The fiberboard insulators in prewar electric train motors can break or warp over time. If that happens, you have options besides plundering one from another motor. Here’s how to replace or repair the fiberboard insulator in a train motor.
Gluing fiberboard back together
If I have the piece that broke off, I just glue it back on. I prefer to use epoxy rather than a PVA-type glue like Elmer’s. Epoxy won’t cause the piece to warp.
For added strength, I add some plastic to back up the repair. Ideally I put it on the side that goes inside the motor, to make it harder to see. Old used-up gift cards are great for this. A piece repaired this way will be stronger than the original, though it won’t be period-correct.
If I don’t have the piece that broke off, I’ll sometimes just fashion a replacement from gift-card plastic, then epoxy it in, using a backer piece. This obviously gives you something that doesn’t at all resemble an invisible repair. Sometimes I’ll paint it afterward, to make the new material blend in better. You can still tell it was repaired, but it may take a second glance to notice. At least you don’t have modern corporate logos screaming out that the part was fixed in 2019.
Creating a new fiberboard with period-correct material
If the board is unsalvageable and reproduction parts aren’t available, you can make your own. You can easily fashion a replacement from an old gift card. And while I probably wouldn’t mind that solution for a postwar Marx locomotive, that just doesn’t quite feel right for a prewar locomotive.
But the material train makers used in the 1920s is still available. The composition might not be exactly the same as it was then, but modern 100% cotton material looks and feels like the old stuff did. It just doesn’t have anything else mixed in.
McMaster Carr sells hard fiber in its fishpaper section. For toy train applications, you’ll want one of the thicker versions, probably .02 or .03 inch. Measure your existing part’s thickness and pick the closer match. .03 inches is about .75 mm, and .02 inches is about .5 mm, if you find it easier to measure in metric. Remove the pickup assembly, then trace the old part onto the board, then cut it out to the rough shape and then shape it to match the old with files or a rotary tool.
The smallest section McMaster Carr sells is 12×12 inches, so you can make a lot of replacements from one sheet. Be sure to store your leftovers in a dry place to keep it from warping. The McMaster Carr material is gray, which probably won’t match the original. It will work as-is of course, or you can paint it to more closely match the original color before you put the pickup assembly on it.