7/8" UNF tap drill size
The tap drill for 7/8-14 UNF is 13/16" (20.64 mm). That’s the standard chart size and gives about 67% 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% | 20.811 mm 0.8193" | 20.75 mm (63%) | 13/16" 0.8125" (67%) |
| 65% | 20.693 mm 0.8147" | 20.75 mm (63%) | 13/16" 0.8125" (67%) |
| 70% | 20.575 mm 0.8100" | 20.5 mm (73%) | 13/16" 0.8125" (67%) |
| 75% | 20.457 mm 0.8054" | 20.5 mm (73%) | 13/16" 0.8125" (67%) |
| 80% | 20.34 mm 0.8008" | 20.25 mm (84%) | 51/64" 0.7969" (84%) |
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 20.75 mm drill gives 63%. For mild steel, brass and cast iron the 13/16" drill is fine.
- For a blind hole with 1.5 × D thread engagement (33.3 mm), drill about 40.6 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 7/8" UNF
| Fit | Drill |
|---|---|
| Close | 59/64" (23.42 mm) |
| Medium (normal) | 61/64" (24.21 mm) |
| Free (loose) | 1-1/64" (25.8 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.
7/8" UNF thread data
| Designation | 7/8-14 UNF |
|---|---|
| Standard | ASME B1.1 Unified basic profile (same 60° basic form as ISO metric) |
| Pitch | 14 TPI (1.814 mm pitch) |
| Thread angle | 60° |
| Major diameter | 0.875" (22.225 mm) |
| Pitch (effective) diameter | 0.829" (21.047 mm) |
| Minor diameter, external | 0.787" (19.999 mm) |
| Minor diameter, internal | 0.798" (20.261 mm) |
| Thread depth (internal) | 0.982 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:
- 20.25 mm drill → about 84% thread
- 20.5 mm drill → about 73% thread
- 20.75 mm drill → about 63% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 7/8" UNC – 22.22 mm major, 9 TPI (2.822 mm pitch)
- 7/8" BSW – 22.22 mm major, 9 TPI (2.822 mm pitch)
- 7/8" BSF – 22.22 mm major, 11 TPI (2.309 mm pitch)
- M22 – 22 mm major, 2.5 mm pitch (10.2 TPI equivalent)
- M22 × 1.5 – 22 mm major, 1.5 mm pitch (16.9 TPI equivalent)
Questions
What size drill for a 7/8" UNF tap?
The tap drill for 7/8-14 UNF is 13/16" (20.64 mm). That’s the standard chart size and gives about 67% of full thread.
What size clearance hole for 7/8" UNF?
For a 7/8" UNF screw or bolt, drill 59/64" for a close fit, 61/64" for a medium (normal) fit or 1-1/64" for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 7/8" UNF?
7/8-14 UNF has a 14 TPI (1.814 mm pitch), a 60° thread angle and a 22.225 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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