1-8 UNC tap drill size and thread dimensions
1-8 UNC has a major diameter of 25.400 mm and a pitch of 8 threads per inch (3.1750 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 27.432
- Pitch diameter
- 23.3378
- Minor diameter of the nut thread
- 21.9630
- Minor diameter of the bolt thread
- 21.5047
- Tensile stress area
- 394.829 mm2
- Breaking load at that tensile strength
- 315 863
- The old "diameter minus pitch" drill
- 22.225
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 12.00
- Equivalent threads per inch
- 8.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 38.1
- Material tensile strength = 800
- major = 25.4 = 25.4
- p = 3.175 = 3.175
- h = 3.175 * 0.8660254037844386 = 2.749 63
- Tap drill = 25.4 - 75/100 * 1.082532 * 3.175 = 22.8222
- The old "diameter minus pitch" drill = 25.4 - 3.175 = 22.225
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 25.4 - 0.649519 * 3.175 = 23.3378
- Minor diameter of the nut thread = 25.4 - 1.082532 * 3.175 = 21.963
- Minor diameter of the bolt thread = 25.4 - 1.226869 * 3.175 = 21.5047
- Tensile stress area = pi/4 * (25.4 - 0.938194 * 3.175)^2 = 394.829
- Clearance hole = 25.4 * (1 + 8/100) = 27.432
- Breaking load at that tensile strength = 394.829 * 800 = 315 863
- shear_area = pi * 21.963 * 38.1 * (0.5 + 0.57735 * (23.3378 - 21.963)/3.175) = 1971.64
- Threads engaged over that length = 3.175 > 0 ? 38.1 / 3.175 : 0 = 12
- Equivalent threads per inch = 3.175 > 0 ? 25.4 / 3.175 : 0 = 8
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.
1-8 UNC at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 23.166 | 0.9120 |
| 70% | 22.994 | 0.9053 |
| 75% | 22.822 | 0.8985 |
| 80% | 22.650 | 0.8917 |
| 85% | 22.479 | 0.8850 |
| 90% | 22.307 | 0.8782 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 1-8 UNC?
22.82 mm (0.8985 in) for 75% thread engagement. The traditional rule gives 22.22 mm, which is a tighter and harder-tapping hole.
What is the pitch of 1-8 UNC?
8 threads per inch (3.1750 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.