Harry,
Like you I think replacing screws with the original thread pattern is important to a proper restoration/repair. Here is an option. Locate a supply of "engravers acid" used in the commercial trade primarily to take off background in coin dies and similar work.
The acid I have been using will dissolve a common iron finishing nail (diam .060") in about 4 minutes!
Recently made a bridal screw for a Purdey hammer gun using this acid. Like your issue, the threads were 40 TPI but the diameter was about .010" smaller than our standard 4-40 screw. By immersing the screw shank in the acid with a forceps and measuring the erroded screw a few times it worked perfectly. Also, the acid is useful for fitting small hardened pins/parts otherwise difficult to chuck up for grinding or machining.
Sadly, I bought the acid about 15 years ago from an chemical supplier on the US east coast and it is nearly gone now. The label on the bottle has long since disappeared and I can not find a new supply. This acid is light sky blue in color which no suppliers seem to have.
Jim Westberg
Jim, I think the screw end is the part that's damaged but it may also be the female thread that the screw goes into. I'll have to check that out with my friend.
That's a good tip though, thanks.
Harry
