M56 tap drill size and thread dimensions
M56 has a major diameter of 56.000 mm and a pitch of 5.5 mm. Every other dimension below follows from those two numbers and the 60-degree thread form.
- Clearance hole
- 60.480
- Pitch diameter
- 52.4276
- Minor diameter of the nut thread
- 50.0461
- Minor diameter of the bolt thread
- 49.2522
- Tensile stress area
- 2030.018 mm2
- Breaking load at that tensile strength
- 1 624 014
- The old "diameter minus pitch" drill
- 50.500
- Engagement that rule actually gives
- 92.4 %
- Threads engaged over that length
- 15.27
- Equivalent threads per inch
- 4.618
Show the working
- Thread engagement = 75
- Clearance hole allowance = 8
- Length of engagement = 84
- Material tensile strength = 800
- major = 56 = 56
- p = 5.5 = 5.5
- h = 5.5 * 0.8660254037844386 = 4.763 14
- Tap drill = 56 - 75/100 * 1.082532 * 5.5 = 51.5346
- The old "diameter minus pitch" drill = 56 - 5.5 = 50.5
- Engagement that rule actually gives = 100 / 1.082532 = 92.376
- Pitch diameter = 56 - 0.649519 * 5.5 = 52.4276
- Minor diameter of the nut thread = 56 - 1.082532 * 5.5 = 50.0461
- Minor diameter of the bolt thread = 56 - 1.226869 * 5.5 = 49.2522
- Tensile stress area = pi/4 * (56 - 0.938194 * 5.5)^2 = 2030.02
- Clearance hole = 56 * (1 + 8/100) = 60.48
- Breaking load at that tensile strength = 2030.02 * 800 = 1 624 010
- shear_area = pi * 50.0461 * 84 * (0.5 + 0.57735 * (52.4276 - 50.0461)/5.5) = 9905.14
- Threads engaged over that length = 5.5 > 0 ? 84 / 5.5 : 0 = 15.2727
- Equivalent threads per inch = 5.5 > 0 ? 25.4 / 5.5 : 0 = 4.618 18
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.
M56 at common engagements
| Engagement | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| 65% | 52.130 | 2.0524 |
| 70% | 51.832 | 2.0406 |
| 75% | 51.535 | 2.0289 |
| 80% | 51.237 | 2.0172 |
| 85% | 50.939 | 2.0055 |
| 90% | 50.641 | 1.9938 |
Pick the nearest drill above the figure, never below it.
Frequently asked
What drill size do I need for M56?
51.53 mm (2.0289 in) for 75% thread engagement. The traditional rule gives 50.50 mm, which is a tighter and harder-tapping hole.
What is the pitch of M56?
5.5 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.