The tap drill for an M24 x 3 thread is a 21 mm drill (0.8268 in), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 21.08 mm, and 21 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 22.05 mm | 22 mm | 22.00 mm / 0.8661 in | 51% |
| 60% | 21.66 mm | 21.5 mm | 21.50 mm / 0.8465 in | 64% |
| 65% | 21.47 mm | 21.5 mm | 21.50 mm / 0.8465 in | 64% |
| 70% | 21.27 mm | 21.5 mm | 21.50 mm / 0.8465 in | 64% |
| 75% | 21.08 mm | 21 mm | 21.00 mm / 0.8268 in | 77% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 13 points of thread apart. 21 mm (0.8268 in) gives 77% and a stronger but harder-to-tap thread; 21.5 mm (0.8465 in) gives 64% and less tapping torque. On M24 x 3 the choice is mostly about tapping torque and machine power.
M24 appears on structural connections, heavy equipment frames and large flanges. Its basic pitch diameter is 22.05 mm and its tensile stress area is 352.5 mm². A bolt of this size passes through a 26.00 mm normal-fit clearance hole (ISO 273).
Example: M24 x 3 at 75% thread
Result: 21 mm (0.8268 in), 77% thread
| Major (nominal) diameter | 24.00 mm / 0.9449 in |
|---|---|
| Pitch | 3 mm |
| Basic pitch diameter (d − 0.6495·P) | 22.05 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 20.75 mm |
| Tensile stress area | 352.5 mm² |
| Fit | Hole diameter |
|---|---|
| Close | 25.00 mm / 0.9843 in |
| Normal | 26.00 mm / 1.0236 in |
| Loose | 28.00 mm / 1.1024 in |
All Metric coarse sizes:
Use 21 mm (0.8268 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 21.08 mm. For an easier-to-tap 50% thread, 22 mm (0.8661 in) gives 51%.
21 mm is 0.8268 in. The nearest inch drill to the 75% target is 53/64" (0.8281 in), which gives 76% thread.
A forming tap needs a bigger hole: 22.67 mm for 65% thread, so drill 22.5 mm (0.8858 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M24 x 3, ISO 273 gives close fit 25.00 mm, normal fit 26.00 mm and loose fit 28.00 mm. Normal fit suits most bolted joints; close fit locates parts more accurately.
352.5 mm², from As = π/4 × (d − 0.9382 × P)². Multiply it by the bolt material's proof or tensile strength to get the load the thread can carry.
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