M8x1 tap drill size and thread dimensions
M8x1 has a major diameter of 8.000 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
- 8.640
- Pitch diameter
- 7.3505
- Minor diameter of the nut thread
- 6.9175
- Minor diameter of the bolt thread
- 6.7731
- Tensile stress area
- 39.167 mm2
- Breaking load at that tensile strength
- 31 334
- The old "diameter minus pitch" drill
- 7.000
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 12.00
- Equivalent threads per inch
- 25.400
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 12
- Material tensile strength = 800
- major = 8 = 8
- p = 1 = 1
- h = 1 * 0.8660254037844386 = 0.866 025
- Tap drill = 8 - 75/100 * 1.082532 * 1 = 7.1881
- The old "diameter minus pitch" drill = 8 - 1 = 7
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 8 - 0.649519 * 1 = 7.350 48
- Minor diameter of the nut thread = 8 - 1.082532 * 1 = 6.917 47
- Minor diameter of the bolt thread = 8 - 1.226869 * 1 = 6.773 13
- Tensile stress area = pi/4 * (8 - 0.938194 * 1)^2 = 39.1671
- Clearance hole = 8 * (1 + 8/100) = 8.64
- Breaking load at that tensile strength = 39.1671 * 800 = 31 333.7
- shear_area = pi * 6.917 47 * 12 * (0.5 + 0.57735 * (7.350 48 - 6.917 47)/1) = 195.587
- Threads engaged over that length = 1 > 0 ? 12 / 1 : 0 = 12
- 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.
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.
M8x1 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 7.296 | 0.2873 |
| 70% | 7.242 | 0.2851 |
| 75% | 7.188 | 0.2830 |
| 80% | 7.134 | 0.2809 |
| 85% | 7.080 | 0.2787 |
| 90% | 7.026 | 0.2766 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M8x1?
7.19 mm (0.2830 in) for 75% thread engagement. The traditional rule gives 7.00 mm, which is a tighter and harder-tapping hole.
What is the pitch of M8x1?
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.