center of the pad for a 2.0 mm screw. Now I know at least some readers who are asking themselves, where the heck would I get metric screws and taps and all that. For the 2.0 mm size, a 1/16-inch drill is the tap drill, but you still need a tap.
NorthWest Short Line has sold these types of screws, along with tap drills and metric taps, for many years, and they still do (the screw offerings are here: https://nwsl.com/collections/screws ). The assortments are the bargain. But many on-line hardware suppliers sell metric screws and taps, so shop around.
I should also mention that I know U.S. modelers who hate metric screws and simply drill out the 2.0 mm hole and tap it for 2-56. That’s your call, of course. I confess to having done that on the brass locomotive tender I mentioned above, the coupler screw for which I just could not identify (and it wasn’t 1-72 or 0-80, either). But ordinarily I prefer using the screw holes that are provided.