10-24 UNC tap drill size and thread dimensions
10-24 UNC has a major diameter of 4.826 mm and a pitch of 24 threads per inch (1.0583 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 5.212
- Pitch diameter
- 4.1386
- Minor diameter of the nut thread
- 3.6803
- Minor diameter of the bolt thread
- 3.5276
- Tensile stress area
- 11.539 mm2
- Breaking load at that tensile strength
- 9232
- The old "diameter minus pitch" drill
- 3.768
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 6.80
- Equivalent threads per inch
- 24.000
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 = 1.0583333333333333 = 1.058 33
- h = 1.058 33 * 0.8660254037844386 = 0.916 544
- Tap drill = 4.826 - 75/100 * 1.082532 * 1.058 33 = 3.966 74
- The old "diameter minus pitch" drill = 4.826 - 1.058 33 = 3.767 67
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 4.826 - 0.649519 * 1.058 33 = 4.138 59
- Minor diameter of the nut thread = 4.826 - 1.082532 * 1.058 33 = 3.680 32
- Minor diameter of the bolt thread = 4.826 - 1.226869 * 1.058 33 = 3.527 56
- Tensile stress area = pi/4 * (4.826 - 0.938194 * 1.058 33)^2 = 11.5395
- Clearance hole = 4.826 * (1 + 8/100) = 5.212 08
- Breaking load at that tensile strength = 11.5395 * 800 = 9231.56
- shear_area = pi * 3.680 32 * 7.2 * (0.5 + 0.57735 * (4.138 59 - 3.680 32)/1.058 33) = 62.4352
- Threads engaged over that length = 1.058 33 > 0 ? 7.2 / 1.058 33 : 0 = 6.803 15
- Equivalent threads per inch = 1.058 33 > 0 ? 25.4 / 1.058 33 : 0 = 24
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.
10-24 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 4.081 | 0.1607 |
| 70% | 4.024 | 0.1584 |
| 75% | 3.967 | 0.1562 |
| 80% | 3.909 | 0.1539 |
| 85% | 3.852 | 0.1517 |
| 90% | 3.795 | 0.1494 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 10-24 UNC?
3.97 mm (0.1562 in) for 75% thread engagement. The traditional rule gives 3.77 mm, which is a tighter and harder-tapping hole.
What is the pitch of 10-24 UNC?
24 threads per inch (1.0583 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.