G2-1/2 BSP Thread Dimensions and Tap Drill
G2-1/2 is the parallel British Standard Pipe thread (BSPP) of nominal size 2-1/2: 11 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 75.184 mm (2.9600 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 2-1/2 inch.
- Tap drill72.8 mm2.8661 in, 81% thread
- Threads per inch11pitch 2.309 mm
- Major diameter75.184 mm2.9600 in
- Pitch diameter73.705 mm2.9018 in
- Minor diameter72.227 mm2.8436 in
- Thread height1.479 mm0.0582 in
Thread profile
G2-1/2 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 75.184 | 2.9600 |
| Pitch diameter d2 = D2 | 73.705 | 2.9018 |
| Minor diameter d1 = D1 | 72.227 | 2.8436 |
| 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.
G2-1/2 tap drill size
Tap drill for G2-1/2: 72.8 mm (2.8661 in)
This is 0.57 mm over the basic minor diameter of 72.227 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 72.227 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 |
|---|---|---|---|---|
| 72.5 mm | 72.50 | 2.8543 | 91% | |
| 72.8 mmrecommended | 72.80 | 2.8661 | 81% | |
| 73 mm | 73.00 | 2.8740 | 74% | |
| 73.5 mm | 73.50 | 2.8937 | 57% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
G2-1/2 and the tapered R2-1/2 (BSPT)
The taper thread of ISO 7-1 (EN 10226), written R2-1/2 for the male and Rc2-1/2 or Rp2-1/2 for the female, has the same 11 threads per inch and the same 55° form as G2-1/2, 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 R2-1/2 mates with a taper Rc2-1/2 or a parallel Rp2-1/2 female thread. Rp is not the same thing as G: the basic sizes agree but the tolerances do not, so a G2-1/2 port is not a specified mate for a taper male.
How G2-1/2 seals
A G2-1/2 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.
G2-1/2 against 2-1/2 NPT
The American NPT thread of the same nominal size has 8 threads per inch against 11, a 60° flank angle and an outside diameter of 73.025 mm against 75.184 mm. The parts do not fit each other. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| G2-1/2 (this thread) | 11 | 75.184 | 2.9600 | 55° | parallel |
| 2-1/2-8 NPT | 8 | 73.025 | 2.8750 | 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.