G3-1/2 BSP Thread Dimensions and Tap Drill
G3-1/2 is the parallel British Standard Pipe thread (BSPP) of nominal size 3-1/2: 11 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 100.330 mm (3.9500 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 3-1/2 inch.
- Tap drill97.7 mm3.8465 in, 89% thread
- Threads per inch11pitch 2.309 mm
- Major diameter100.330 mm3.9500 in
- Pitch diameter98.851 mm3.8918 in
- Minor diameter97.373 mm3.8336 in
- Thread height1.479 mm0.0582 in
Thread profile
G3-1/2 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 100.330 | 3.9500 |
| Pitch diameter d2 = D2 | 98.851 | 3.8918 |
| Minor diameter d1 = D1 | 97.373 | 3.8336 |
| 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.
G3-1/2 tap drill size
Tap drill for G3-1/2: 97.7 mm (3.8465 in)
This is 0.33 mm over the basic minor diameter of 97.373 mm and leaves about 89% 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 97.373 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 |
|---|---|---|---|---|
| 97.7 mmrecommended | 97.70 | 3.8465 | 89% | |
| 98 mm | 98.00 | 3.8583 | 79% | |
| 98.5 mm | 98.50 | 3.8780 | 62% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
Is there a tapered 3-1/2 BSP?
ISO 7-1, the standard for taper BSP threads (R, Rc and Rp), does not include the 3-1/2 size. G3-1/2 exists only as a parallel thread.
How G3-1/2 seals
A G3-1/2 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.
G3-1/2 against 3-1/2 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 101.600 mm against 100.330 mm. The parts do not fit each other. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| G3-1/2 (this thread) | 11 | 100.330 | 3.9500 | 55° | parallel |
| 3-1/2-8 NPT | 8 | 101.600 | 4.0000 | 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.