G7/8 BSP Thread Dimensions and Tap Drill
G7/8 is the parallel British Standard Pipe thread (BSPP) of nominal size 7/8: 14 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 30.201 mm (1.1890 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 7/8 inch.
- Tap drill28.25 mm1.1122 in, 84% thread
- Threads per inch14pitch 1.814 mm
- Major diameter30.201 mm1.1890 in
- Pitch diameter29.039 mm1.1433 in
- Minor diameter27.878 mm1.0975 in
- Thread height1.162 mm0.0457 in
Thread profile
G7/8 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 30.201 | 1.1890 |
| Pitch diameter d2 = D2 | 29.039 | 1.1433 |
| Minor diameter d1 = D1 | 27.878 | 1.0975 |
| Pitch P | 1.814 | 0.07143 |
| Thread height h (0.640327 × P) | 1.162 | 0.0457 |
| Crest and root radius r (0.137329 × P) | 0.249 | 0.0098 |
| Height of fundamental triangle H | 1.743 | 0.0686 |
| Property | Value |
|---|---|
| Threads per inch | 14 |
| Flank angle | 55° (27.5° each side) |
| Taper | none (parallel thread) |
Basic dimensions from ISO 228-1 Table 1. Inch values are conversions.
G7/8 tap drill size
Tap drill for G7/8: 28.25 mm (1.1122 in)
This is 0.37 mm over the basic minor diameter of 27.878 mm and leaves about 84% 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 27.878 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 |
|---|---|---|---|---|
| 28 mm | 28.00 | 1.1024 | 95% | |
| 28.25 mmrecommended | 28.25 | 1.1122 | 84% | |
| 28.5 mm | 28.50 | 1.1220 | 73% | |
| 28.75 mm | 28.75 | 1.1319 | 62% | |
| 29 mm | 29.00 | 1.1417 | 52% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
Is there a tapered 7/8 BSP?
ISO 7-1, the standard for taper BSP threads (R, Rc and Rp), does not include the 7/8 size. G7/8 exists only as a parallel thread.
How G7/8 seals
A G7/8 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.