The tap drill for an M12 x 1.75 thread is a 10.2 mm drill (0.4016 in), which leaves about 79% of full thread with a cutting tap. The ideal hole for 75% thread is 10.30 mm, and 10.2 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 10.86 mm | 10.8 mm | 10.80 mm / 0.4252 in | 53% |
| 60% | 10.64 mm | 10.5 mm | 10.50 mm / 0.4134 in | 66% |
| 65% | 10.52 mm | 10.5 mm | 10.50 mm / 0.4134 in | 66% |
| 70% | 10.41 mm | 10.5 mm | 10.50 mm / 0.4134 in | 66% |
| 75% | 10.30 mm | 10.2 mm | 10.20 mm / 0.4016 in | 79% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 13 points of thread apart. 10.2 mm (0.4016 in) gives 79% and a stronger but harder-to-tap thread; 10.5 mm (0.4134 in) gives 66% and less tapping torque. On M12 x 1.75 the choice is mostly about tap life in hard materials.
M12 covers heavy brackets, machine tool fixturing, steel structures and vehicle chassis parts. Its basic pitch diameter is 10.86 mm and its tensile stress area is 84.3 mm². A bolt of this size passes through a 13.50 mm normal-fit clearance hole (ISO 273).
Example: M12 x 1.75 at 75% thread
Result: 10.2 mm (0.4016 in), 79% thread
| Major (nominal) diameter | 12.00 mm / 0.4724 in |
|---|---|
| Pitch | 1.75 mm |
| Basic pitch diameter (d − 0.6495·P) | 10.86 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 10.11 mm |
| Tensile stress area | 84.3 mm² |
| Fit | Hole diameter |
|---|---|
| Close | 13.00 mm / 0.5118 in |
| Normal | 13.50 mm / 0.5315 in |
| Loose | 14.50 mm / 0.5709 in |
All Metric coarse sizes:
Use 10.2 mm (0.4016 in). It leaves 79% of full thread with a cutting tap; the ideal 75% hole is 10.30 mm. For an easier-to-tap 50% thread, 10.8 mm (0.4252 in) gives 53%.
10.2 mm is 0.4016 in. The nearest inch drill to the 75% target is 13/32" (0.4063 in), which gives 74% thread.
A forming tap needs a bigger hole: 11.23 mm for 65% thread, so drill 11 mm (0.4331 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M12 x 1.75, ISO 273 gives close fit 13.00 mm, normal fit 13.50 mm and loose fit 14.50 mm. The hole depends only on the diameter, so M12 x 1.5 and M12 x 1.25 use the same sizes.
84.3 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.
No. The drill depends on the pitch, not only the diameter. M12 x 1.75 takes 10.2 mm (0.4016 in); M12 x 1.5 takes 10.5 mm (0.4134 in); M12 x 1.25 takes 10.8 mm (0.4252 in). Check the pitch on the drawing before you drill.
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