0-80 UNF tap drill size and thread dimensions
0-80 UNF has a major diameter of 1.524 mm and a pitch of 80 threads per inch (0.3175 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 1.646
- Pitch diameter
- 1.3178
- Minor diameter of the nut thread
- 1.1803
- Minor diameter of the bolt thread
- 1.1345
- Tensile stress area
- 1.181 mm2
- Breaking load at that tensile strength
- 945
- The old "diameter minus pitch" drill
- 1.206
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 7.24
- Equivalent threads per inch
- 80.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 2.3
- Material tensile strength = 800
- major = 1.5239999999999998 = 1.524
- p = 0.3175 = 0.3175
- h = 0.3175 * 0.8660254037844386 = 0.274 963
- Tap drill = 1.524 - 75/100 * 1.082532 * 0.3175 = 1.266 22
- The old "diameter minus pitch" drill = 1.524 - 0.3175 = 1.2065
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 1.524 - 0.649519 * 0.3175 = 1.317 78
- Minor diameter of the nut thread = 1.524 - 1.082532 * 0.3175 = 1.1803
- Minor diameter of the bolt thread = 1.524 - 1.226869 * 0.3175 = 1.134 47
- Tensile stress area = pi/4 * (1.524 - 0.938194 * 0.3175)^2 = 1.180 75
- Clearance hole = 1.524 * (1 + 8/100) = 1.645 92
- Breaking load at that tensile strength = 1.180 75 * 800 = 944.601
- shear_area = pi * 1.1803 * 2.3 * (0.5 + 0.57735 * (1.317 78 - 1.1803)/0.3175) = 6.396 32
- Threads engaged over that length = 0.3175 > 0 ? 2.3 / 0.3175 : 0 = 7.244 09
- Equivalent threads per inch = 0.3175 > 0 ? 25.4 / 0.3175 : 0 = 80
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.
0-80 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 1.301 | 0.0512 |
| 70% | 1.283 | 0.0505 |
| 75% | 1.266 | 0.0499 |
| 80% | 1.249 | 0.0492 |
| 85% | 1.232 | 0.0485 |
| 90% | 1.215 | 0.0478 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 0-80 UNF?
1.27 mm (0.0499 in) for 75% thread engagement. The traditional rule gives 1.21 mm, which is a tighter and harder-tapping hole.
What is the pitch of 0-80 UNF?
80 threads per inch (0.3175 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.