A question for 1/2a historians.
In looking at some of the older half A engines that I have sitting around I've noticed that they all seem to have a propeller shaft that has a 5-40 thread. My sample is a bunch of Cox engines, a couple of OK Cubs and a Baby Spitfire. I'm wondering if there was a specific reason for using the 5-40 thread. Was it a more common thread at the time? It seems like using a more common thread like the 4-40 would reduce production cost.Any engine historians out there?