Whitworth BSW and BSF threads: tap drill chart
Full chart with the tap drill for every size. Press “Calculate” on any row to open the calculator with that thread.
Whitworth is the British 55° thread with rounded crests and roots. BSW is the coarse series and BSF the fine one. It is hardly used in new designs today, but it turns up on old machinery, classic British motorcycles and cars, and in repair work. Do not confuse BSW with BSP pipe thread: they share the 55° profile, but BSP is a pipe thread and its sizes are different.
| Thread | Threads per inch | Pitch (mm) | Ø major | Tap drill | Calculate |
|---|---|---|---|---|---|
| BSW 1/8"-40 | 40 | 0.635 | 0.125" · 3.175 mm | ⌀ 2.55 | |
| BSW 3/16"-24 | 24 | 1.058 | 0.1875" · 4.763 mm | ⌀ 3.7 | |
| BSW 1/4"-20 | 20 | 1.27 | 0.25" · 6.35 mm | ⌀ 5.1 | |
| BSW 5/16"-18 | 18 | 1.411 | 0.3125" · 7.938 mm | ⌀ 6.5 | |
| BSW 3/8"-16 | 16 | 1.587 | 0.375" · 9.525 mm | ⌀ 7.9 | |
| BSW 7/16"-14 | 14 | 1.814 | 0.4375" · 11.113 mm | ⌀ 9.3 | |
| BSW 1/2"-12 | 12 | 2.117 | 0.5" · 12.7 mm | ⌀ 10.5 | |
| BSW 5/8"-11 | 11 | 2.309 | 0.625" · 15.875 mm | ⌀ 13.5 | |
| BSW 3/4"-10 | 10 | 2.54 | 0.75" · 19.05 mm | ⌀ 16.25 | |
| BSW 7/8"-9 | 9 | 2.822 | 0.875" · 22.225 mm | ⌀ 19.25 | |
| BSW 1"-8 | 8 | 3.175 | 1" · 25.4 mm | ⌀ 22 |
| Thread | Threads per inch | Pitch (mm) | Ø major | Tap drill | Calculate |
|---|---|---|---|---|---|
| BSF 3/16"-32 | 32 | 0.794 | 0.1875" · 4.763 mm | ⌀ 4 | |
| BSF 1/4"-26 | 26 | 0.977 | 0.25" · 6.35 mm | ⌀ 5.3 | |
| BSF 5/16"-22 | 22 | 1.155 | 0.3125" · 7.938 mm | ⌀ 6.8 | |
| BSF 3/8"-20 | 20 | 1.27 | 0.375" · 9.525 mm | ⌀ 8.3 | |
| BSF 7/16"-18 | 18 | 1.411 | 0.4375" · 11.113 mm | ⌀ 9.7 | |
| BSF 1/2"-16 | 16 | 1.587 | 0.5" · 12.7 mm | ⌀ 11.1 | |
| BSF 5/8"-14 | 14 | 1.814 | 0.625" · 15.875 mm | ⌀ 14 | |
| BSF 3/4"-12 | 12 | 2.117 | 0.75" · 19.05 mm | ⌀ 16.75 | |
| BSF 7/8"-11 | 11 | 2.309 | 0.875" · 22.225 mm | ⌀ 19.75 | |
| BSF 1"-10 | 10 | 2.54 | 1" · 25.4 mm | ⌀ 22.75 |
How to read the chart
Drill: the one from the usual Whitworth tapping tables used in the shop. The profile has rounded crests and roots, so the drill does not come from subtracting the pitch.
Threads per inch: 1/4" BSW has 20, like UNC, but the profile is 55° and not 60°; do not mix screws and nuts of the two.
"Calculate" opens the calculator with that size. Guide values; on critical parts, check against the tap maker's data sheet.
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