10-32 UNF tap drill size and thread dimensions
10-32 UNF has a major diameter of 4.826 mm and a pitch of 32 threads per inch (0.7937 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 5.212
- Pitch diameter
- 4.3104
- Minor diameter of the nut thread
- 3.9667
- Minor diameter of the bolt thread
- 3.8522
- Tensile stress area
- 13.082 mm2
- Breaking load at that tensile strength
- 10 466
- The old "diameter minus pitch" drill
- 4.032
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 9.07
- Equivalent threads per inch
- 32.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 7.2
- Material tensile strength = 800
- major = 4.826 = 4.826
- p = 0.79375 = 0.793 75
- h = 0.793 75 * 0.8660254037844386 = 0.687 408
- Tap drill = 4.826 - 75/100 * 1.082532 * 0.793 75 = 4.181 56
- The old "diameter minus pitch" drill = 4.826 - 0.793 75 = 4.032 25
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 4.826 - 0.649519 * 0.793 75 = 4.310 44
- Minor diameter of the nut thread = 4.826 - 1.082532 * 0.793 75 = 3.966 74
- Minor diameter of the bolt thread = 4.826 - 1.226869 * 0.793 75 = 3.852 17
- Tensile stress area = pi/4 * (4.826 - 0.938194 * 0.793 75)^2 = 13.0824
- Clearance hole = 4.826 * (1 + 8/100) = 5.212 08
- Breaking load at that tensile strength = 13.0824 * 800 = 10 466
- shear_area = pi * 3.966 74 * 7.2 * (0.5 + 0.57735 * (4.310 44 - 3.966 74)/0.793 75) = 67.2942
- Threads engaged over that length = 0.793 75 > 0 ? 7.2 / 0.793 75 : 0 = 9.070 87
- Equivalent threads per inch = 0.793 75 > 0 ? 25.4 / 0.793 75 : 0 = 32
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.
10-32 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 4.267 | 0.1680 |
| 70% | 4.225 | 0.1663 |
| 75% | 4.182 | 0.1646 |
| 80% | 4.139 | 0.1629 |
| 85% | 4.096 | 0.1612 |
| 90% | 4.053 | 0.1596 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 10-32 UNF?
4.18 mm (0.1646 in) for 75% thread engagement. The traditional rule gives 4.03 mm, which is a tighter and harder-tapping hole.
What is the pitch of 10-32 UNF?
32 threads per inch (0.7937 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.