5/16-18 UNC tap drill size and thread dimensions
5/16-18 UNC has a major diameter of 7.938 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
- 8.573
- Pitch diameter
- 7.0210
- Minor diameter of the nut thread
- 6.4099
- Minor diameter of the bolt thread
- 6.2063
- Tensile stress area
- 34.353 mm2
- Breaking load at that tensile strength
- 27 483
- The old "diameter minus pitch" drill
- 6.526
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 8.43
- Equivalent threads per inch
- 18.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 11.9
- Material tensile strength = 800
- major = 7.9375 = 7.9375
- p = 1.411111111111111 = 1.411 11
- h = 1.411 11 * 0.8660254037844386 = 1.222 06
- Tap drill = 7.9375 - 75/100 * 1.082532 * 1.411 11 = 6.791 82
- The old "diameter minus pitch" drill = 7.9375 - 1.411 11 = 6.526 39
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 7.9375 - 0.649519 * 1.411 11 = 7.020 96
- Minor diameter of the nut thread = 7.9375 - 1.082532 * 1.411 11 = 6.409 93
- Minor diameter of the bolt thread = 7.9375 - 1.226869 * 1.411 11 = 6.206 25
- Tensile stress area = pi/4 * (7.9375 - 0.938194 * 1.411 11)^2 = 34.3531
- Clearance hole = 7.9375 * (1 + 8/100) = 8.5725
- Breaking load at that tensile strength = 34.3531 * 800 = 27 482.5
- shear_area = pi * 6.409 93 * 11.9 * (0.5 + 0.57735 * (7.020 96 - 6.409 93)/1.411 11) = 179.726
- Threads engaged over that length = 1.411 11 > 0 ? 11.9 / 1.411 11 : 0 = 8.433 07
- 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.
5/16-18 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 6.945 | 0.2734 |
| 70% | 6.868 | 0.2704 |
| 75% | 6.792 | 0.2674 |
| 80% | 6.715 | 0.2644 |
| 85% | 6.639 | 0.2614 |
| 90% | 6.563 | 0.2584 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 5/16-18 UNC?
6.79 mm (0.2674 in) for 75% thread engagement. The traditional rule gives 6.53 mm, which is a tighter and harder-tapping hole.
What is the pitch of 5/16-18 UNC?
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.