threadsizes

3/8 NPT Tap Drill Size

3/8-18 NPT · American National Standard taper pipe thread · ASME B1.20.1

What size drill bit for a 3/8 NPT tap?

37/64"fractional drill
0.5781inch
14.69mm
4.3turns by hand
Drill size for tapping without a taper pipe reamer. A 3/8 NPT hole is much bigger than 3/8 in: the size is the nominal pipe size, and the pipe itself is 0.675 in across.

To scale male thread, side section

3/8 NPT male thread to scale: OD 0.675 in, hand-tight length L1 0.24 in, effective thread L2 0.4078 in. L1 0.24 in, hand tight L2 0.4078 in, effective OD 0.675 in
The thread tapers 1 in 16 on the diameter (3/4 in per foot), drawn to scale. L1 is how far the parts go together by hand; L2 is the effective thread length.

3/8-18 NPT ASME B1.20.1

Pipe outside diameter0.675 in / 17.15 mm
Threads per inch18 TPI (pitch 0.0556 in)
Taper1:16 on diameter
Pitch dia at pipe end (E0)0.6120 in
Pitch dia at L1 (E1)0.6270 in
Hand-tight engagement (L1)0.24 in, 4.32 turns
Effective thread (L2)0.4078 in
Wrench makeup0.1667 in, 3 turns
Thread height0.0444 in

Shop notes

The drill for tapping 3/8 NPT without a reamer is 37/64 (0.5781 in, 14.7 mm), and that is the awkward part. It sits above the range of a typical jobber drill index, so you may have to buy it, or step up through what you have and accept a rougher bore. Do not round it down to a size you own: a smaller hole makes the taper tap cut a full thread from the first turn, and the torque climbs fast.

The male thread measures 0.675 in across at 18 TPI, the same pitch as 1/4 NPT. If you have to tell the two apart on the bench, measure the diameter, not the pitch.

Hand-tight engagement is 0.240 in, about 4.32 turns, and the effective thread length (L2) is 0.4078 in. That second number matters when tapping into a boss or a thin plate. If the wall is shorter than L2, the fitting runs out of material before it reaches its full engagement, and the usual fix of cranking harder only spreads the hole or cracks a casting.

As with any NPT thread, the seal is on the flanks, so use PTFE tape or a pipe thread sealant.