Greetings ,
My question .. seeing as we live in a metric World
What say you
NOT so easy
4mm drill bit size is 3.3mm = special order
3.3 does not remove ALL the old thread (3.51)
SO
4mm iffy
No one keeps American UNS ( non standard ) UNF .. maybe ?
#8-36 IS standard UNF - where to find ?
5mm is much too big
Watch this space
I play around in my garage with making stuff and fixing stuff. A lot is with screws of thin diameter, M3-M5. Tapping and threading. I still haven't seen a 3.3mm drill bit off the shelf, but I have found the following experience:
With soft materials, like aluminum, or plastics, I drill a 3.2mm hole, and tap an M4. Its a bit of a squish to get started, but it works. You can work a 3.2mm drill around a bit to make the hole closer to 3.3mm
With harder materials, mild steel, and steel with a bit of carbon I drill a 3.0mm hole, and then ream out to 3.5mm, so there is no extra scraping and rubbing to make the 3.5 hole bigger than it should.
1) the 3.5mm m4 tapped holes still manage to hold an m4 screw very well, even though the pitch is 0.5mm instead of 0.7mm . I have not had any of them pull loose and strip the threads.
2) It is a lot easier to tap an M4 with a 3.5mm hole, and there is less chance that you are going to break your die off in the hole and cause even bigger problems than you had. I imagine gun metal often has quite a bit of carbon in it, so this is a real threat.
If a perfect thread is important, I would perhaps look at an M4 fine thread. This taps into a 3.5mm hole. Here the drill bit is easy to find, but the matching dies and screws are harder to find.