M42 tap drill size and thread dimensions
M42 has a major diameter of 42.000 mm and a pitch of 4.5 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 45.360
- Pitch diameter
- 39.0772
- Minor diameter of the nut thread
- 37.1286
- Minor diameter of the bolt thread
- 36.4791
- Tensile stress area
- 1120.910 mm2
- Breaking load at that tensile strength
- 896 728
- The old "diameter minus pitch" drill
- 37.500
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 14.00
- Equivalent threads per inch
- 5.644
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 63
- Material tensile strength = 800
- major = 42 = 42
- p = 4.5 = 4.5
- h = 4.5 * 0.8660254037844386 = 3.897 11
- Tap drill = 42 - 75/100 * 1.082532 * 4.5 = 38.3465
- The old "diameter minus pitch" drill = 42 - 4.5 = 37.5
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 42 - 0.649519 * 4.5 = 39.0772
- Minor diameter of the nut thread = 42 - 1.082532 * 4.5 = 37.1286
- Minor diameter of the bolt thread = 42 - 1.226869 * 4.5 = 36.4791
- Tensile stress area = pi/4 * (42 - 0.938194 * 4.5)^2 = 1120.91
- Clearance hole = 42 * (1 + 8/100) = 45.36
- Breaking load at that tensile strength = 1120.91 * 800 = 896 728
- shear_area = pi * 37.1286 * 63 * (0.5 + 0.57735 * (39.0772 - 37.1286)/4.5) = 5511.38
- Threads engaged over that length = 4.5 > 0 ? 63 / 4.5 : 0 = 14
- Equivalent threads per inch = 4.5 > 0 ? 25.4 / 4.5 : 0 = 5.644 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.
M42 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 38.834 | 1.5289 |
| 70% | 38.590 | 1.5193 |
| 75% | 38.346 | 1.5097 |
| 80% | 38.103 | 1.5001 |
| 85% | 37.859 | 1.4905 |
| 90% | 37.616 | 1.4809 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M42?
38.35 mm (1.5097 in) for 75% thread engagement. The traditional rule gives 37.50 mm, which is a tighter and harder-tapping hole.
What is the pitch of M42?
4.5 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.