7/16" BSF tap drill size
The tap drill for 7/16" BSF (18 TPI) is 9.8 mm, which gives about 73% 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% | 10.028 mm 0.3948" | 10 mm (62%) | letter X 0.3970" (57%) |
| 65% | 9.938 mm 0.3913" | 9.9 mm (67%) | 25/64" 0.3906" (66%) |
| 70% | 9.847 mm 0.3877" | 9.8 mm (73%) | letter W 0.3860" (72%) |
| 75% | 9.757 mm 0.3841" | 9.8 mm (73%) | letter W 0.3860" (72%) |
| 80% | 9.667 mm 0.3806" | 9.7 mm (78%) | letter V 0.3770" (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 9.9 mm drill gives 67%. For mild steel, brass and cast iron the 9.8 mm is fine.
- For a blind hole with 1.5 × D thread engagement (16.7 mm), drill about 22.3 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 7/16" BSF
| Fit | Drill |
|---|---|
| Close | 15/32" (11.91 mm) |
| Medium (normal) | 31/64" (12.3 mm) |
| Free (loose) | 33/64" (13.1 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.
7/16" BSF thread data
| Designation | 7/16" BSF (18 TPI) |
|---|---|
| Standard | BS 84 Whitworth form (55°) |
| Pitch | 18 TPI (1.411 mm pitch) |
| Thread angle | 55° |
| Major diameter | 0.438" (11.112 mm) |
| Pitch (effective) diameter | 0.402" (10.209 mm) |
| Minor diameter, external | 0.366" (9.305 mm) |
| Minor diameter, internal | 0.366" (9.305 mm) |
| Thread depth (internal) | 0.904 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:
- 9.7 mm drill → about 78% thread
- 9.8 mm drill → about 73% thread
- 9.9 mm drill → about 67% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 7/16" UNC – 11.11 mm major, 14 TPI (1.814 mm pitch)
- 7/16" UNF – 11.11 mm major, 20 TPI (1.27 mm pitch)
- 7/16" BSW – 11.11 mm major, 14 TPI (1.814 mm pitch)
- M12 – 12 mm major, 1.75 mm pitch (14.5 TPI equivalent)
- M12 × 1.5 – 12 mm major, 1.5 mm pitch (16.9 TPI equivalent)
Questions
What size drill for a 7/16" BSF tap?
The tap drill for 7/16" BSF (18 TPI) is 9.8 mm, which gives about 73% 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 7/16" BSF?
For a 7/16" BSF screw or bolt, drill 15/32" for a close fit, 31/64" for a medium (normal) fit or 33/64" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 7/16" BSF?
7/16" BSF (18 TPI) has a 18 TPI (1.411 mm pitch), a 55° thread angle and a 11.112 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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