3/8-24 UNF tap drill size and thread dimensions
3/8-24 UNF has a major diameter of 9.525 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
- 10.287
- Pitch diameter
- 8.8376
- Minor diameter of the nut thread
- 8.3793
- Minor diameter of the bolt thread
- 8.2266
- Tensile stress area
- 57.174 mm2
- Breaking load at that tensile strength
- 45 739
- The old "diameter minus pitch" drill
- 8.467
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 13.51
- Equivalent threads per inch
- 24.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 14.3
- Material tensile strength = 800
- major = 9.524999999999999 = 9.525
- p = 1.0583333333333333 = 1.058 33
- h = 1.058 33 * 0.8660254037844386 = 0.916 544
- Tap drill = 9.525 - 75/100 * 1.082532 * 1.058 33 = 8.665 74
- The old "diameter minus pitch" drill = 9.525 - 1.058 33 = 8.466 67
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 9.525 - 0.649519 * 1.058 33 = 8.837 59
- Minor diameter of the nut thread = 9.525 - 1.082532 * 1.058 33 = 8.379 32
- Minor diameter of the bolt thread = 9.525 - 1.226869 * 1.058 33 = 8.226 56
- Tensile stress area = pi/4 * (9.525 - 0.938194 * 1.058 33)^2 = 57.1741
- Clearance hole = 9.525 * (1 + 8/100) = 10.287
- Breaking load at that tensile strength = 57.1741 * 800 = 45 739.3
- shear_area = pi * 8.379 32 * 14.3 * (0.5 + 0.57735 * (8.837 59 - 8.379 32)/1.058 33) = 282.329
- Threads engaged over that length = 1.058 33 > 0 ? 14.3 / 1.058 33 : 0 = 13.5118
- 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.
3/8-24 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 8.780 | 0.3457 |
| 70% | 8.723 | 0.3434 |
| 75% | 8.666 | 0.3412 |
| 80% | 8.608 | 0.3389 |
| 85% | 8.551 | 0.3367 |
| 90% | 8.494 | 0.3344 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 3/8-24 UNF?
8.67 mm (0.3412 in) for 75% thread engagement. The traditional rule gives 8.47 mm, which is a tighter and harder-tapping hole.
What is the pitch of 3/8-24 UNF?
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.