# 1 1/4 — Plumbing fitting and pipe thread standards

For 1 1/4, thread standard is BSPT; threads per inch (tpi) is 11; thread pitch (mm) is 2.309; major (outside) diameter (mm) is 41.91; minor diameter (mm) is 38.95, recorded from its source on 2026-08-12.

- **Thread standard:** BSPT _(our reading, not quoted from the source)_
- **Nominal size:** 1 1/4 _(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):** 41.91 _(verified: appears in the quote below)_
- **Minor diameter (mm):** 38.95 _(verified: appears in the quote below)_
- **Thread angle (degrees):** 55 _(our reading, not quoted from the source)_
- **Governing standard:** ISO 7-1 _(our reading, not quoted from the source)_

## What the source says

> <https://www.machiningdoctor.com/threadinfo-bspt/?tid=4537> BSPT Rp 1 1/4 | 1 1/4" | 11 | 2.309 | 12.7 | 0 | 41.91 | 40.43 | 38.95 | 0.180 | 39.0 | Rp | <https://www.machiningdoctor.com/threadinfo-bspt/?tid=4538> BSPT Rp 1 1/2 | 1 1/2" | 11 | 2.309 | 12.7 | 0 | 47.80 | 46.32 | 44.85 | 0.180 | 45.0 | Rp | <https://www.machiningdoctor.com/threadinfo-bspt/?tid=4539> BSPT Rp 2 | 2" | 11 | 2.309 | 15.9 | 0 | 59.61 | 58.14 | 56.66 | 0.180 | 56.4 | Rp | <https://www.machiningdoctor.com/threadinfo-bspt/?tid=4540> BSPT Rp 2 1/2 | 2 1/2" | 11 | 2.309 | 17.5 | 0 | 75.18 | 73.71 | 72.23 | 0.216 | -999 | Rp | <https://www.machiningdoctor.com/threadinfo-bspt/?tid=4541> BSPT Rp 3 | 3" | 11 | 2.309 | 20.6 | 0 | 87.88 | 86.41 | 84.93 | 0.216 | -999 | Rp | <https://www.machiningdoctor.com/threadinfo-bspt/?tid=4542> BSPT Rp 4 | 4" | 11 | 2.309 | 25.4 | 0 | 113.03 | 111.55 | 110.07 | 0.216 | -999 | Rp | <https://www.machiningdoctor.com/threadinfo-bspt/?tid=4543> BSPT Rp 5 | 5" | 11 | 2.309 | 28.6 | 0 | 138.43 | 136.95 | 135.47 | 0.216 | -999 | Rp | <https://www.machiningdoctor.com/threadinfo-bspt/?tid=4544> BSPT Rp 6 | 6" | 11 | 2.309 | 28.6 | 0 | 163.83 | 162.35 | 160.87 | 0.216 | -999 | Tolerances and Inspection The ISO 7-1 provides tolerances only for the following dimensions:

## Source

- https://www.machiningdoctor.com/charts/bsp/

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/).