G1-3/8 BSP Thread Dimensions and Tap Drill
G1-3/8 is the parallel British Standard Pipe thread (BSPP) of nominal size 1-3/8: 11 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 44.323 mm (1.7450 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 1-3/8 inch.
G1-3/8 is not one of the sizes in ISO 228-1. It appears in manufacturers’ thread tables with the dimensions below, which follow the same Whitworth proportions, but fittings in this size are uncommon.
- Tap drill41.75 mm1.6437 in, 87% thread
- Threads per inch11pitch 2.309 mm
- Major diameter44.323 mm1.7450 in
- Pitch diameter42.844 mm1.6868 in
- Minor diameter41.366 mm1.6286 in
- Thread height1.479 mm0.0582 in
Thread profile
G1-3/8 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 44.323 | 1.7450 |
| Pitch diameter d2 = D2 | 42.844 | 1.6868 |
| Minor diameter d1 = D1 | 41.366 | 1.6286 |
| Pitch P | 2.309 | 0.09091 |
| Thread height h (0.640327 × P) | 1.479 | 0.0582 |
| Crest and root radius r (0.137329 × P) | 0.317 | 0.0125 |
| Height of fundamental triangle H | 2.218 | 0.0873 |
| Property | Value |
|---|---|
| Threads per inch | 11 |
| Flank angle | 55° (27.5° each side) |
| Taper | none (parallel thread) |
Basic dimensions from manufacturers’ tables (size not in ISO 228-1). Inch values are conversions.
G1-3/8 tap drill size
Tap drill for G1-3/8: 41.75 mm (1.6437 in)
This is 0.38 mm over the basic minor diameter of 41.366 mm and leaves about 87% thread.
ISO 228-1 puts the whole tolerance on the minor diameter of the internal thread above the basic size, so the hole is always drilled larger than 41.366 mm. Drilling to the basic minor diameter itself leaves the tap cutting on its full profile and risks breaking it.
| Drill | mm | in | % thread | Thread engagement |
|---|---|---|---|---|
| 41.75 mmrecommended | 41.75 | 1.6437 | 87% | |
| 42 mm | 42.00 | 1.6535 | 79% | |
| 42.5 mm | 42.50 | 1.6732 | 62% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
Is there a tapered 1-3/8 BSP?
ISO 7-1, the standard for taper BSP threads (R, Rc and Rp), does not include the 1-3/8 size. G1-3/8 exists only as a parallel thread.
How G1-3/8 seals
A G1-3/8 joint does not seal on its threads. ISO 228-1 is the standard for pipe threads where the pressure-tight joint is made outside the thread: the male fitting clamps a bonded washer, an O-ring or a flat gasket against the face of the port. The thread only supplies the clamping force, so thread sealant is not what makes the joint tight.
Related sizes and tools
Sources: ISO 228-1 (dimensions and sealing principle), ISO 7-1 (taper threads), tap manufacturers’ drill charts. How this site checks its data. Last reviewed 29 September 2026.