M14 tap drill size and thread dimensions
M14 has a major diameter of 14.000 mm and a pitch of 2.0 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 15.120
- Pitch diameter
- 12.7010
- Minor diameter of the nut thread
- 11.8349
- Minor diameter of the bolt thread
- 11.5463
- Tensile stress area
- 115.439 mm2
- Breaking load at that tensile strength
- 92 351
- The old "diameter minus pitch" drill
- 12.000
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 10.50
- Equivalent threads per inch
- 12.700
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 21
- Material tensile strength = 800
- major = 14 = 14
- p = 2 = 2
- h = 2 * 0.8660254037844386 = 1.732 05
- Tap drill = 14 - 75/100 * 1.082532 * 2 = 12.3762
- The old "diameter minus pitch" drill = 14 - 2 = 12
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 14 - 0.649519 * 2 = 12.701
- Minor diameter of the nut thread = 14 - 1.082532 * 2 = 11.8349
- Minor diameter of the bolt thread = 14 - 1.226869 * 2 = 11.5463
- Tensile stress area = pi/4 * (14 - 0.938194 * 2)^2 = 115.439
- Clearance hole = 14 * (1 + 8/100) = 15.12
- Breaking load at that tensile strength = 115.439 * 800 = 92 351.5
- shear_area = pi * 11.8349 * 21 * (0.5 + 0.57735 * (12.701 - 11.8349)/2) = 585.594
- Threads engaged over that length = 2 > 0 ? 21 / 2 : 0 = 10.5
- Equivalent threads per inch = 2 > 0 ? 25.4 / 2 : 0 = 12.7
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.
M14 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 12.593 | 0.4958 |
| 70% | 12.484 | 0.4915 |
| 75% | 12.376 | 0.4873 |
| 80% | 12.268 | 0.4830 |
| 85% | 12.160 | 0.4787 |
| 90% | 12.051 | 0.4745 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M14?
12.38 mm (0.4873 in) for 75% thread engagement. The traditional rule gives 12.00 mm, which is a tighter and harder-tapping hole.
What is the pitch of M14?
2.0 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.