8-36 UNF tap drill size and thread dimensions
8-36 UNF has a major diameter of 4.166 mm and a pitch of 36 threads per inch (0.7056 mm). Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 4.499
- Pitch diameter
- 3.7073
- Minor diameter of the nut thread
- 3.4018
- Minor diameter of the bolt thread
- 3.3000
- Tensile stress area
- 9.641 mm2
- Breaking load at that tensile strength
- 7713
- The old "diameter minus pitch" drill
- 3.460
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 8.79
- Equivalent threads per inch
- 36.000
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 6.2
- Material tensile strength = 800
- major = 4.1655999999999995 = 4.1656
- p = 0.7055555555555555 = 0.705 556
- h = 0.705 556 * 0.8660254037844386 = 0.611 029
- Tap drill = 4.1656 - 75/100 * 1.082532 * 0.705 556 = 3.592 76
- The old "diameter minus pitch" drill = 4.1656 - 0.705 556 = 3.460 04
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 4.1656 - 0.649519 * 0.705 556 = 3.707 33
- Minor diameter of the nut thread = 4.1656 - 1.082532 * 0.705 556 = 3.401 81
- Minor diameter of the bolt thread = 4.1656 - 1.226869 * 0.705 556 = 3.299 98
- Tensile stress area = pi/4 * (4.1656 - 0.938194 * 0.705 556)^2 = 9.641 22
- Clearance hole = 4.1656 * (1 + 8/100) = 4.498 85
- Breaking load at that tensile strength = 9.641 22 * 800 = 7712.97
- shear_area = pi * 3.401 81 * 6.2 * (0.5 + 0.57735 * (3.707 33 - 3.401 81)/0.705 556) = 49.6951
- Threads engaged over that length = 0.705 556 > 0 ? 6.2 / 0.705 556 : 0 = 8.7874
- Equivalent threads per inch = 0.705 556 > 0 ? 25.4 / 0.705 556 : 0 = 36
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.
8-36 UNF at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 3.669 | 0.1445 |
| 70% | 3.631 | 0.1430 |
| 75% | 3.593 | 0.1414 |
| 80% | 3.555 | 0.1399 |
| 85% | 3.516 | 0.1384 |
| 90% | 3.478 | 0.1369 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for 8-36 UNF?
3.59 mm (0.1414 in) for 75% thread engagement. The traditional rule gives 3.46 mm, which is a tighter and harder-tapping hole.
What is the pitch of 8-36 UNF?
36 threads per inch (0.7056 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.