#4 UNF tap drill size
The tap drill for #4-48 UNF is #42 (2.37 mm). That’s the standard chart size and gives about 68% 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% | 2.432 mm 0.0958" | 2.45 mm (57%) | #41 0.0960" (59%) |
| 65% | 2.398 mm 0.0944" | 2.4 mm (65%) | 3/32" 0.0937" (67%) |
| 70% | 2.364 mm 0.0931" | 2.35 mm (72%) | #42 0.0935" (68%) |
| 75% | 2.329 mm 0.0917" | 2.35 mm (72%) | #42 0.0935" (68%) |
| 80% | 2.295 mm 0.0903" | 2.3 mm (79%) | #43 0.0890" (85%) |
About this size
UNF is the fine Unified series, common on older British and American vehicles, aircraft and instruments.
- Taps this small snap easily. By hand, or in stainless steel, a 2.4 mm drill (about 65% thread) makes tapping much easier for very little loss of strength.
- For a blind hole with 1.5 × D thread engagement (4.3 mm), drill about 6.4 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 #4 UNF
| Fit | Drill |
|---|---|
| Close | 1/8" (3.17 mm) |
| Medium (normal) | 9/64" (3.57 mm) |
| Free (loose) | 5/32" (3.97 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.
#4 UNF thread data
| Designation | #4-48 UNF |
|---|---|
| Standard | ASME B1.1 Unified basic profile (same 60° basic form as ISO metric) |
| Pitch | 48 TPI (0.529 mm pitch) |
| Thread angle | 60° |
| Major diameter | 0.112" (2.845 mm) |
| Pitch (effective) diameter | 0.098" (2.501 mm) |
| Minor diameter, external | 0.086" (2.196 mm) |
| Minor diameter, internal | 0.089" (2.272 mm) |
| Thread depth (internal) | 0.286 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:
- 2.3 mm drill → about 79% thread
- 2.35 mm drill → about 72% thread
- 2.4 mm drill → about 65% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- #4 UNC – 2.84 mm major, 40 TPI (0.635 mm pitch)
- 6BA – 2.8 mm major, 0.53 mm pitch (47.9 TPI equivalent)
- M3 – 3 mm major, 0.5 mm pitch (50.8 TPI equivalent)
Questions
What size drill for a #4 UNF tap?
The tap drill for #4-48 UNF is #42 (2.37 mm). That’s the standard chart size and gives about 68% of full thread.
What size clearance hole for #4 UNF?
For a #4 UNF screw or bolt, drill 1/8" for a close fit, 9/64" for a medium (normal) fit or 5/32" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of #4 UNF?
#4-48 UNF has a 48 TPI (0.529 mm pitch), a 60° thread angle and a 2.845 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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