G3 BSP Thread Dimensions and Tap Drill
G3 is the parallel British Standard Pipe thread (BSPP) of nominal size 3: 11 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 87.884 mm (3.4600 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 3 inch.
- Tap drill85.5 mm3.3661 in, 81% thread
- Threads per inch11pitch 2.309 mm
- Major diameter87.884 mm3.4600 in
- Pitch diameter86.405 mm3.4018 in
- Minor diameter84.927 mm3.3436 in
- Thread height1.479 mm0.0582 in
Thread profile
G3 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 87.884 | 3.4600 |
| Pitch diameter d2 = D2 | 86.405 | 3.4018 |
| Minor diameter d1 = D1 | 84.927 | 3.3436 |
| 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.
G3 tap drill size
Tap drill for G3: 85.5 mm (3.3661 in)
This is 0.57 mm over the basic minor diameter of 84.927 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 84.927 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 |
|---|---|---|---|---|
| 85.5 mmrecommended | 85.50 | 3.3661 | 81% | |
| 86 mm | 86.00 | 3.3858 | 64% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
G3 and the tapered R3 (BSPT)
The taper thread of ISO 7-1 (EN 10226), written R3 for the male and Rc3 or Rp3 for the female, has the same 11 threads per inch and the same 55° form as G3, 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 R3 mates with a taper Rc3 or a parallel Rp3 female thread. Rp is not the same thing as G: the basic sizes agree but the tolerances do not, so a G3 port is not a specified mate for a taper male.
How G3 seals
A G3 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.
G3 against 3 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 88.900 mm against 87.884 mm. The parts do not fit each other. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| G3 (this thread) | 11 | 87.884 | 3.4600 | 55° | parallel |
| 3-8 NPT | 8 | 88.900 | 3.5000 | 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.