M8 tap drill size and thread dimensions
M8 has a major diameter of 8.000 mm and a pitch of 1.25 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 8.640
- Pitch diameter
- 7.1881
- Minor diameter of the nut thread
- 6.6468
- Minor diameter of the bolt thread
- 6.4664
- Tensile stress area
- 36.609 mm2
- Breaking load at that tensile strength
- 29 287
- The old "diameter minus pitch" drill
- 6.750
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 9.60
- Equivalent threads per inch
- 20.320
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 12
- Material tensile strength = 800
- major = 8 = 8
- p = 1.25 = 1.25
- h = 1.25 * 0.8660254037844386 = 1.082 53
- Tap drill = 8 - 75/100 * 1.082532 * 1.25 = 6.985 13
- The old "diameter minus pitch" drill = 8 - 1.25 = 6.75
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 8 - 0.649519 * 1.25 = 7.1881
- Minor diameter of the nut thread = 8 - 1.082532 * 1.25 = 6.646 84
- Minor diameter of the bolt thread = 8 - 1.226869 * 1.25 = 6.466 41
- Tensile stress area = pi/4 * (8 - 0.938194 * 1.25)^2 = 36.6085
- Clearance hole = 8 * (1 + 8/100) = 8.64
- Breaking load at that tensile strength = 36.6085 * 800 = 29 286.8
- shear_area = pi * 6.646 84 * 12 * (0.5 + 0.57735 * (7.1881 - 6.646 84)/1.25) = 187.935
- Threads engaged over that length = 1.25 > 0 ? 12 / 1.25 : 0 = 9.6
- Equivalent threads per inch = 1.25 > 0 ? 25.4 / 1.25 : 0 = 20.32
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.
M8 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 7.120 | 0.2803 |
| 70% | 7.053 | 0.2777 |
| 75% | 6.985 | 0.2750 |
| 80% | 6.917 | 0.2723 |
| 85% | 6.850 | 0.2697 |
| 90% | 6.782 | 0.2670 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M8?
6.99 mm (0.2750 in) for 75% thread engagement. The traditional rule gives 6.75 mm, which is a tighter and harder-tapping hole.
What is the pitch of M8?
1.25 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.