NPT taper and dimensions
The 1:16 taper is diametral: diameter changes by 1 mm over 16 mm of length. Do not apply straight-thread pitch-diameter formulas to this reference. The gauge plane and depth define where dimensions are checked.
01What 3/8-18 NPT means
3/8 is a nominal pipe size, not a 9.525 mm outside diameter. The reference pipe outside diameter is 0.675 inch (17.145 mm); the actual thread diameter changes along the taper. The thread has 18 TPI, approximately 1.4111 mm pitch and a 1:16 diametral taper.
029/16 versus 37/64 inch tap drills
Allied Machine specifies 9/16 inch without a taper reamer. This website’s calculator also includes a different 37/64 inch (14.6844 mm) preparation. They are not equivalent or a universal rule: confirm drilling, reaming and depth with the tap manufacturer.
03Identification, sealing and inspection
A 3/8 BSP thread has 19 TPI and a 55° profile, compared with 18 TPI and 60° for NPT: they are not compatible. Do not replace NPT with NPTF just because dimensions are close. Check the drawing, pipe-thread gauge and specified connection sealing method.
Similar threads: check diameter and pitch
NPT and BSP differ in profile, pitch and dimensions. Reference pipe diameter is not a constant thread diameter: these figures do not establish interchangeability.
| Thread | Ø mm | Pitch |
|---|---|---|
| 3/8-18 NPT | 17.145 | 18 TPI · 1.4111 mm |
| G 3/8-19 BSP | 16.662 | 19 TPI · 1.3368 mm |
Frequently asked questions · 3/8-18 NPT
What tap drill size is used for 3/8-18 NPT?
Allied Machine lists 9/16 inch, 14.2875 mm, without a taper reamer. Other preparations use 37/64 inch, 14.6844 mm. Follow the tap and process instructions.
Is 3/8 NPT 9.525 mm in diameter?
No. 3/8 is a nominal pipe size. The reference pipe outside diameter is 17.145 mm, while thread diameter varies along the taper.
Are 3/8 NPT and 3/8 BSP interchangeable?
No. This NPT size has 18 TPI and a 60° profile; BSP has 19 TPI and a 55° profile. They are not interchangeable.
Sources and standard reference
Cutting-tap drill:
Allied Machine · National Pipe Threads (without taper reamer)Standard reference: ASME B1.20.1 · NPT pipe threads
Nominal reference data. The drawing, tolerance class and tool instructions govern hole preparation and acceptance. Do not automatically use a cutting-tap drill size for form tapping.
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