4-48 UNF tap drill size and thread dimensions
4-48 UNF has a major diameter of 2.845 mm and a pitch of 48 threads per inch (0.5292 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 3.072
- Pitch diameter
- 2.5011
- Minor diameter of the nut thread
- 2.2720
- Minor diameter of the bolt thread
- 2.1956
- Tensile stress area
- 4.331 mm2
- Breaking load at that tensile strength
- 3465
- The old "diameter minus pitch" drill
- 2.316
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 8.13
- Equivalent threads per inch
- 48.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 4.3
- Material tensile strength = 800
- major = 2.8447999999999993 = 2.8448
- p = 0.5291666666666667 = 0.529 167
- h = 0.529 167 * 0.8660254037844386 = 0.458 272
- Tap drill = 2.8448 - 75/100 * 1.082532 * 0.529 167 = 2.415 17
- The old "diameter minus pitch" drill = 2.8448 - 0.529 167 = 2.315 63
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 2.8448 - 0.649519 * 0.529 167 = 2.5011
- Minor diameter of the nut thread = 2.8448 - 1.082532 * 0.529 167 = 2.271 96
- Minor diameter of the bolt thread = 2.8448 - 1.226869 * 0.529 167 = 2.195 58
- Tensile stress area = pi/4 * (2.8448 - 0.938194 * 0.529 167)^2 = 4.331 23
- Clearance hole = 2.8448 * (1 + 8/100) = 3.072 38
- Breaking load at that tensile strength = 4.331 23 * 800 = 3464.99
- shear_area = pi * 2.271 96 * 4.3 * (0.5 + 0.57735 * (2.5011 - 2.271 96)/0.529 167) = 23.0187
- Threads engaged over that length = 0.529 167 > 0 ? 4.3 / 0.529 167 : 0 = 8.125 98
- Equivalent threads per inch = 0.529 167 > 0 ? 25.4 / 0.529 167 : 0 = 48
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.
4-48 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 2.472 | 0.0973 |
| 70% | 2.444 | 0.0962 |
| 75% | 2.415 | 0.0951 |
| 80% | 2.387 | 0.0940 |
| 85% | 2.358 | 0.0928 |
| 90% | 2.329 | 0.0917 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 4-48 UNF?
2.42 mm (0.0951 in) for 75% thread engagement. The traditional rule gives 2.32 mm, which is a tighter and harder-tapping hole.
What is the pitch of 4-48 UNF?
48 threads per inch (0.5292 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.