The tap drill for an M8 x 1 thread is a 7 mm drill (0.2756 in), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 7.03 mm, and 7 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 7.35 mm | 7.4 mm | 7.40 mm / 0.2913 in | 46% |
| 60% | 7.22 mm | 7.2 mm | 7.20 mm / 0.2835 in | 62% |
| 65% | 7.16 mm | 7.2 mm | 7.20 mm / 0.2835 in | 62% |
| 70% | 7.09 mm | 7.1 mm | 7.10 mm / 0.2795 in | 69% |
| 75% | 7.03 mm | 7 mm | 7.00 mm / 0.2756 in | 77% |
The drills either side of the target are 8 points of thread apart, so either is a reasonable choice. 7 mm (0.2756 in) gives 77% and a stronger but harder-to-tap thread; 7.1 mm (0.2795 in) gives 69% and less tapping torque. On M8 x 1 the choice is mostly about tap life in hard materials.
M8 x 1 is used for adjusting screws, hydraulic and automotive fittings, and thin-wall parts where a coarse thread would leave too few threads. Its basic pitch diameter is 7.35 mm and its tensile stress area is 39.2 mm². A bolt of this size passes through a 9.00 mm normal-fit clearance hole (ISO 273).
Example: M8 x 1 at 75% thread
Result: 7 mm (0.2756 in), 77% thread
| Major (nominal) diameter | 8.00 mm / 0.3150 in |
|---|---|
| Pitch | 1 mm |
| Basic pitch diameter (d − 0.6495·P) | 7.35 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 6.92 mm |
| Tensile stress area | 39.2 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 fine sizes:
Use 7 mm (0.2756 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 7.03 mm. For an easier-to-tap 50% thread, 7.4 mm (0.2913 in) gives 46%.
7 mm is 0.2756 in. The nearest inch drill to the 75% target is J (0.2770 in), which gives 74% thread.
A forming tap needs a bigger hole: 7.56 mm for 65% thread, so drill 7.6 mm (0.2992 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M8 x 1, 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 1.25 uses the same sizes.
39.2 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 takes 7 mm (0.2756 in); M8 x 1.25 takes 6.8 mm (0.2677 in). Check the pitch on the drawing before you drill.
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