M3.5 tap drill size and thread dimensions
M3.5 has a major diameter of 3.500 mm and a pitch of 0.6 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 3.780
- Pitch diameter
- 3.1103
- Minor diameter of the nut thread
- 2.8505
- Minor diameter of the bolt thread
- 2.7639
- Tensile stress area
- 6.775 mm2
- Breaking load at that tensile strength
- 5420
- The old "diameter minus pitch" drill
- 2.900
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 8.83
- Equivalent threads per inch
- 42.333
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 5.3
- Material tensile strength = 800
- major = 3.5 = 3.5
- p = 0.6 = 0.6
- h = 0.6 * 0.8660254037844386 = 0.519 615
- Tap drill = 3.5 - 75/100 * 1.082532 * 0.6 = 3.012 86
- The old "diameter minus pitch" drill = 3.5 - 0.6 = 2.9
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 3.5 - 0.649519 * 0.6 = 3.110 29
- Minor diameter of the nut thread = 3.5 - 1.082532 * 0.6 = 2.850 48
- Minor diameter of the bolt thread = 3.5 - 1.226869 * 0.6 = 2.763 88
- Tensile stress area = pi/4 * (3.5 - 0.938194 * 0.6)^2 = 6.775 21
- Clearance hole = 3.5 * (1 + 8/100) = 3.78
- Breaking load at that tensile strength = 6.775 21 * 800 = 5420.16
- shear_area = pi * 2.850 48 * 5.3 * (0.5 + 0.57735 * (3.110 29 - 2.850 48)/0.6) = 35.5963
- Threads engaged over that length = 0.6 > 0 ? 5.3 / 0.6 : 0 = 8.833 33
- Equivalent threads per inch = 0.6 > 0 ? 25.4 / 0.6 : 0 = 42.3333
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.
M3.5 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 3.078 | 0.1212 |
| 70% | 3.045 | 0.1199 |
| 75% | 3.013 | 0.1186 |
| 80% | 2.980 | 0.1173 |
| 85% | 2.948 | 0.1161 |
| 90% | 2.915 | 0.1148 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M3.5?
3.01 mm (0.1186 in) for 75% thread engagement. The traditional rule gives 2.90 mm, which is a tighter and harder-tapping hole.
What is the pitch of M3.5?
0.6 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.