G1-1/4 BSP Thread Dimensions and Tap Drill
G1-1/4 is the parallel British Standard Pipe thread (BSPP) of nominal size 1-1/4: 11 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 41.910 mm (1.6500 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 1-1/4 inch.
- Tap drill39.5 mm1.5551 in, 81% thread
- Threads per inch11pitch 2.309 mm
- Major diameter41.910 mm1.6500 in
- Pitch diameter40.431 mm1.5918 in
- Minor diameter38.953 mm1.5336 in
- Thread height1.479 mm0.0582 in
Thread profile
G1-1/4 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 41.910 | 1.6500 |
| Pitch diameter d2 = D2 | 40.431 | 1.5918 |
| Minor diameter d1 = D1 | 38.953 | 1.5336 |
| 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 ISO 228-1 Table 1. Inch values are conversions.
G1-1/4 tap drill size
Tap drill for G1-1/4: 39.5 mm (1.5551 in)
This is 0.55 mm over the basic minor diameter of 38.953 mm and leaves about 81% 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 38.953 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 |
|---|---|---|---|---|
| 39.5 mmrecommended | 39.50 | 1.5551 | 81% | |
| 40 mm | 40.00 | 1.5748 | 65% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
G1-1/4 and the tapered R1-1/4 (BSPT)
The taper thread of ISO 7-1 (EN 10226), written R1-1/4 for the male and Rc1-1/4 or Rp1-1/4 for the female, has the same 11 threads per inch and the same 55° form as G1-1/4, and at its gauge plane it has the same diameters. It differs in being cut on a 1 in 16 taper and in sealing on the thread with a sealant. The male R1-1/4 mates with a taper Rc1-1/4 or a parallel Rp1-1/4 female thread. Rp is not the same thing as G: the basic sizes agree but the tolerances do not, so a G1-1/4 port is not a specified mate for a taper male.
How G1-1/4 seals
A G1-1/4 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.
G1-1/4 against 1-1/4 NPT
The American NPT thread of the same nominal size has 11.5 threads per inch against 11, a 60° flank angle and an outside diameter of 42.164 mm against 41.910 mm. The parts do not fit each other. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| G1-1/4 (this thread) | 11 | 41.910 | 1.6500 | 55° | parallel |
| 1-1/4-11.5 NPT | 11.5 | 42.164 | 1.6600 | 60° | 1 in 16 |
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.