M1.6 tap drill size and thread dimensions
M1.6 has a major diameter of 1.600 mm and a pitch of 0.35 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 1.728
- Pitch diameter
- 1.3727
- Minor diameter of the nut thread
- 1.2211
- Minor diameter of the bolt thread
- 1.1706
- Tensile stress area
- 1.270 mm2
- Breaking load at that tensile strength
- 1016
- The old "diameter minus pitch" drill
- 1.250
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 6.86
- Equivalent threads per inch
- 72.571
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 2.4
- Material tensile strength = 800
- major = 1.6 = 1.6
- p = 0.35 = 0.35
- h = 0.35 * 0.8660254037844386 = 0.303 109
- Tap drill = 1.6 - 75/100 * 1.082532 * 0.35 = 1.315 84
- The old "diameter minus pitch" drill = 1.6 - 0.35 = 1.25
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 1.6 - 0.649519 * 0.35 = 1.372 67
- Minor diameter of the nut thread = 1.6 - 1.082532 * 0.35 = 1.221 11
- Minor diameter of the bolt thread = 1.6 - 1.226869 * 0.35 = 1.1706
- Tensile stress area = pi/4 * (1.6 - 0.938194 * 0.35)^2 = 1.270 03
- Clearance hole = 1.6 * (1 + 8/100) = 1.728
- Breaking load at that tensile strength = 1.270 03 * 800 = 1016.02
- shear_area = pi * 1.221 11 * 2.4 * (0.5 + 0.57735 * (1.372 67 - 1.221 11)/0.35) = 6.905 24
- Threads engaged over that length = 0.35 > 0 ? 2.4 / 0.35 : 0 = 6.857 14
- Equivalent threads per inch = 0.35 > 0 ? 25.4 / 0.35 : 0 = 72.5714
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.
M1.6 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 1.354 | 0.0533 |
| 70% | 1.335 | 0.0526 |
| 75% | 1.316 | 0.0518 |
| 80% | 1.297 | 0.0511 |
| 85% | 1.278 | 0.0503 |
| 90% | 1.259 | 0.0496 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M1.6?
1.32 mm (0.0518 in) for 75% thread engagement. The traditional rule gives 1.25 mm, which is a tighter and harder-tapping hole.
What is the pitch of M1.6?
0.35 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.