1/8" NPT tap drill size
For a 1/8" NPT taper pipe tap, drill 11/32" (8.73 mm). The nearest metric drill is 8.7 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 4.1 mm (4.4 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.
1/8" NPT thread data
| Threads per inch | 27 TPI (0.941 mm pitch) |
|---|---|
| Thread angle | 60° |
| Taper | 1 in 16 on diameter (1.7899° half-angle) |
| Pipe outside diameter | 0.405" (10.287 mm) |
| Pitch diameter at small end (E0) | 0.364" (9.233 mm) |
| Pitch diameter at gauge plane (E1) | 0.374" (9.49 mm) |
| Hand-tight engagement (L1) | 0.162" (4.1 mm) |
| Effective thread length (L2) | 0.264" (6.7 mm) |
| Thread height | 0.753 mm |
Quick calculator: taper diameter
Nearest alternatives
Same nominal size, British parallel pipe: G1/8 (1/8" BSP).
Questions
What size drill for a 1/8" NPT tap?
For a 1/8" NPT taper pipe tap, drill 11/32" (8.73 mm). The nearest metric drill is 8.7 mm.
Will 1/8" NPT screw into 1/8" BSP?
Not properly. 1/8" NPT has 27 TPI and a 60° thread; 1/8" BSP has 28 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 1/8" NPT?
About 4.1 mm (0.162"), 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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