6-40 UNF tap drill size and thread dimensions
6-40 UNF has a major diameter of 3.505 mm and a pitch of 40 threads per inch (0.6350 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 3.786
- Pitch diameter
- 3.0928
- Minor diameter of the nut thread
- 2.8178
- Minor diameter of the bolt thread
- 2.7261
- Tensile stress area
- 6.648 mm2
- Breaking load at that tensile strength
- 5319
- The old "diameter minus pitch" drill
- 2.870
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 8.35
- Equivalent threads per inch
- 40.000
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 = 0.635 = 0.635
- h = 0.635 * 0.8660254037844386 = 0.549 926
- Tap drill = 3.5052 - 75/100 * 1.082532 * 0.635 = 2.989 64
- The old "diameter minus pitch" drill = 3.5052 - 0.635 = 2.8702
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 3.5052 - 0.649519 * 0.635 = 3.092 76
- Minor diameter of the nut thread = 3.5052 - 1.082532 * 0.635 = 2.817 79
- Minor diameter of the bolt thread = 3.5052 - 1.226869 * 0.635 = 2.726 14
- Tensile stress area = pi/4 * (3.5052 - 0.938194 * 0.635)^2 = 6.6483
- Clearance hole = 3.5052 * (1 + 8/100) = 3.785 62
- Breaking load at that tensile strength = 6.6483 * 800 = 5318.64
- shear_area = pi * 2.817 79 * 5.3 * (0.5 + 0.57735 * (3.092 76 - 2.817 79)/0.635) = 35.1881
- Threads engaged over that length = 0.635 > 0 ? 5.3 / 0.635 : 0 = 8.346 46
- Equivalent threads per inch = 0.635 > 0 ? 25.4 / 0.635 : 0 = 40
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.
6-40 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 3.058 | 0.1204 |
| 70% | 3.024 | 0.1191 |
| 75% | 2.990 | 0.1177 |
| 80% | 2.955 | 0.1163 |
| 85% | 2.921 | 0.1150 |
| 90% | 2.887 | 0.1136 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 6-40 UNF?
2.99 mm (0.1177 in) for 75% thread engagement. The traditional rule gives 2.87 mm, which is a tighter and harder-tapping hole.
What is the pitch of 6-40 UNF?
40 threads per inch (0.6350 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.