I am in the process of commissioning a XIVth bascinet with brass trim that has Latin prayers engraved on it. I also wanted a screw thread in it for a plume holder and/or a crest. I know I have seen information on screw in plume holders as well as threading the helmet, but can't seem to find it now. I would appreciate it if anyone knew where I could find this information.

Thanks,
Al

Not sure if this is much help, but normally, I believe, the threading (if any) would be on the top of the plume holder, not on the helmet itself. The metal of the helmet would be to thin to hold any effective threading (I would think.) The period examples I've seen seem to have a hole on tope into which a plume holder might be inserted.
Sorry, no pics, and others may know more, but I hope this helps.
David