M64 tap drill size and thread dimensions
M64 has a major diameter of 64.000 mm and a pitch of 6.0 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 69.120
- Pitch diameter
- 60.1029
- Minor diameter of the nut thread
- 57.5048
- Minor diameter of the bolt thread
- 56.6388
- Tensile stress area
- 2675.973 mm2
- Breaking load at that tensile strength
- 2 140 778
- The old "diameter minus pitch" drill
- 58.000
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 16.00
- Equivalent threads per inch
- 4.233
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 96
- Material tensile strength = 800
- major = 64 = 64
- p = 6 = 6
- h = 6 * 0.8660254037844386 = 5.196 15
- Tap drill = 64 - 75/100 * 1.082532 * 6 = 59.1286
- The old "diameter minus pitch" drill = 64 - 6 = 58
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 64 - 0.649519 * 6 = 60.1029
- Minor diameter of the nut thread = 64 - 1.082532 * 6 = 57.5048
- Minor diameter of the bolt thread = 64 - 1.226869 * 6 = 56.6388
- Tensile stress area = pi/4 * (64 - 0.938194 * 6)^2 = 2675.97
- Clearance hole = 64 * (1 + 8/100) = 69.12
- Breaking load at that tensile strength = 2675.97 * 800 = 2 140 780
- shear_area = pi * 57.5048 * 96 * (0.5 + 0.57735 * (60.1029 - 57.5048)/6) = 13 007.3
- Threads engaged over that length = 6 > 0 ? 96 / 6 : 0 = 16
- Equivalent threads per inch = 6 > 0 ? 25.4 / 6 : 0 = 4.233 33
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.
M64 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 59.778 | 2.3535 |
| 70% | 59.453 | 2.3407 |
| 75% | 59.129 | 2.3279 |
| 80% | 58.804 | 2.3151 |
| 85% | 58.479 | 2.3023 |
| 90% | 58.154 | 2.2895 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M64?
59.13 mm (2.3279 in) for 75% thread engagement. The traditional rule gives 58.00 mm, which is a tighter and harder-tapping hole.
What is the pitch of M64?
6.0 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.