I recently bought an Altas FP-7 from Ebay which came with horn-hook couplers held in place with pins. I want to use newer knuckle couplers, so I carefully removed the pins, but am unsure what is the best screw size to use for replacement. Did not want test with what I have in fear of cracking the plastic. Any suggestions? Thanks!
You can use the pin to hold the replacement Kadee coupler in the box(pin is "press-fit"),or
tap for a 1/4 inch 2-56 screw,or(last resort)use a tiny self tapping screw/wood screw.
You should be able to find 2-56 and tiny screws at the local hobby shop,or in the "doll house" hardware section at Michels,or Hobby Lobby.