The tap drill for an M6 x 1 thread is a 5 mm drill (0.1969 in), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 5.03 mm, and 5 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 5.35 mm | 5.4 mm | 5.40 mm / 0.2126 in | 46% |
| 60% | 5.22 mm | 5.2 mm | 5.20 mm / 0.2047 in | 62% |
| 65% | 5.16 mm | 5.2 mm | 5.20 mm / 0.2047 in | 62% |
| 70% | 5.09 mm | 5.1 mm | 5.10 mm / 0.2008 in | 69% |
| 75% | 5.03 mm | 5 mm | 5.00 mm / 0.1969 in | 77% |
The drills either side of the target are 8 points of thread apart, so either is a reasonable choice. 5 mm (0.1969 in) gives 77% and a stronger but harder-to-tap thread; 5.1 mm (0.2008 in) gives 69% and less tapping torque. On M6 x 1 the choice is mostly about tap life in hard materials.
M6 is one of the most common machine screw sizes, found on fixtures, guards, machine frames and automotive trim. Its basic pitch diameter is 5.35 mm and its tensile stress area is 20.1 mm². A bolt of this size passes through a 6.60 mm normal-fit clearance hole (ISO 273).
Example: M6 x 1 at 75% thread
Result: 5 mm (0.1969 in), 77% thread
| Major (nominal) diameter | 6.00 mm / 0.2362 in |
|---|---|
| Pitch | 1 mm |
| Basic pitch diameter (d − 0.6495·P) | 5.35 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 4.92 mm |
| Tensile stress area | 20.1 mm² |
| Fit | Hole diameter |
|---|---|
| Close | 6.40 mm / 0.2520 in |
| Normal | 6.60 mm / 0.2598 in |
| Loose | 7.00 mm / 0.2756 in |
All Metric coarse sizes:
Use 5 mm (0.1969 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 5.03 mm. For an easier-to-tap 50% thread, 5.4 mm (0.2126 in) gives 46%.
5 mm is 0.1969 in. The nearest inch drill to the 75% target is #8 (0.1990 in), which gives 73% thread.
A forming tap needs a bigger hole: 5.56 mm for 65% thread, so drill 5.6 mm (0.2205 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M6 x 1, ISO 273 gives close fit 6.40 mm, normal fit 6.60 mm and loose fit 7.00 mm. Normal fit suits most bolted joints; close fit locates parts more accurately.
20.1 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.
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