M1.2 tap drill size and thread dimensions
M1.2 has a major diameter of 1.200 mm and a pitch of 0.25 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 1.296
- Pitch diameter
- 1.0376
- Minor diameter of the nut thread
- 0.9294
- Minor diameter of the bolt thread
- 0.8933
- Tensile stress area
- 0.732 mm2
- Breaking load at that tensile strength
- 586
- The old "diameter minus pitch" drill
- 0.950
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 7.20
- Equivalent threads per inch
- 101.600
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 1.8
- Material tensile strength = 800
- major = 1.2 = 1.2
- p = 0.25 = 0.25
- h = 0.25 * 0.8660254037844386 = 0.216 506
- Tap drill = 1.2 - 75/100 * 1.082532 * 0.25 = 0.997 025
- The old "diameter minus pitch" drill = 1.2 - 0.25 = 0.95
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 1.2 - 0.649519 * 0.25 = 1.037 62
- Minor diameter of the nut thread = 1.2 - 1.082532 * 0.25 = 0.929 367
- Minor diameter of the bolt thread = 1.2 - 1.226869 * 0.25 = 0.893 283
- Tensile stress area = pi/4 * (1.2 - 0.938194 * 0.25)^2 = 0.732 067
- Clearance hole = 1.2 * (1 + 8/100) = 1.296
- Breaking load at that tensile strength = 0.732 067 * 800 = 585.654
- shear_area = pi * 0.929 367 * 1.8 * (0.5 + 0.57735 * (1.037 62 - 0.929 367)/0.25) = 3.941 59
- Threads engaged over that length = 0.25 > 0 ? 1.8 / 0.25 : 0 = 7.2
- Equivalent threads per inch = 0.25 > 0 ? 25.4 / 0.25 : 0 = 101.6
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
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.
M1.2 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 1.024 | 0.0403 |
| 70% | 1.011 | 0.0398 |
| 75% | 0.997 | 0.0393 |
| 80% | 0.983 | 0.0387 |
| 85% | 0.970 | 0.0382 |
| 90% | 0.956 | 0.0377 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M1.2?
1.00 mm (0.0393 in) for 75% thread engagement. The traditional rule gives 0.95 mm, which is a tighter and harder-tapping hole.
What is the pitch of M1.2?
0.25 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.