1-1/4" NPT tap drill size
For a 1-1/4" NPT taper pipe tap, drill 1-1/2" (38.1 mm). The nearest metric drill is 38 mm.
Open in the full tap drill finder
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 10.7 mm (4.8 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.
1-1/4" NPT thread data
| Threads per inch | 11.5 TPI (2.209 mm pitch) |
|---|---|
| Thread angle | 60° |
| Taper | 1 in 16 on diameter (1.7899° half-angle) |
| Pipe outside diameter | 1.660" (42.164 mm) |
| Pitch diameter at small end (E0) | 1.557" (39.551 mm) |
| Pitch diameter at gauge plane (E1) | 1.583" (40.218 mm) |
| Hand-tight engagement (L1) | 0.420" (10.67 mm) |
| Effective thread length (L2) | 0.707" (17.95 mm) |
| Thread height | 1.767 mm |
Quick calculator: taper diameter
Nearest alternatives
Questions
What size drill for a 1-1/4" NPT tap?
For a 1-1/4" NPT taper pipe tap, drill 1-1/2" (38.1 mm). The nearest metric drill is 38 mm.
Will 1-1/4" NPT screw into 1-1/4" BSP?
Not properly. 1-1/4" NPT has 11.5 TPI and a 60° thread; 1-1/4" BSP has a different 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 1-1/4" NPT?
About 10.67 mm (0.420"), 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.
Spotted a mistake? Report an error.