7/16-14 UNC tap drill size and thread dimensions
7/16-14 UNC has a major diameter of 11.112 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
- 12.001
- Pitch diameter
- 9.9341
- Minor diameter of the nut thread
- 9.1485
- Minor diameter of the bolt thread
- 8.8866
- Tensile stress area
- 69.551 mm2
- Breaking load at that tensile strength
- 55 641
- The old "diameter minus pitch" drill
- 9.298
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 9.20
- Equivalent threads per inch
- 14.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 16.7
- Material tensile strength = 800
- major = 11.112499999999999 = 11.1125
- p = 1.8142857142857143 = 1.814 29
- h = 1.814 29 * 0.8660254037844386 = 1.571 22
- Tap drill = 11.1125 - 75/100 * 1.082532 * 1.814 29 = 9.639 48
- The old "diameter minus pitch" drill = 11.1125 - 1.814 29 = 9.298 21
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 11.1125 - 0.649519 * 1.814 29 = 9.934 09
- Minor diameter of the nut thread = 11.1125 - 1.082532 * 1.814 29 = 9.148 48
- Minor diameter of the bolt thread = 11.1125 - 1.226869 * 1.814 29 = 8.886 61
- Tensile stress area = pi/4 * (11.1125 - 0.938194 * 1.814 29)^2 = 69.5507
- Clearance hole = 11.1125 * (1 + 8/100) = 12.0015
- Breaking load at that tensile strength = 69.5507 * 800 = 55 640.5
- shear_area = pi * 9.148 48 * 16.7 * (0.5 + 0.57735 * (9.934 09 - 9.148 48)/1.814 29) = 359.978
- Threads engaged over that length = 1.814 29 > 0 ? 16.7 / 1.814 29 : 0 = 9.204 72
- 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.
7/16-14 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 9.836 | 0.3872 |
| 70% | 9.738 | 0.3834 |
| 75% | 9.639 | 0.3795 |
| 80% | 9.541 | 0.3756 |
| 85% | 9.443 | 0.3718 |
| 90% | 9.345 | 0.3679 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 7/16-14 UNC?
9.64 mm (0.3795 in) for 75% thread engagement. The traditional rule gives 9.30 mm, which is a tighter and harder-tapping hole.
What is the pitch of 7/16-14 UNC?
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.