5/8-18 UNF tap drill size and thread dimensions
5/8-18 UNF has a major diameter of 15.875 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
- 17.145
- Pitch diameter
- 14.9585
- Minor diameter of the nut thread
- 14.3474
- Minor diameter of the bolt thread
- 14.1438
- Tensile stress area
- 166.296 mm2
- Breaking load at that tensile strength
- 133 037
- The old "diameter minus pitch" drill
- 14.464
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 16.87
- Equivalent threads per inch
- 18.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 23.8
- Material tensile strength = 800
- major = 15.875 = 15.875
- p = 1.411111111111111 = 1.411 11
- h = 1.411 11 * 0.8660254037844386 = 1.222 06
- Tap drill = 15.875 - 75/100 * 1.082532 * 1.411 11 = 14.7293
- The old "diameter minus pitch" drill = 15.875 - 1.411 11 = 14.4639
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 15.875 - 0.649519 * 1.411 11 = 14.9585
- Minor diameter of the nut thread = 15.875 - 1.082532 * 1.411 11 = 14.3474
- Minor diameter of the bolt thread = 15.875 - 1.226869 * 1.411 11 = 14.1438
- Tensile stress area = pi/4 * (15.875 - 0.938194 * 1.411 11)^2 = 166.296
- Clearance hole = 15.875 * (1 + 8/100) = 17.145
- Breaking load at that tensile strength = 166.296 * 800 = 133 037
- shear_area = pi * 14.3474 * 23.8 * (0.5 + 0.57735 * (14.9585 - 14.3474)/1.411 11) = 804.567
- Threads engaged over that length = 1.411 11 > 0 ? 23.8 / 1.411 11 : 0 = 16.8661
- 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.
5/8-18 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 14.882 | 0.5859 |
| 70% | 14.806 | 0.5829 |
| 75% | 14.729 | 0.5799 |
| 80% | 14.653 | 0.5769 |
| 85% | 14.577 | 0.5739 |
| 90% | 14.500 | 0.5709 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 5/8-18 UNF?
14.73 mm (0.5799 in) for 75% thread engagement. The traditional rule gives 14.46 mm, which is a tighter and harder-tapping hole.
What is the pitch of 5/8-18 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.