The tap drill for an M12 x 1.25 thread is a 10.8 mm drill (0.4252 in), which leaves about 74% of full thread with a cutting tap. The ideal hole for 75% thread is 10.78 mm, and 10.8 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 11.19 mm | 11 mm | 11.00 mm / 0.4331 in | 62% |
| 60% | 11.03 mm | 11 mm | 11.00 mm / 0.4331 in | 62% |
| 65% | 10.94 mm | 11 mm | 11.00 mm / 0.4331 in | 62% |
| 70% | 10.86 mm | 10.8 mm | 10.80 mm / 0.4252 in | 74% |
| 75% | 10.78 mm | 10.8 mm | 10.80 mm / 0.4252 in | 74% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 18 points of thread apart. 10.5 mm (0.4134 in) gives 92% and a stronger but harder-to-tap thread; 10.8 mm (0.4252 in) gives 74% and less tapping torque. On M12 x 1.25 the choice is mostly about tap life in hard materials.
M12 x 1.25 is the extra-fine M12, used on some Japanese vehicle wheel studs, sensors and precision adjusters. Its basic pitch diameter is 11.19 mm and its tensile stress area is 92.1 mm². A bolt of this size passes through a 13.50 mm normal-fit clearance hole (ISO 273).
Example: M12 x 1.25 at 75% thread
Result: 10.8 mm (0.4252 in), 74% thread
| Major (nominal) diameter | 12.00 mm / 0.4724 in |
|---|---|
| Pitch | 1.25 mm |
| Basic pitch diameter (d − 0.6495·P) | 11.19 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 10.65 mm |
| Tensile stress area | 92.1 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 fine sizes:
Use 10.8 mm (0.4252 in). It leaves 74% of full thread with a cutting tap; the ideal 75% hole is 10.78 mm. For an easier-to-tap 50% thread, 11 mm (0.4331 in) gives 62%.
10.8 mm is 0.4252 in. The nearest inch drill to the 75% target is 27/64" (0.4219 in), which gives 79% thread.
A forming tap needs a bigger hole: 11.45 mm for 65% thread, so drill 11.5 mm (0.4528 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M12 x 1.25, 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 1.75 and M12 x 1.5 use the same sizes.
92.1 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 1.25 takes 10.8 mm (0.4252 in); M12 x 1.75 takes 10.2 mm (0.4016 in); M12 x 1.5 takes 10.5 mm (0.4134 in). Check the pitch on the drawing before you drill.
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