6-32 UNC tap drill size and thread dimensions
6-32 UNC has a major diameter of 3.505 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
- 3.786
- Pitch diameter
- 2.9896
- Minor diameter of the nut thread
- 2.6459
- Minor diameter of the bolt thread
- 2.5314
- Tensile stress area
- 5.985 mm2
- Breaking load at that tensile strength
- 4788
- The old "diameter minus pitch" drill
- 2.711
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 6.68
- Equivalent threads per inch
- 32.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 5.3
- Material tensile strength = 800
- major = 3.5052000000000003 = 3.5052
- p = 0.79375 = 0.793 75
- h = 0.793 75 * 0.8660254037844386 = 0.687 408
- Tap drill = 3.5052 - 75/100 * 1.082532 * 0.793 75 = 2.860 76
- The old "diameter minus pitch" drill = 3.5052 - 0.793 75 = 2.711 45
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 3.5052 - 0.649519 * 0.793 75 = 2.989 64
- Minor diameter of the nut thread = 3.5052 - 1.082532 * 0.793 75 = 2.645 94
- Minor diameter of the bolt thread = 3.5052 - 1.226869 * 0.793 75 = 2.531 37
- Tensile stress area = pi/4 * (3.5052 - 0.938194 * 0.793 75)^2 = 5.985 05
- Clearance hole = 3.5052 * (1 + 8/100) = 3.785 62
- Breaking load at that tensile strength = 5.985 05 * 800 = 4788.04
- shear_area = pi * 2.645 94 * 5.3 * (0.5 + 0.57735 * (2.989 64 - 2.645 94)/0.793 75) = 33.0421
- Threads engaged over that length = 0.793 75 > 0 ? 5.3 / 0.793 75 : 0 = 6.677 17
- 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.
6-32 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 2.947 | 0.1160 |
| 70% | 2.904 | 0.1143 |
| 75% | 2.861 | 0.1126 |
| 80% | 2.818 | 0.1109 |
| 85% | 2.775 | 0.1092 |
| 90% | 2.732 | 0.1076 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 6-32 UNC?
2.86 mm (0.1126 in) for 75% thread engagement. The traditional rule gives 2.71 mm, which is a tighter and harder-tapping hole.
What is the pitch of 6-32 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.