# 1-1/4 Inch BSP — Plumbing fitting and pipe thread standards

For 1-1/4 Inch BSP, thread standard is BSP; threads per inch (tpi) is 11; major (outside) diameter (mm) is 41.91, recorded from its source on 2026-08-12.

- **Thread standard:** BSP _(our reading, not quoted from the source)_
- **Nominal size:** 1-1/4 Inch BSP _(verified: appears in the quote below)_
- **Threads per inch (TPI):** 11 _(verified: appears in the quote below)_
- **Major (outside) diameter (mm):** 41.91 _(verified: appears in the quote below)_

## What the source says

> 1-1/4 Inch BSP | 41.91 mm | 39.6 mm | 11

## Source

- https://www.lsengineers.co.uk/advice-centre/bsp-thread-sizes-explained

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/).