3/8" BSF tap drill size
The tap drill for 3/8" BSF (20 TPI) is 8.3 mm, which gives about 75% 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% | 8.549 mm 0.3366" | 8.5 mm (63%) | letter R 0.3390" (56%) |
| 65% | 8.468 mm 0.3334" | 8.5 mm (63%) | letter Q 0.3320" (67%) |
| 70% | 8.386 mm 0.3302" | 8.4 mm (69%) | letter Q 0.3320" (67%) |
| 75% | 8.305 mm 0.3270" | 8.3 mm (75%) | 21/64" 0.3281" (73%) |
| 80% | 8.224 mm 0.3238" | 8.2 mm (81%) | letter P 0.3230" (81%) |
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 8.5 mm drill gives 63%. For mild steel, brass and cast iron the 8.3 mm is fine.
- For a blind hole with 1.5 × D thread engagement (14.3 mm), drill about 19.4 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 3/8" BSF
| Fit | Drill |
|---|---|
| Close | 25/64" (9.92 mm) |
| Medium (normal) | 13/32" (10.32 mm) |
| Free (loose) | 7/16" (11.11 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.
3/8" BSF thread data
| Designation | 3/8" BSF (20 TPI) |
|---|---|
| Standard | BS 84 Whitworth form (55°) |
| Pitch | 20 TPI (1.27 mm pitch) |
| Thread angle | 55° |
| Major diameter | 0.375" (9.525 mm) |
| Pitch (effective) diameter | 0.343" (8.712 mm) |
| Minor diameter, external | 0.311" (7.899 mm) |
| Minor diameter, internal | 0.311" (7.899 mm) |
| Thread depth (internal) | 0.813 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
Quick calculator
Nearest alternatives
If you don’t have the right drill, these give:
- 8.2 mm drill → about 81% thread
- 8.3 mm drill → about 75% thread
- 8.4 mm drill → about 69% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 3/8" UNC – 9.52 mm major, 16 TPI (1.587 mm pitch)
- 3/8" UNF – 9.52 mm major, 24 TPI (1.058 mm pitch)
- 3/8" BSW – 9.52 mm major, 16 TPI (1.587 mm pitch)
- G1/8 (1/8" BSP) – 9.73 mm major, 28 TPI (0.907 mm pitch)
- M10 – 10 mm major, 1.5 mm pitch (16.9 TPI equivalent)
Questions
What size drill for a 3/8" BSF tap?
The tap drill for 3/8" BSF (20 TPI) is 8.3 mm, which gives about 75% 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 3/8" BSF?
For a 3/8" BSF screw or bolt, drill 25/64" for a close fit, 13/32" for a medium (normal) fit or 7/16" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 3/8" BSF?
3/8" BSF (20 TPI) has a 20 TPI (1.27 mm pitch), a 55° thread angle and a 9.525 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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