The tap drill for an M8 x 1.25 thread is a 6.8 mm drill (0.2677 in), which leaves about 74% of full thread with a cutting tap. The ideal hole for 75% thread is 6.78 mm, and 6.8 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 7.19 mm | 7.2 mm | 7.20 mm / 0.2835 in | 49% |
| 60% | 7.03 mm | 7 mm | 7.00 mm / 0.2756 in | 62% |
| 65% | 6.94 mm | 6.9 mm | 6.90 mm / 0.2717 in | 68% |
| 70% | 6.86 mm | 6.9 mm | 6.90 mm / 0.2717 in | 68% |
| 75% | 6.78 mm | 6.8 mm | 6.80 mm / 0.2677 in | 74% |
The drills either side of the target are 6 points of thread apart, so either is a reasonable choice. 6.7 mm (0.2638 in) gives 80% and a stronger but harder-to-tap thread; 6.8 mm (0.2677 in) gives 74% and less tapping torque. On M8 x 1.25 the choice is mostly about tap life in hard materials.
M8 is the default bolt for machine bases, brackets, motor mounts and automotive assemblies. Its basic pitch diameter is 7.19 mm and its tensile stress area is 36.6 mm². A bolt of this size passes through a 9.00 mm normal-fit clearance hole (ISO 273).
Example: M8 x 1.25 at 75% thread
Result: 6.8 mm (0.2677 in), 74% thread
| Major (nominal) diameter | 8.00 mm / 0.3150 in |
|---|---|
| Pitch | 1.25 mm |
| Basic pitch diameter (d − 0.6495·P) | 7.19 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 6.65 mm |
| Tensile stress area | 36.6 mm² |
| Fit | Hole diameter |
|---|---|
| Close | 8.40 mm / 0.3307 in |
| Normal | 9.00 mm / 0.3543 in |
| Loose | 10.00 mm / 0.3937 in |
All Metric coarse sizes:
Use 6.8 mm (0.2677 in). It leaves 74% of full thread with a cutting tap; the ideal 75% hole is 6.78 mm. For an easier-to-tap 50% thread, 7.2 mm (0.2835 in) gives 49%.
6.8 mm is 0.2677 in. The nearest inch drill to the 75% target is H (0.2660 in), which gives 77% thread.
A forming tap needs a bigger hole: 7.45 mm for 65% thread, so drill 7.4 mm (0.2913 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M8 x 1.25, ISO 273 gives close fit 8.40 mm, normal fit 9.00 mm and loose fit 10.00 mm. The hole depends only on the diameter, so M8 x 1 uses the same sizes.
36.6 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. M8 x 1.25 takes 6.8 mm (0.2677 in); M8 x 1 takes 7 mm (0.2756 in). Check the pitch on the drawing before you drill.
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