The tap drill for an M2.5 x 0.45 thread is a 2.05 mm drill (0.0807 in), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 2.06 mm, and 2.05 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 2.21 mm | 2.2 mm | 2.20 mm / 0.0866 in | 51% |
| 60% | 2.15 mm | 2.15 mm | 2.15 mm / 0.0846 in | 60% |
| 65% | 2.12 mm | 2.1 mm | 2.10 mm / 0.0827 in | 68% |
| 70% | 2.09 mm | 2.1 mm | 2.10 mm / 0.0827 in | 68% |
| 75% | 2.06 mm | 2.05 mm | 2.05 mm / 0.0807 in | 77% |
The drills either side of the target are 9 points of thread apart, so either is a reasonable choice. 2.05 mm (0.0807 in) gives 77% and a stronger but harder-to-tap thread; 2.1 mm (0.0827 in) gives 68% and less tapping torque. On M2.5 x 0.45 the choice is mostly about tap breakage.
M2.5 is common on single-board computer mounts, 3D printer parts and small instrument housings. Its basic pitch diameter is 2.21 mm and its tensile stress area is 3.4 mm². A bolt of this size passes through a 2.90 mm normal-fit clearance hole (ISO 273).
Example: M2.5 x 0.45 at 75% thread
Result: 2.05 mm (0.0807 in), 77% thread
| Major (nominal) diameter | 2.50 mm / 0.0984 in |
|---|---|
| Pitch | 0.45 mm |
| Basic pitch diameter (d − 0.6495·P) | 2.21 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 2.01 mm |
| Tensile stress area | 3.4 mm² |
| Fit | Hole diameter |
|---|---|
| Close | 2.70 mm / 0.1063 in |
| Normal | 2.90 mm / 0.1142 in |
| Loose | 3.10 mm / 0.1220 in |
All Metric coarse sizes:
Use 2.05 mm (0.0807 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 2.06 mm. For an easier-to-tap 50% thread, 2.2 mm (0.0866 in) gives 51%.
2.05 mm is 0.0807 in. The nearest inch drill to the 75% target is #46 (0.0810 in), which gives 76% thread.
A forming tap needs a bigger hole: 2.30 mm for 65% thread, so drill 2.3 mm (0.0906 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M2.5 x 0.45, ISO 273 gives close fit 2.70 mm, normal fit 2.90 mm and loose fit 3.10 mm. Normal fit suits most bolted joints; close fit locates parts more accurately.
3.4 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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