5-44 UNF tap drill size and thread dimensions
5-44 UNF has a major diameter of 3.175 mm and a pitch of 44 threads per inch (0.5773 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 3.429
- Pitch diameter
- 2.8001
- Minor diameter of the nut thread
- 2.5501
- Minor diameter of the bolt thread
- 2.4668
- Tensile stress area
- 5.447 mm2
- Breaking load at that tensile strength
- 4357
- The old "diameter minus pitch" drill
- 2.598
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 8.31
- Equivalent threads per inch
- 44.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 4.8
- Material tensile strength = 800
- major = 3.175 = 3.175
- p = 0.5772727272727273 = 0.577 273
- h = 0.577 273 * 0.8660254037844386 = 0.499 933
- Tap drill = 3.175 - 75/100 * 1.082532 * 0.577 273 = 2.706 31
- The old "diameter minus pitch" drill = 3.175 - 0.577 273 = 2.597 73
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 3.175 - 0.649519 * 0.577 273 = 2.800 05
- Minor diameter of the nut thread = 3.175 - 1.082532 * 0.577 273 = 2.550 08
- Minor diameter of the bolt thread = 3.175 - 1.226869 * 0.577 273 = 2.466 76
- Tensile stress area = pi/4 * (3.175 - 0.938194 * 0.577 273)^2 = 5.4466
- Clearance hole = 3.175 * (1 + 8/100) = 3.429
- Breaking load at that tensile strength = 5.4466 * 800 = 4357.28
- shear_area = pi * 2.550 08 * 4.8 * (0.5 + 0.57735 * (2.800 05 - 2.550 08)/0.577 273) = 28.8408
- Threads engaged over that length = 0.577 273 > 0 ? 4.8 / 0.577 273 : 0 = 8.314 96
- Equivalent threads per inch = 0.577 273 > 0 ? 25.4 / 0.577 273 : 0 = 44
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.
5-44 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 2.769 | 0.1090 |
| 70% | 2.738 | 0.1078 |
| 75% | 2.706 | 0.1065 |
| 80% | 2.675 | 0.1053 |
| 85% | 2.644 | 0.1041 |
| 90% | 2.613 | 0.1029 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 5-44 UNF?
2.71 mm (0.1065 in) for 75% thread engagement. The traditional rule gives 2.60 mm, which is a tighter and harder-tapping hole.
What is the pitch of 5-44 UNF?
44 threads per inch (0.5773 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.