8-32 UNC tap drill size and thread dimensions
8-32 UNC has a major diameter of 4.166 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
- 4.499
- Pitch diameter
- 3.6500
- Minor diameter of the nut thread
- 3.3063
- Minor diameter of the bolt thread
- 3.1918
- Tensile stress area
- 9.191 mm2
- Breaking load at that tensile strength
- 7353
- The old "diameter minus pitch" drill
- 3.372
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 7.81
- Equivalent threads per inch
- 32.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 6.2
- Material tensile strength = 800
- major = 4.1655999999999995 = 4.1656
- p = 0.79375 = 0.793 75
- h = 0.793 75 * 0.8660254037844386 = 0.687 408
- Tap drill = 4.1656 - 75/100 * 1.082532 * 0.793 75 = 3.521 16
- The old "diameter minus pitch" drill = 4.1656 - 0.793 75 = 3.371 85
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 4.1656 - 0.649519 * 0.793 75 = 3.650 04
- Minor diameter of the nut thread = 4.1656 - 1.082532 * 0.793 75 = 3.306 34
- Minor diameter of the bolt thread = 4.1656 - 1.226869 * 0.793 75 = 3.191 77
- Tensile stress area = pi/4 * (4.1656 - 0.938194 * 0.793 75)^2 = 9.191 21
- Clearance hole = 4.1656 * (1 + 8/100) = 4.498 85
- Breaking load at that tensile strength = 9.191 21 * 800 = 7352.97
- shear_area = pi * 3.306 34 * 6.2 * (0.5 + 0.57735 * (3.650 04 - 3.306 34)/0.793 75) = 48.3004
- Threads engaged over that length = 0.793 75 > 0 ? 6.2 / 0.793 75 : 0 = 7.811 02
- 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.
Scenarios & local history
Compare a saved scenario
Recent calculations
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.
8-32 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 3.607 | 0.1420 |
| 70% | 3.564 | 0.1403 |
| 75% | 3.521 | 0.1386 |
| 80% | 3.478 | 0.1369 |
| 85% | 3.435 | 0.1352 |
| 90% | 3.392 | 0.1336 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 8-32 UNC?
3.52 mm (0.1386 in) for 75% thread engagement. The traditional rule gives 3.37 mm, which is a tighter and harder-tapping hole.
What is the pitch of 8-32 UNC?
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.