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BSPP (G) Thread Chart — ISO 228-1
All 24 sizes, G 1/16 to G 6. Parallel Whitworth pipe thread, 55°.
Every drill here is the nearest stocked size to 80% thread
engagement. That sentence is missing from almost every other BSPP table, and it
is exactly why they disagree with each other by up to half a millimetre — each one
has quietly assumed a different engagement, anywhere from 71% to 88%. Each size page
lists every drill you can buy for that thread with its true engagement, so you can pick
deliberately rather than inherit someone else's assumption.
| Size | TPI | Pitch mm | Major mm | Pitch dia mm | Minor mm | Drill (~80%) |
|---|---|---|---|---|---|---|
| G 1/16 | 28 | 0.907 | 7.723 | 7.142 | 6.561 | 6.80 mm |
| G 1/8 | 28 | 0.907 | 9.728 | 9.147 | 8.566 | 8.80 mm |
| G 1/4 | 19 | 1.337 | 13.157 | 12.301 | 11.445 | 11.80 mm |
| G 3/8 | 19 | 1.337 | 16.662 | 15.806 | 14.950 | 15.30 mm |
| G 1/2 | 14 | 1.814 | 20.955 | 19.793 | 18.632 | 19.10 mm |
| G 5/8 | 14 | 1.814 | 22.911 | 21.749 | 20.588 | 21.00 mm |
| G 3/4 | 14 | 1.814 | 26.441 | 25.279 | 24.118 | 24.50 mm |
| G 7/8 | 14 | 1.814 | 30.201 | 29.039 | 27.878 | 28.25 mm |
| G 1 | 11 | 2.309 | 33.249 | 31.770 | 30.292 | 31.00 mm |
| G 1-1/8 | 11 | 2.309 | 37.897 | 36.418 | 34.940 | 35.50 mm |
| G 1-1/4 | 11 | 2.309 | 41.910 | 40.431 | 38.953 | 39.50 mm |
| G 1-1/2 | 11 | 2.309 | 47.803 | 46.324 | 44.846 | 45.50 mm |
| G 1-3/4 | 11 | 2.309 | 53.746 | 52.267 | 50.789 | 51.50 mm |
| G 2 | 11 | 2.309 | 59.614 | 58.135 | 56.657 | 57.00 mm |
| G 2-1/4 | 11 | 2.309 | 65.710 | 64.231 | 62.753 | 62.753 mm bore |
| G 2-1/2 | 11 | 2.309 | 75.184 | 73.705 | 72.227 | 72.227 mm bore |
| G 2-3/4 | 11 | 2.309 | 81.534 | 80.055 | 78.577 | 78.577 mm bore |
| G 3 | 11 | 2.309 | 87.884 | 86.405 | 84.927 | 84.927 mm bore |
| G 3-1/2 | 11 | 2.309 | 100.330 | 98.851 | 97.373 | 97.373 mm bore |
| G 4 | 11 | 2.309 | 113.030 | 111.551 | 110.073 | 110.073 mm bore |
| G 4-1/2 | 11 | 2.309 | 125.730 | 124.251 | 122.773 | 122.773 mm bore |
| G 5 | 11 | 2.309 | 138.430 | 136.951 | 135.473 | 135.473 mm bore |
| G 5-1/2 | 11 | 2.309 | 151.130 | 149.651 | 148.173 | 148.173 mm bore |
| G 6 | 11 | 2.309 | 163.830 | 162.351 | 160.873 | 160.873 mm bore |
Sizes above G 2 are marked bore and show the minor diameter instead. There is no hand tap catalogued that large, so the hole is bored and the thread single-point turned or cut with a die head — there is no drill size to give, and tables that print one anyway are guessing.
BSPP does not seal on the thread. The flanks are parallel, so tightening only pulls metal against metal and never wedges. The seal has to come from somewhere else: a bonded seal (Dowty washer) under the hex, an O-ring in a face groove per ISO 1179-2, or a 60° cone seat. PTFE tape on a BSPP thread accomplishes nothing except making the joint harder to undo.