5/16" BSW tap drill size
The tap drill for 5/16" BSW (18 TPI) is 6.6 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% | 6.853 mm 0.2698" | 6.9 mm (57%) | letter I 0.2720" (57%) |
| 65% | 6.763 mm 0.2663" | 6.8 mm (63%) | letter H 0.2660" (65%) |
| 70% | 6.672 mm 0.2627" | 6.7 mm (68%) | letter G 0.2610" (72%) |
| 75% | 6.582 mm 0.2591" | 6.6 mm (74%) | letter G 0.2610" (72%) |
| 80% | 6.492 mm 0.2556" | 6.5 mm (80%) | letter F 0.2570" (78%) |
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 6.8 mm drill gives 63%. For mild steel, brass and cast iron the 6.6 mm is fine.
- For a blind hole with 1.5 × D thread engagement (11.9 mm), drill about 17.6 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 5/16" BSW
| Fit | Drill |
|---|---|
| Close | 21/64" (8.33 mm) |
| Medium (normal) | 11/32" (8.73 mm) |
| Free (loose) | 23/64" (9.13 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.
5/16" BSW thread data
| Designation | 5/16" BSW (18 TPI) |
|---|---|
| Standard | BS 84 Whitworth form (55°) |
| Pitch | 18 TPI (1.411 mm pitch) |
| Thread angle | 55° |
| Major diameter | 0.313" (7.938 mm) |
| Pitch (effective) diameter | 0.277" (7.034 mm) |
| Minor diameter, external | 0.241" (6.13 mm) |
| Minor diameter, internal | 0.241" (6.13 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.
Quick calculator
Nearest alternatives
If you don’t have the right drill, these give:
- 6.5 mm drill → about 80% thread
- 6.6 mm drill → about 74% thread
- 6.7 mm drill → about 68% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 5/16" UNC – 7.94 mm major, 18 TPI (1.411 mm pitch)
- 5/16" UNF – 7.94 mm major, 24 TPI (1.058 mm pitch)
- 5/16" BSF – 7.94 mm major, 22 TPI (1.155 mm pitch)
- M8 – 8 mm major, 1.25 mm pitch (20.3 TPI equivalent)
- M8 × 1 – 8 mm major, 1 mm pitch (25.4 TPI equivalent)
Questions
What size drill for a 5/16" BSW tap?
The tap drill for 5/16" BSW (18 TPI) is 6.6 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 5/16" BSW?
For a 5/16" BSW screw or bolt, drill 21/64" for a close fit, 11/32" for a medium (normal) fit or 23/64" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 5/16" BSW?
5/16" BSW (18 TPI) has a 18 TPI (1.411 mm pitch), a 55° thread angle and a 7.938 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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