9/16-18 UNF tap drill size and thread dimensions
9/16-18 UNF has a major diameter of 14.287 mm and a pitch of 18 threads per inch (1.4111 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 15.431
- Pitch diameter
- 13.3710
- Minor diameter of the nut thread
- 12.7599
- Minor diameter of the bolt thread
- 12.5563
- Tensile stress area
- 131.990 mm2
- Breaking load at that tensile strength
- 105 592
- The old "diameter minus pitch" drill
- 12.876
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 15.17
- Equivalent threads per inch
- 18.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 21.4
- Material tensile strength = 800
- major = 14.2875 = 14.2875
- p = 1.411111111111111 = 1.411 11
- h = 1.411 11 * 0.8660254037844386 = 1.222 06
- Tap drill = 14.2875 - 75/100 * 1.082532 * 1.411 11 = 13.1418
- The old "diameter minus pitch" drill = 14.2875 - 1.411 11 = 12.8764
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 14.2875 - 0.649519 * 1.411 11 = 13.371
- Minor diameter of the nut thread = 14.2875 - 1.082532 * 1.411 11 = 12.7599
- Minor diameter of the bolt thread = 14.2875 - 1.226869 * 1.411 11 = 12.5563
- Tensile stress area = pi/4 * (14.2875 - 0.938194 * 1.411 11)^2 = 131.99
- Clearance hole = 14.2875 * (1 + 8/100) = 15.4305
- Breaking load at that tensile strength = 131.99 * 800 = 105 592
- shear_area = pi * 12.7599 * 21.4 * (0.5 + 0.57735 * (13.371 - 12.7599)/1.411 11) = 643.388
- Threads engaged over that length = 1.411 11 > 0 ? 21.4 / 1.411 11 : 0 = 15.1654
- Equivalent threads per inch = 1.411 11 > 0 ? 25.4 / 1.411 11 : 0 = 18
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.
9/16-18 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 13.295 | 0.5234 |
| 70% | 13.218 | 0.5204 |
| 75% | 13.142 | 0.5174 |
| 80% | 13.065 | 0.5144 |
| 85% | 12.989 | 0.5114 |
| 90% | 12.913 | 0.5084 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 9/16-18 UNF?
13.14 mm (0.5174 in) for 75% thread engagement. The traditional rule gives 12.88 mm, which is a tighter and harder-tapping hole.
What is the pitch of 9/16-18 UNF?
18 threads per inch (1.4111 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.