NPT sizes are deceiving. The inside diameter of the 1/4" pipe is close to 1/4", but the outside is a tad over 3/8.

I have neve found it necessary to predrill before tapping. I have always just started the tap, and did not tap deep at all (about 3/16"), and the bpt fitting would still engage leak free with a little teflon tape, so it's reversible.


Not responsible for advice not taken...