1/4" BSW tap drill size
The tap drill for 1/4" BSW (20 TPI) is 5.1 mm, which gives about 77% 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% | 5.374 mm 0.2116" | 5.4 mm (58%) | #3 0.2130" (58%) |
| 65% | 5.293 mm 0.2084" | 5.3 mm (65%) | #4 0.2090" (64%) |
| 70% | 5.211 mm 0.2052" | 5.2 mm (71%) | #5 0.2055" (69%) |
| 75% | 5.13 mm 0.2020" | 5.1 mm (77%) | #7 0.2010" (77%) |
| 80% | 5.049 mm 0.1988" | 5 mm (83%) | #8 0.1990" (80%) |
About this size
BSW (Whitworth) has a 55° thread with rounded crests and roots. You will mostly meet it on older British machines and in restoration work. The rounded form means a Whitworth tap gives a slightly stronger root than a 60° thread of the same pitch.
- For stainless steel, duplex and other tough materials, go to about 65% thread: a 5.3 mm drill gives 65%. For mild steel, brass and cast iron the 5.1 mm is fine.
- For a blind hole with 1.5 × D thread engagement (9.5 mm), drill about 14.6 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 1/4" BSW
| Fit | Drill |
|---|---|
| Close | 17/64" (6.75 mm) |
| Medium (normal) | 9/32" (7.14 mm) |
| Free (loose) | 19/64" (7.54 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.
1/4" BSW thread data
| Designation | 1/4" BSW (20 TPI) |
|---|---|
| Standard | BS 84 Whitworth form (55°) |
| Pitch | 20 TPI (1.27 mm pitch) |
| Thread angle | 55° |
| Major diameter | 0.250" (6.35 mm) |
| Pitch (effective) diameter | 0.218" (5.537 mm) |
| Minor diameter, external | 0.186" (4.724 mm) |
| Minor diameter, internal | 0.186" (4.724 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:
- 5 mm drill → about 83% thread
- 5.1 mm drill → about 77% thread
- 5.2 mm drill → about 71% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 1/4" UNC – 6.35 mm major, 20 TPI (1.27 mm pitch)
- 1/4" UNF – 6.35 mm major, 28 TPI (0.907 mm pitch)
- 1/4" BSF – 6.35 mm major, 26 TPI (0.977 mm pitch)
- 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 1/4" BSW tap?
The tap drill for 1/4" BSW (20 TPI) is 5.1 mm, which gives about 77% 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 1/4" BSW?
For a 1/4" BSW screw or bolt, drill 17/64" for a close fit, 9/32" for a medium (normal) fit or 19/64" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 1/4" BSW?
1/4" BSW (20 TPI) has a 20 TPI (1.27 mm pitch), a 55° thread angle and a 6.35 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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