# 4 — Plumbing fitting and pipe thread standards

For 4, thread standard is NPT; threads per inch (tpi) is 8; thread pitch (mm) is 3.1750; major (outside) diameter (mm) is 114.3000; thread angle (degrees) is 60, recorded from its source on 2026-08-12.

- **Thread standard:** NPT _(our reading, not quoted from the source)_
- **Nominal size:** 4 _(verified: appears in the quote below)_
- **Threads per inch (TPI):** 8
- **Thread pitch (mm):** 3.1750
- **Major (outside) diameter (mm):** 114.3000 _(verified: appears in the quote below)_
- **Thread angle (degrees):** 60 _(our reading, not quoted from the source)_
- **Governing standard:** ASME B1.20.1 _(our reading, not quoted from the source)_

## What the source says

> 4.500 | 114.3000 |

## Sources disagree

More than one authority states this, and they do not state the same
thing. Both are reproduced with the source each came from.

### Thread pitch (mm)

en.wikipedia.org says thread pitch (mm) is **3.1750**, as of 2026-08-12.

> 4.500 | 114.3000 |

Source: https://en.wikipedia.org/wiki/National_pipe_thread

geminivalve.com says thread pitch (mm) is **3.175**, as of 2026-08-12.

> 4 | 8 | 0.125 | 3.175 | 4.5 | 114.3

Source: https://www.geminivalve.com/bsp-thread-vs-npt-fitting/


### Major (outside) diameter (mm)

en.wikipedia.org says major (outside) diameter (mm) is **114.3000**, as of 2026-08-12.

> 4.500 | 114.3000 |

Source: https://en.wikipedia.org/wiki/National_pipe_thread

geminivalve.com says major (outside) diameter (mm) is **114.3**, as of 2026-08-12.

> 4 | 8 | 0.125 | 3.175 | 4.5 | 114.3

Source: https://www.geminivalve.com/bsp-thread-vs-npt-fitting/


## Source

- https://en.wikipedia.org/wiki/National_pipe_thread
- https://www.geminivalve.com/bsp-thread-vs-npt-fitting/

Last verified: 2026-08-12. Review by: 2028-08-11.
Part of [Plumbing fitting and pipe thread standards](https://referencesource.org/plumbing-fitting-thread-standards/).