M2 tap drill size and thread dimensions
M2 has a major diameter of 2.000 mm and a pitch of 0.4 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 2.160
- Pitch diameter
- 1.7402
- Minor diameter of the nut thread
- 1.5670
- Minor diameter of the bolt thread
- 1.5093
- Tensile stress area
- 2.073 mm2
- Breaking load at that tensile strength
- 1659
- The old "diameter minus pitch" drill
- 1.600
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 7.50
- Equivalent threads per inch
- 63.500
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 3
- Material tensile strength = 800
- major = 2 = 2
- p = 0.4 = 0.4
- h = 0.4 * 0.8660254037844386 = 0.346 41
- Tap drill = 2 - 75/100 * 1.082532 * 0.4 = 1.675 24
- The old "diameter minus pitch" drill = 2 - 0.4 = 1.6
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 2 - 0.649519 * 0.4 = 1.740 19
- Minor diameter of the nut thread = 2 - 1.082532 * 0.4 = 1.566 99
- Minor diameter of the bolt thread = 2 - 1.226869 * 0.4 = 1.509 25
- Tensile stress area = pi/4 * (2 - 0.938194 * 0.4)^2 = 2.073 23
- Clearance hole = 2 * (1 + 8/100) = 2.16
- Breaking load at that tensile strength = 2.073 23 * 800 = 1658.59
- shear_area = pi * 1.566 99 * 3 * (0.5 + 0.57735 * (1.740 19 - 1.566 99)/0.4) = 11.0764
- Threads engaged over that length = 0.4 > 0 ? 3 / 0.4 : 0 = 7.5
- Equivalent threads per inch = 0.4 > 0 ? 25.4 / 0.4 : 0 = 63.5
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.
M2 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 1.719 | 0.0677 |
| 70% | 1.697 | 0.0668 |
| 75% | 1.675 | 0.0660 |
| 80% | 1.654 | 0.0651 |
| 85% | 1.632 | 0.0642 |
| 90% | 1.610 | 0.0634 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M2?
1.68 mm (0.0660 in) for 75% thread engagement. The traditional rule gives 1.60 mm, which is a tighter and harder-tapping hole.
What is the pitch of M2?
0.4 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.