M7 tap drill size and thread dimensions
M7 has a major diameter of 7.000 mm and a pitch of 1.0 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 7.560
- Pitch diameter
- 6.3505
- Minor diameter of the nut thread
- 5.9175
- Minor diameter of the bolt thread
- 5.7731
- Tensile stress area
- 28.860 mm2
- Breaking load at that tensile strength
- 23 088
- The old "diameter minus pitch" drill
- 6.000
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 10.50
- Equivalent threads per inch
- 25.400
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 10.5
- Material tensile strength = 800
- major = 7 = 7
- p = 1 = 1
- h = 1 * 0.8660254037844386 = 0.866 025
- Tap drill = 7 - 75/100 * 1.082532 * 1 = 6.1881
- The old "diameter minus pitch" drill = 7 - 1 = 6
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 7 - 0.649519 * 1 = 6.350 48
- Minor diameter of the nut thread = 7 - 1.082532 * 1 = 5.917 47
- Minor diameter of the bolt thread = 7 - 1.226869 * 1 = 5.773 13
- Tensile stress area = pi/4 * (7 - 0.938194 * 1)^2 = 28.8598
- Clearance hole = 7 * (1 + 8/100) = 7.56
- Breaking load at that tensile strength = 28.8598 * 800 = 23 087.9
- shear_area = pi * 5.917 47 * 10.5 * (0.5 + 0.57735 * (6.350 48 - 5.917 47)/1) = 146.398
- Threads engaged over that length = 1 > 0 ? 10.5 / 1 : 0 = 10.5
- Equivalent threads per inch = 1 > 0 ? 25.4 / 1 : 0 = 25.4
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.
M7 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 6.296 | 0.2479 |
| 70% | 6.242 | 0.2458 |
| 75% | 6.188 | 0.2436 |
| 80% | 6.134 | 0.2415 |
| 85% | 6.080 | 0.2394 |
| 90% | 6.026 | 0.2372 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M7?
6.19 mm (0.2436 in) for 75% thread engagement. The traditional rule gives 6.00 mm, which is a tighter and harder-tapping hole.
What is the pitch of M7?
1.0 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.