3/4-10 UNC tap drill size and thread dimensions
3/4-10 UNC has a major diameter of 19.050 mm and a pitch of 10 threads per inch (2.5400 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 20.574
- Pitch diameter
- 17.4002
- Minor diameter of the nut thread
- 16.3004
- Minor diameter of the bolt thread
- 15.9338
- Tensile stress area
- 218.175 mm2
- Breaking load at that tensile strength
- 174 540
- The old "diameter minus pitch" drill
- 16.510
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 11.26
- Equivalent threads per inch
- 10.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 28.6
- Material tensile strength = 800
- major = 19.049999999999997 = 19.05
- p = 2.54 = 2.54
- h = 2.54 * 0.8660254037844386 = 2.1997
- Tap drill = 19.05 - 75/100 * 1.082532 * 2.54 = 16.9878
- The old "diameter minus pitch" drill = 19.05 - 2.54 = 16.51
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 19.05 - 0.649519 * 2.54 = 17.4002
- Minor diameter of the nut thread = 19.05 - 1.082532 * 2.54 = 16.3004
- Minor diameter of the bolt thread = 19.05 - 1.226869 * 2.54 = 15.9338
- Tensile stress area = pi/4 * (19.05 - 0.938194 * 2.54)^2 = 218.175
- Clearance hole = 19.05 * (1 + 8/100) = 20.574
- Breaking load at that tensile strength = 218.175 * 800 = 174 540
- shear_area = pi * 16.3004 * 28.6 * (0.5 + 0.57735 * (17.4002 - 16.3004)/2.54) = 1098.44
- Threads engaged over that length = 2.54 > 0 ? 28.6 / 2.54 : 0 = 11.2598
- Equivalent threads per inch = 2.54 > 0 ? 25.4 / 2.54 : 0 = 10
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.
3/4-10 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 17.263 | 0.6796 |
| 70% | 17.125 | 0.6742 |
| 75% | 16.988 | 0.6688 |
| 80% | 16.850 | 0.6634 |
| 85% | 16.713 | 0.6580 |
| 90% | 16.575 | 0.6526 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 3/4-10 UNC?
16.99 mm (0.6688 in) for 75% thread engagement. The traditional rule gives 16.51 mm, which is a tighter and harder-tapping hole.
What is the pitch of 3/4-10 UNC?
10 threads per inch (2.5400 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.