3/8-24 and 1/2-20 are common threads for hand held drills and chucks.. How old is the drill and who and where manufactured?
Unknown for origin and age.
Agreed with the threads above as well as age. I have several connections that are
almost but not quite a "standard" thread. Among them are a couple of "egg-beater" manual drills. I have had to replace the chuck on one, it was smashed up pretty bad. What I did was use a hex drive chuck, unscrewed the drive bit, and rethreaded the drill. It was smaller that your's, 1/4 chuck with 1/4-20 thread. The original thread was a little larger than 1/4 inch but less than 5/16 or 8mm. Since it was very old, I suspect it was a machine screw thread, Nr 15 or Nr 16. In which case, the TPI could be
anywhere around a standard.
Converting a "brace" from 2 point to 3 point grip was a little more difficult. I couldn't fit it to the lathe for rethreading. Did it with a die and very careful alignment. That one got a 3/8 chuck because that was what I had handy. I wouldn't want to drive a 3/8 drill with a brace but have that capacity should the need arise. On very old equipment, machine screw threads were often used. Further, there were "specialty" threads that were used in some areas. It is not known to me, but a 3/8-22 thread is certainly plausable.
Prior to WW-2, British and American threads differed on several points. The most notable was
"Whitworth", having 55 degree threads versus
USS or SAE having 60 degree. In that particular case, the threads would match American threads but were not interchangable. This difference was resolved in the '50s, sorta. In the mid-'70s, I crossed paths with a Norton motorcycle manufactured in the late '60s that had a mix of Unified threads and Whitworth. "Standard" wrenches would fit, but "standard" bolts wouldn't. It is rare these days to cross Whitworth, but for older equipment, they very well could be used.
I apologize for getting so wound up. But often a thought drags up old memories. I am not particularly fond of Norton or BSA so haven't thought about them for years. The BSA 442 was a low speed single that would shake your teeth out. But the memories came up here. The solution to your problem might well be to run a die down the thread to convert from Whitworth to Unified. The pitch would be aligned but the threads wouldn't mate. I'm not saying that is the absolute solution, but is something to consider.
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