#8 UNC tap drill size
The tap drill for #8-32 UNC is #29 (3.45 mm). That’s the standard chart size and gives about 69% 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% | 3.547 mm 0.1396" | 3.5 mm (65%) | #28 0.1405" (58%) |
| 65% | 3.495 mm 0.1376" | 3.5 mm (65%) | #29 0.1360" (69%) |
| 70% | 3.444 mm 0.1356" | 3.4 mm (74%) | #29 0.1360" (69%) |
| 75% | 3.392 mm 0.1336" | 3.4 mm (74%) | #29 0.1360" (69%) |
| 80% | 3.341 mm 0.1315" | 3.3 mm (84%) | #30 0.1285" (87%) |
About this size
UNC is the general-purpose American inch thread. It shares the 60° basic profile with ISO metric, so the same tap-drill rule applies.
- For stainless steel, duplex and other tough materials, go to about 65% thread: a 3.5 mm drill gives 65%. For mild steel, brass and cast iron the #29 drill is fine.
- For a blind hole with 1.5 × D thread engagement (6.2 mm), drill about 9.4 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 #8 UNC
| Fit | Drill |
|---|---|
| Close | 11/64" (4.37 mm) |
| Medium (normal) | 3/16" (4.76 mm) |
| Free (loose) | 13/64" (5.16 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.
#8 UNC thread data
| Designation | #8-32 UNC |
|---|---|
| Standard | ASME B1.1 Unified basic profile (same 60° basic form as ISO metric) |
| Pitch | 32 TPI (0.794 mm pitch) |
| Thread angle | 60° |
| Major diameter | 0.164" (4.166 mm) |
| Pitch (effective) diameter | 0.144" (3.65 mm) |
| Minor diameter, external | 0.126" (3.192 mm) |
| Minor diameter, internal | 0.130" (3.306 mm) |
| Thread depth (internal) | 0.43 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:
- 3.3 mm drill → about 84% thread
- 3.4 mm drill → about 74% thread
- 3.5 mm drill → about 65% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- #8 UNF – 4.17 mm major, 36 TPI (0.706 mm pitch)
- 3BA – 4.1 mm major, 0.73 mm pitch (34.8 TPI equivalent)
- M4 – 4 mm major, 0.7 mm pitch (36.3 TPI equivalent)
Questions
What size drill for a #8 UNC tap?
The tap drill for #8-32 UNC is #29 (3.45 mm). That’s the standard chart size and gives about 69% of full thread.
What size clearance hole for #8 UNC?
For a #8 UNC screw or bolt, drill 11/64" for a close fit, 3/16" for a medium (normal) fit or 13/64" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of #8 UNC?
#8-32 UNC has a 32 TPI (0.794 mm pitch), a 60° thread angle and a 4.166 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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