M33 tap drill size and thread dimensions
M33 has a major diameter of 33.000 mm and a pitch of 3.5 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 35.640
- Pitch diameter
- 30.7267
- Minor diameter of the nut thread
- 29.2111
- Minor diameter of the bolt thread
- 28.7060
- Tensile stress area
- 693.553 mm2
- Breaking load at that tensile strength
- 554 843
- The old "diameter minus pitch" drill
- 29.500
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 14.14
- Equivalent threads per inch
- 7.257
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 49.5
- Material tensile strength = 800
- major = 33 = 33
- p = 3.5 = 3.5
- h = 3.5 * 0.8660254037844386 = 3.031 09
- Tap drill = 33 - 75/100 * 1.082532 * 3.5 = 30.1584
- The old "diameter minus pitch" drill = 33 - 3.5 = 29.5
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 33 - 0.649519 * 3.5 = 30.7267
- Minor diameter of the nut thread = 33 - 1.082532 * 3.5 = 29.2111
- Minor diameter of the bolt thread = 33 - 1.226869 * 3.5 = 28.706
- Tensile stress area = pi/4 * (33 - 0.938194 * 3.5)^2 = 693.553
- Clearance hole = 33 * (1 + 8/100) = 35.64
- Breaking load at that tensile strength = 693.553 * 800 = 554 843
- shear_area = pi * 29.2111 * 49.5 * (0.5 + 0.57735 * (30.7267 - 29.2111)/3.5) = 3406.94
- Threads engaged over that length = 3.5 > 0 ? 49.5 / 3.5 : 0 = 14.1429
- Equivalent threads per inch = 3.5 > 0 ? 25.4 / 3.5 : 0 = 7.257 14
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.
M33 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 30.537 | 1.2023 |
| 70% | 30.348 | 1.1948 |
| 75% | 30.158 | 1.1873 |
| 80% | 29.969 | 1.1799 |
| 85% | 29.779 | 1.1724 |
| 90% | 29.590 | 1.1650 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M33?
30.16 mm (1.1873 in) for 75% thread engagement. The traditional rule gives 29.50 mm, which is a tighter and harder-tapping hole.
What is the pitch of M33?
3.5 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.