The tap drill for an M5 x 0.8 thread is a 4.2 mm drill (0.1654 in), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 4.22 mm, and 4.2 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 4.48 mm | 4.5 mm | 4.50 mm / 0.1772 in | 48% |
| 60% | 4.38 mm | 4.4 mm | 4.40 mm / 0.1732 in | 58% |
| 65% | 4.32 mm | 4.3 mm | 4.30 mm / 0.1693 in | 67% |
| 70% | 4.27 mm | 4.3 mm | 4.30 mm / 0.1693 in | 67% |
| 75% | 4.22 mm | 4.2 mm | 4.20 mm / 0.1654 in | 77% |
The drills either side of the target are 10 points of thread apart, so either is a reasonable choice. 4.2 mm (0.1654 in) gives 77% and a stronger but harder-to-tap thread; 4.3 mm (0.1693 in) gives 67% and less tapping torque. On M5 x 0.8 the choice is mostly about tap life in hard materials.
M5 is common on pneumatic fittings, linear rail carriages, bicycle components and fixture clamps. Its basic pitch diameter is 4.48 mm and its tensile stress area is 14.2 mm². A bolt of this size passes through a 5.50 mm normal-fit clearance hole (ISO 273).
Example: M5 x 0.8 at 75% thread
Result: 4.2 mm (0.1654 in), 77% thread
| Major (nominal) diameter | 5.00 mm / 0.1969 in |
|---|---|
| Pitch | 0.8 mm |
| Basic pitch diameter (d − 0.6495·P) | 4.48 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 4.13 mm |
| Tensile stress area | 14.2 mm² |
| Fit | Hole diameter |
|---|---|
| Close | 5.30 mm / 0.2087 in |
| Normal | 5.50 mm / 0.2165 in |
| Loose | 5.80 mm / 0.2283 in |
All Metric coarse sizes:
Use 4.2 mm (0.1654 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 4.22 mm. For an easier-to-tap 50% thread, 4.5 mm (0.1772 in) gives 48%.
4.2 mm is 0.1654 in. The nearest inch drill to the 75% target is #19 (0.1660 in), which gives 75% thread.
A forming tap needs a bigger hole: 4.65 mm for 65% thread, so drill 4.6 mm (0.1811 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M5 x 0.8, ISO 273 gives close fit 5.30 mm, normal fit 5.50 mm and loose fit 5.80 mm. Normal fit suits most bolted joints; close fit locates parts more accurately.
14.2 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.
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