3/8" NPT tap drill size
For a 3/8" NPT taper pipe tap, drill 37/64" (14.68 mm). The nearest metric drill is 14.75 mm.
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Drilling and tapping
Taper pipe taps cut the full thread form as they go deeper, so there is no “% of thread” choice: use the chart drill. For a better taper form, drill a few thou smaller and finish with a 1:16 taper pipe reamer before tapping.
- Tap until a fitting screws in hand-tight by about 6.1 mm (4.3 turns), then check with the mating part.
- NPT seals on the thread but needs PTFE tape or a thread sealant in the gaps.
About this size
NPT is the American taper pipe thread (1 in 16 on diameter, 60° form). It seals on the thread with PTFE tape or a thread sealant.
3/8" NPT thread data
| Threads per inch | 18 TPI (1.411 mm pitch) |
|---|---|
| Thread angle | 60° |
| Taper | 1 in 16 on diameter (1.7899° half-angle) |
| Pipe outside diameter | 0.675" (17.145 mm) |
| Pitch diameter at small end (E0) | 0.612" (15.545 mm) |
| Pitch diameter at gauge plane (E1) | 0.627" (15.926 mm) |
| Hand-tight engagement (L1) | 0.240" (6.1 mm) |
| Effective thread length (L2) | 0.408" (10.36 mm) |
| Thread height | 1.129 mm |
Quick calculator: taper diameter
Nearest alternatives
Same nominal size, British parallel pipe: G3/8 (3/8" BSP).
Questions
What size drill for a 3/8" NPT tap?
For a 3/8" NPT taper pipe tap, drill 37/64" (14.68 mm). The nearest metric drill is 14.75 mm.
Will 3/8" NPT screw into 3/8" BSP?
Not properly. 3/8" NPT has 18 TPI and a 60° thread; 3/8" BSP has 19 TPI and a 55° thread. The pitches differ, so they will cross-thread after a turn or two. Use an adaptor.
What is the hand-tight engagement of 3/8" NPT?
About 6.1 mm (0.240"), the L1 length in ASME B1.20.1. Wrench make-up adds a few more turns.
Sources: Basic dimensions per ASME B1.20.1. Drill from the common taper-pipe-tap chart (drill only, no reamer). Values are for reference only – check your tap maker’s chart. See the disclaimer.
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