#6 UNF tap drill size
The tap drill for #6-40 UNF is #33 (2.87 mm). That’s the standard chart size and gives about 77% of full thread.
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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% | 3.01 mm 0.1185" | 3 mm (61%) | #31 0.1200" (55%) |
| 65% | 2.969 mm 0.1169" | 2.95 mm (67%) | #32 0.1160" (68%) |
| 70% | 2.928 mm 0.1153" | 2.95 mm (67%) | #32 0.1160" (68%) |
| 75% | 2.887 mm 0.1136" | 2.9 mm (73%) | #33 0.1130" (77%) |
| 80% | 2.845 mm 0.1120" | 2.85 mm (79%) | #33 0.1130" (77%) |
About this size
UNF is the fine Unified series, common on older British and American vehicles, aircraft and instruments.
- For stainless steel, duplex and other tough materials, go to about 65% thread: a 2.95 mm drill gives 67%. For mild steel, brass and cast iron the #33 drill is fine.
- For a blind hole with 1.5 × D thread engagement (5.3 mm), drill about 7.8 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 #6 UNF
| Fit | Drill |
|---|---|
| Close | 5/32" (3.97 mm) |
| Medium (normal) | 5/32" (3.97 mm) |
| Free (loose) | 11/64" (4.37 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.
#6 UNF thread data
| Designation | #6-40 UNF |
|---|---|
| Standard | ASME B1.1 Unified basic profile (same 60° basic form as ISO metric) |
| Pitch | 40 TPI (0.635 mm pitch) |
| Thread angle | 60° |
| Major diameter | 0.138" (3.505 mm) |
| Pitch (effective) diameter | 0.122" (3.093 mm) |
| Minor diameter, external | 0.107" (2.726 mm) |
| Minor diameter, internal | 0.111" (2.818 mm) |
| Thread depth (internal) | 0.344 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:
- 2.85 mm drill → about 79% thread
- 2.9 mm drill → about 73% thread
- 2.95 mm drill → about 67% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- #6 UNC – 3.51 mm major, 32 TPI (0.794 mm pitch)
- M3.5 – 3.5 mm major, 0.6 mm pitch (42.3 TPI equivalent)
- 4BA – 3.6 mm major, 0.66 mm pitch (38.5 TPI equivalent)
- 5BA – 3.2 mm major, 0.59 mm pitch (43.1 TPI equivalent)
- 1/8" BSW – 3.17 mm major, 40 TPI (0.635 mm pitch)
Questions
What size drill for a #6 UNF tap?
The tap drill for #6-40 UNF is #33 (2.87 mm). That’s the standard chart size and gives about 77% of full thread.
What size clearance hole for #6 UNF?
For a #6 UNF screw or bolt, drill 5/32" for a close fit, 5/32" for a medium (normal) fit or 11/64" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of #6 UNF?
#6-40 UNF has a 40 TPI (0.635 mm pitch), a 60° thread angle and a 3.505 mm major diameter.
Sources: Basic sizes per ASME B1.1. Chart drill from the common UN 75% tapping chart where one exists. Number and letter drill sizes checked 25/10/2026. Values are for reference only – check your tap maker’s chart. See the disclaimer.
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