M22 tap drill size and thread dimensions
M22 has a major diameter of 22.000 mm and a pitch of 2.5 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 23.760
- Pitch diameter
- 20.3762
- Minor diameter of the nut thread
- 19.2937
- Minor diameter of the bolt thread
- 18.9328
- Tensile stress area
- 303.399 mm2
- Breaking load at that tensile strength
- 242 719
- The old "diameter minus pitch" drill
- 19.500
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 13.20
- Equivalent threads per inch
- 10.160
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 33
- Material tensile strength = 800
- major = 22 = 22
- p = 2.5 = 2.5
- h = 2.5 * 0.8660254037844386 = 2.165 06
- Tap drill = 22 - 75/100 * 1.082532 * 2.5 = 19.9703
- The old "diameter minus pitch" drill = 22 - 2.5 = 19.5
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 22 - 0.649519 * 2.5 = 20.3762
- Minor diameter of the nut thread = 22 - 1.082532 * 2.5 = 19.2937
- Minor diameter of the bolt thread = 22 - 1.226869 * 2.5 = 18.9328
- Tensile stress area = pi/4 * (22 - 0.938194 * 2.5)^2 = 303.399
- Clearance hole = 22 * (1 + 8/100) = 23.76
- Breaking load at that tensile strength = 303.399 * 800 = 242 719
- shear_area = pi * 19.2937 * 33 * (0.5 + 0.57735 * (20.3762 - 19.2937)/2.5) = 1500.17
- Threads engaged over that length = 2.5 > 0 ? 33 / 2.5 : 0 = 13.2
- Equivalent threads per inch = 2.5 > 0 ? 25.4 / 2.5 : 0 = 10.16
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.
M22 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 20.241 | 0.7969 |
| 70% | 20.106 | 0.7916 |
| 75% | 19.970 | 0.7862 |
| 80% | 19.835 | 0.7809 |
| 85% | 19.700 | 0.7756 |
| 90% | 19.564 | 0.7702 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M22?
19.97 mm (0.7862 in) for 75% thread engagement. The traditional rule gives 19.50 mm, which is a tighter and harder-tapping hole.
What is the pitch of M22?
2.5 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.