1" BSF tap drill size
The tap drill for 1" BSF (10 TPI) is 23 mm, which gives about 74% of full thread. There’s no single standard chart size, so this is worked out from the thread profile for 75%.
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Tap drill by thread percentage
75% is the usual workshop figure. Less thread is easier to tap; more adds little strength.
| Thread % | Ideal drill | Nearest metric drill | Nearest imperial drill |
|---|---|---|---|
| 60% | 23.448 mm 0.9232" | 23.5 mm (58%) | 59/64" 0.9219" (61%) |
| 65% | 23.286 mm 0.9168" | 23.25 mm (66%) | 59/64" 0.9219" (61%) |
| 70% | 23.123 mm 0.9104" | 23 mm (74%) | 29/32" 0.9062" (73%) |
| 75% | 22.96 mm 0.9040" | 23 mm (74%) | 29/32" 0.9062" (73%) |
| 80% | 22.798 mm 0.8975" | 22.75 mm (81%) | 57/64" 0.8906" (85%) |
About this size
BSF uses the same 55° Whitworth form as BSW but with a finer pitch. It was the standard steel-bolt thread on British vehicles and machine tools until Unified and metric threads took over.
- For stainless steel, duplex and other tough materials, go to about 65% thread: a 23.25 mm drill gives 66%. For mild steel, brass and cast iron the 23 mm is fine.
- At this size many people bore the hole and screwcut the thread on the lathe instead of using a hand tap. See the thread calculator for depths and the infeed.
- For a blind hole with 1.5 × D thread engagement (38.1 mm), drill about 48.3 mm deep. That leaves room for the 3–4 tapered lead threads on a second or plug tap. In steel, 1 × D engagement is usually enough; in aluminium or cast iron allow 1.5–2 × D.
Clearance holes for 1" BSF
| Fit | Drill |
|---|---|
| Close | 1-3/64" (26.59 mm) |
| Medium (normal) | 1-3/32" (27.78 mm) |
| Free (loose) | 1-5/32" (29.37 mm) |
Worked out with our own rule, not taken from a standards table: close = the larger of D + 4% and D + 0.2 mm, medium = the larger of D + 8% and D + 0.4 mm, free = the larger of D + 15% and D + 0.75 mm, each rounded up to the next 1/64". Check against your drawing where the fit matters.
1" BSF thread data
| Designation | 1" BSF (10 TPI) |
|---|---|
| Standard | BS 84 Whitworth form (55°) |
| Pitch | 10 TPI (2.54 mm pitch) |
| Thread angle | 55° |
| Major diameter | 1.000" (25.4 mm) |
| Pitch (effective) diameter | 0.936" (23.774 mm) |
| Minor diameter, external | 0.872" (22.147 mm) |
| Minor diameter, internal | 0.872" (22.147 mm) |
| Thread depth (internal) | 1.626 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
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Nearest alternatives
If you don’t have the right drill, these give:
- 22.75 mm drill → about 81% thread
- 23 mm drill → about 74% thread
- 23.25 mm drill → about 66% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 1" UNC – 25.4 mm major, 8 TPI (3.175 mm pitch)
- 1" UNF – 25.4 mm major, 12 TPI (2.117 mm pitch)
- 1" BSW – 25.4 mm major, 8 TPI (3.175 mm pitch)
- G3/4 (3/4" BSP) – 26.44 mm major, 14 TPI (1.814 mm pitch)
- M24 – 24 mm major, 3 mm pitch (8.5 TPI equivalent)
Questions
What size drill for a 1" BSF tap?
The tap drill for 1" BSF (10 TPI) is 23 mm, which gives about 74% of full thread. There’s no single standard chart size, so this is worked out from the thread profile for 75%.
What size clearance hole for 1" BSF?
For a 1" BSF screw or bolt, drill 1-3/64" for a close fit, 1-3/32" for a medium (normal) fit or 1-5/32" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 1" BSF?
1" BSF (10 TPI) has a 10 TPI (2.54 mm pitch), a 55° thread angle and a 25.4 mm major diameter.
Sources: Profile per BS 84 (55° Whitworth). Threads per inch checked 25/10/2026 against published BSW size tables. There is no single standard tapping chart, so the drill is worked out from the profile. Values are for reference only – check your tap maker’s chart. See the disclaimer.
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