Good afternoon everyone!

I have a need to tap an existing 0.125 inch hole.  What size tap would work best?  I would prefer to not drill the hole any larger, but could possible go up ever so slightly in size.  I'm thinking M4 would be closest but not sure.

I currently have a 9mm mag-block pinned with two roll pins, but would like to thread the holes and use 4 cap-head screws to attach it to the lower instead.

Thanks in advance!
-Parker

M4 x .7 or 8-36 but you'd have to ream it about 10 thou over.

ETA: M4 x .7 should be .1299, 8-36 should be .1360.

That's kind of what I was thinking... So either a #29 drill for the 8-36 or a 3.3mm drill for the M4, right?

Thanks man!!

Not requiring clamping forces, you could get away with 6-32.  If it gives you problems, stepping up to 8-32 would be easy.  6-40 and 8-40 would be marginally better.