# 1 — Plumbing fitting and pipe thread standards

For 1, thread standard is BSP; threads per inch (tpi) is 11; thread pitch (mm) is 2.309; major (outside) diameter (mm) is 33.249; minor diameter (mm) is 30.291, recorded from its source on 2026-08-12.

- **Thread standard:** BSP _(our reading, not quoted from the source)_
- **Nominal size:** 1 _(verified: appears in the quote below)_
- **Threads per inch (TPI):** 11 _(verified: appears in the quote below)_
- **Thread pitch (mm):** 2.309 _(verified: appears in the quote below)_
- **Major (outside) diameter (mm):** 33.249 _(verified: appears in the quote below)_
- **Minor diameter (mm):** 30.291 _(verified: appears in the quote below)_
- **Thread angle (degrees):** 55 _(our reading, not quoted from the source)_
- **Governing standard:** ISO 228-1 _(our reading, not quoted from the source)_

## What the source says

> 1 | 11 | 2.309 | 1.479 | 33.249 | 31.770 | 30.291 |

## 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.

### Major (outside) diameter (mm)

amesweb.info says major (outside) diameter (mm) is **33.249**, as of 2026-08-12.

> 1 | 11 | 2.309 | 1.479 | 33.249 | 31.770 | 30.291 |

Source: https://amesweb.info/Screws/BSP-Thread-Chart.aspx

geminivalve.com says major (outside) diameter (mm) is **33.249**, as of 2026-08-12.

> 1 | 11 | 0.0909 | 2.309 | 1.309 | 33.249 | 1.1926 | 30.292 | 0.4091 | 10.391

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

htsspares.com says major (outside) diameter (mm) is **33.25**, as of 2026-08-12.

> 1 Inch BSP | 33.25 mm / 1-1/4" | 11

Source: https://www.htsspares.com/blog/how-to/how-to-measure-your-bsp-fittings


### Minor diameter (mm)

amesweb.info says minor diameter (mm) is **30.291**, as of 2026-08-12.

> 1 | 11 | 2.309 | 1.479 | 33.249 | 31.770 | 30.291 |

Source: https://amesweb.info/Screws/BSP-Thread-Chart.aspx

geminivalve.com says minor diameter (mm) is **30.292**, as of 2026-08-12.

> 1 | 11 | 0.0909 | 2.309 | 1.309 | 33.249 | 1.1926 | 30.292 | 0.4091 | 10.391

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


## Source

- https://amesweb.info/Screws/BSP-Thread-Chart.aspx
- https://www.geminivalve.com/bsp-thread-vs-npt-fitting/
- https://www.htsspares.com/blog/how-to/how-to-measure-your-bsp-fittings

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/).