7/8-14 UNF tap drill size and thread dimensions
7/8-14 UNF has a major diameter of 22.225 mm and a pitch of 14 threads per inch (1.8143 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 24.003
- Pitch diameter
- 21.0466
- Minor diameter of the nut thread
- 20.2610
- Minor diameter of the bolt thread
- 19.9991
- Tensile stress area
- 330.800 mm2
- Breaking load at that tensile strength
- 264 640
- The old "diameter minus pitch" drill
- 20.411
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 18.35
- Equivalent threads per inch
- 14.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 33.3
- Material tensile strength = 800
- major = 22.224999999999998 = 22.225
- p = 1.8142857142857143 = 1.814 29
- h = 1.814 29 * 0.8660254037844386 = 1.571 22
- Tap drill = 22.225 - 75/100 * 1.082532 * 1.814 29 = 20.752
- The old "diameter minus pitch" drill = 22.225 - 1.814 29 = 20.4107
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 22.225 - 0.649519 * 1.814 29 = 21.0466
- Minor diameter of the nut thread = 22.225 - 1.082532 * 1.814 29 = 20.261
- Minor diameter of the bolt thread = 22.225 - 1.226869 * 1.814 29 = 19.9991
- Tensile stress area = pi/4 * (22.225 - 0.938194 * 1.814 29)^2 = 330.8
- Clearance hole = 22.225 * (1 + 8/100) = 24.003
- Breaking load at that tensile strength = 330.8 * 800 = 264 640
- shear_area = pi * 20.261 * 33.3 * (0.5 + 0.57735 * (21.0466 - 20.261)/1.814 29) = 1589.7
- Threads engaged over that length = 1.814 29 > 0 ? 33.3 / 1.814 29 : 0 = 18.3543
- Equivalent threads per inch = 1.814 29 > 0 ? 25.4 / 1.814 29 : 0 = 14
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- tap drill = D - (engagement / 100) x 1.082532 x P
- pitch diameter = D - 0.649519 P
- internal minor diameter = D - 1.082532 P
- tensile stress area = (pi/4)(D - 0.938194 P)^2
The familiar "drill equals diameter minus pitch" rule actually produces about 92% thread engagement, not 75%. Past roughly 75% the extra holding power is negligible and the tapping torque rises sharply, which is why 75% or even 65% is the shop standard.
The tensile stress area formula is the ISO 898-1 definition, based on the mean of the pitch and minor diameters.
Cut a clearance hole rather than a close fit unless the joint is dowelled: thread clearance is what lets parts line up.
7/8-14 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 20.948 | 0.8247 |
| 70% | 20.850 | 0.8209 |
| 75% | 20.752 | 0.8170 |
| 80% | 20.654 | 0.8131 |
| 85% | 20.556 | 0.8093 |
| 90% | 20.457 | 0.8054 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 7/8-14 UNF?
20.75 mm (0.8170 in) for 75% thread engagement. The traditional rule gives 20.41 mm, which is a tighter and harder-tapping hole.
What is the pitch of 7/8-14 UNF?
14 threads per inch (1.8143 mm).
Where the numbers come from
- Thread geometry follows the ISO 68-1 basic profile shared by ISO metric and Unified inch threads; the stress-area definition is from ISO 898-1.