M39 tap drill size and thread dimensions
M39 has a major diameter of 39.000 mm and a pitch of 4.0 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 42.120
- Pitch diameter
- 36.4019
- Minor diameter of the nut thread
- 34.6699
- Minor diameter of the bolt thread
- 34.0925
- Tensile stress area
- 975.753 mm2
- Breaking load at that tensile strength
- 780 602
- The old "diameter minus pitch" drill
- 35.000
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 14.63
- Equivalent threads per inch
- 6.350
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 58.5
- Material tensile strength = 800
- major = 39 = 39
- p = 4 = 4
- h = 4 * 0.8660254037844386 = 3.4641
- Tap drill = 39 - 75/100 * 1.082532 * 4 = 35.7524
- The old "diameter minus pitch" drill = 39 - 4 = 35
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 39 - 0.649519 * 4 = 36.4019
- Minor diameter of the nut thread = 39 - 1.082532 * 4 = 34.6699
- Minor diameter of the bolt thread = 39 - 1.226869 * 4 = 34.0925
- Tensile stress area = pi/4 * (39 - 0.938194 * 4)^2 = 975.753
- Clearance hole = 39 * (1 + 8/100) = 42.12
- Breaking load at that tensile strength = 975.753 * 800 = 780 602
- shear_area = pi * 34.6699 * 58.5 * (0.5 + 0.57735 * (36.4019 - 34.6699)/4) = 4778.8
- Threads engaged over that length = 4 > 0 ? 58.5 / 4 : 0 = 14.625
- Equivalent threads per inch = 4 > 0 ? 25.4 / 4 : 0 = 6.35
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.
M39 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 36.185 | 1.4246 |
| 70% | 35.969 | 1.4161 |
| 75% | 35.752 | 1.4076 |
| 80% | 35.536 | 1.3991 |
| 85% | 35.319 | 1.3905 |
| 90% | 35.103 | 1.3820 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M39?
35.75 mm (1.4076 in) for 75% thread engagement. The traditional rule gives 35.00 mm, which is a tighter and harder-tapping hole.
What is the pitch of M39?
4.0 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.