7/32" BSF tap drill size
The tap drill for 7/32" BSF (28 TPI) is 4.7 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% | 4.859 mm 0.1913" | 4.9 mm (56%) | #11 0.1910" (61%) |
| 65% | 4.801 mm 0.1890" | 4.8 mm (65%) | #12 0.1890" (65%) |
| 70% | 4.743 mm 0.1867" | 4.7 mm (74%) | 3/16" 0.1875" (68%) |
| 75% | 4.685 mm 0.1844" | 4.7 mm (74%) | #13 0.1850" (74%) |
| 80% | 4.627 mm 0.1822" | 4.6 mm (82%) | #14 0.1820" (80%) |
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 4.8 mm drill gives 65%. For mild steel, brass and cast iron the 4.7 mm is fine.
- For a blind hole with 1.5 × D thread engagement (8.3 mm), drill about 12 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 7/32" BSF
| Fit | Drill |
|---|---|
| Close | 15/64" (5.95 mm) |
| Medium (normal) | 1/4" (6.35 mm) |
| Free (loose) | 17/64" (6.75 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.
7/32" BSF thread data
| Designation | 7/32" BSF (28 TPI) |
|---|---|
| Standard | BS 84 Whitworth form (55°) |
| Pitch | 28 TPI (0.907 mm pitch) |
| Thread angle | 55° |
| Major diameter | 0.219" (5.556 mm) |
| Pitch (effective) diameter | 0.196" (4.975 mm) |
| Minor diameter, external | 0.173" (4.395 mm) |
| Minor diameter, internal | 0.173" (4.395 mm) |
| Thread depth (internal) | 0.581 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:
- 4.6 mm drill → about 82% thread
- 4.7 mm drill → about 74% thread
- 4.8 mm drill → about 65% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- #12 UNC – 5.49 mm major, 24 TPI (1.058 mm pitch)
- #12 UNF – 5.49 mm major, 28 TPI (0.907 mm pitch)
- 1BA – 5.3 mm major, 0.9 mm pitch (28.2 TPI equivalent)
- M6 – 6 mm major, 1 mm pitch (25.4 TPI equivalent)
- 0BA – 6 mm major, 1 mm pitch (25.4 TPI equivalent)
Questions
What size drill for a 7/32" BSF tap?
The tap drill for 7/32" BSF (28 TPI) is 4.7 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 7/32" BSF?
For a 7/32" BSF screw or bolt, drill 15/64" for a close fit, 1/4" for a medium (normal) fit or 17/64" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 7/32" BSF?
7/32" BSF (28 TPI) has a 28 TPI (0.907 mm pitch), a 55° thread angle and a 5.556 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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