threadsizes

1/8 NPT Tap Drill Size

1/8-27 NPT · American National Standard taper pipe thread · ASME B1.20.1

What size drill bit for a 1/8 NPT tap?

Rletter drill
0.3390inch
8.61mm
4.4turns by hand
Drill size for tapping without a taper pipe reamer. A 1/8 NPT hole is much bigger than 1/8 in: the size is the nominal pipe size, and the pipe itself is 0.405 in across.

To scale male thread, side section

1/8 NPT male thread to scale: OD 0.405 in, hand-tight length L1 0.1615 in, effective thread L2 0.2639 in. L1 0.1615 in, hand tight L2 0.2639 in, effective OD 0.405 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.

1/8-27 NPT ASME B1.20.1

Pipe outside diameter0.405 in / 10.29 mm
Threads per inch27 TPI (pitch 0.0370 in)
Taper1:16 on diameter
Pitch dia at pipe end (E0)0.3635 in
Pitch dia at L1 (E1)0.3736 in
Hand-tight engagement (L1)0.1615 in, 4.36 turns
Effective thread (L2)0.2639 in
Wrench makeup0.1111 in, 3 turns
Thread height0.0296 in

Shop notes

Nothing about 1/8 NPT measures 1/8 in. The nominal size is a pipe designation, not a dimension: the outside diameter is 0.405 in, bigger than a 3/8 bolt, and the drill for tapping it without a reamer is a letter R (0.3390 in, 8.6 mm). People who go looking for a small drill to match "1/8" end up with a hole the tap cannot use.

At 27 TPI this is the finest pipe thread on the site and the shortest. Hand-tight engagement (L1) is only 0.1615 in, about 4.36 turns, and the wrench adds just 3 more. That leaves very little room for over-tapping. A taper tap opens the hole along its whole length with every extra turn, so a hole tapped too deep lets the fitting run in further by hand and leaves less taper to tighten against. Tap a few turns, try the actual fitting, and stop when it threads in by hand roughly to L1.

You see 1/8 NPT on small pressure gauges and grease fittings. If a gauge port looks too big for it, try 1/4 NPT.

The seal happens on the thread flanks, so the joint needs PTFE tape or a pipe thread sealant. Fittings marked NPTF (dryseal) are made to seal metal to metal without it, and they still thread into an ordinary NPT hole.