1-64 UNC tap drill size and thread dimensions
1-64 UNC has a major diameter of 1.854 mm and a pitch of 64 threads per inch (0.3969 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 2.003
- Pitch diameter
- 1.5964
- Minor diameter of the nut thread
- 1.4246
- Minor diameter of the bolt thread
- 1.3673
- Tensile stress area
- 1.725 mm2
- Breaking load at that tensile strength
- 1380
- The old "diameter minus pitch" drill
- 1.457
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 7.06
- Equivalent threads per inch
- 64.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 2.8
- Material tensile strength = 800
- major = 1.8541999999999998 = 1.8542
- p = 0.396875 = 0.396 875
- h = 0.396 875 * 0.8660254037844386 = 0.343 704
- Tap drill = 1.8542 - 75/100 * 1.082532 * 0.396 875 = 1.531 98
- The old "diameter minus pitch" drill = 1.8542 - 0.396 875 = 1.457 32
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 1.8542 - 0.649519 * 0.396 875 = 1.596 42
- Minor diameter of the nut thread = 1.8542 - 1.082532 * 0.396 875 = 1.424 57
- Minor diameter of the bolt thread = 1.8542 - 1.226869 * 0.396 875 = 1.367 29
- Tensile stress area = pi/4 * (1.8542 - 0.938194 * 0.396 875)^2 = 1.724 65
- Clearance hole = 1.8542 * (1 + 8/100) = 2.002 54
- Breaking load at that tensile strength = 1.724 65 * 800 = 1379.72
- shear_area = pi * 1.424 57 * 2.8 * (0.5 + 0.57735 * (1.596 42 - 1.424 57)/0.396 875) = 9.398 38
- Threads engaged over that length = 0.396 875 > 0 ? 2.8 / 0.396 875 : 0 = 7.055 12
- Equivalent threads per inch = 0.396 875 > 0 ? 25.4 / 0.396 875 : 0 = 64
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.
1-64 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 1.575 | 0.0620 |
| 70% | 1.553 | 0.0612 |
| 75% | 1.532 | 0.0603 |
| 80% | 1.510 | 0.0595 |
| 85% | 1.489 | 0.0586 |
| 90% | 1.468 | 0.0578 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 1-64 UNC?
1.53 mm (0.0603 in) for 75% thread engagement. The traditional rule gives 1.46 mm, which is a tighter and harder-tapping hole.
What is the pitch of 1-64 UNC?
64 threads per inch (0.3969 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.