Ok, this got a bit long. Short version is you can replace the springs on these, they are called compression springs, you just have to find the right outside diameter, free length (no compression on the spring), and compressed length. The right fitting spring for my drill would have been the .14” outside diameter and 61/128” long or about .4765”.
My Millers Falls 5A was a pretty decent eggbeater drill, in fact I was surprised how well they work considering they’ve been basically abandoned for electric drills. The problem was the jaws would stick and would require taping on them to get them to open up after you unscrewed the chuck to insert a larger bit. Not to bit a deal but annoying. I’d read that you can open up the chuck and polish the inside to get the jaws to retract more easily. I couldn’t find that bit of advice, but just cleaning it with penetrating oil like the first link here didn’t work:
It turned out when opening up the chuck that there was just a lot of grit mixed with the oil and that didn’t come out when trying to clean with penetrant. Anyway I unscrewed the chuck from the drill post, put it in the vise and found a suitable piece of metal to fit in the slots to unscrew the chuck cap.
When I got it apart the jaws and springs had a bit of grime on them so despite being extremely careful with the small parts up to this point, I grabbed a brass brush and without thinking went to brush the grime off. Well of course one of the springs got caught by the brush and shot off across the shop and went wherever small parts go that don’t want to be found.
So at that point I realized I needed to find a replacement. I measured the diameter of the indents where the springs fit by testing different drill bits in them since I’m not very good with a micrometer. A 9/64” bit fit a bit loosely and a 5/32” didn’t fit. I discovered these springs are called compression springs and are fairly standard items. The outside diameters I found for sale close to 9/64” were .14” and .148”. I went with the .148” because I thought the .14 would be too small, but in hindsight I should have tried using a micrometer to measure the spring diameter. I’ve since measured them carefully and they are exactly 9/64” which is just over .14”. The new springs are also a bit too long when compressed so the chuck doesn’t close all the way down anymore, but it does close down enough to hold 1/16” drill bits which is the smallest I have. Also since the new springs have more spring force than the old ones and make it just a little harder than needed to spin the chuck, a spring made from thinner wire might have worked better and solved the compressed length issue. Spring places have a name for the compressed length, but I don’t recall it now.
Fastenal carries compression springs, but they didn’t have any .148”. There were some other online places that sell them but none I recognized. I managed to find the right size and length on Amazon and it happened that they had the .148” OD .5” long springs in a 10 pack for about $5 with free shipping, where most are around $15.
So if you get a drill in good shape where the chuck doesn’t work it’s not too hard to find the right replacement springs. In fact, next I’ll have to try to identify what type of spring in supposed to go in my brace chuck that doesn’t have one. I hope writing this down helps anyone looking for how to fix old hand drill chucks and give them a new life.