The tap drill for an M20 x 1.5 thread is a 18.5 mm drill (0.7283 in), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 18.54 mm, and 18.5 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 19.03 mm | 19 mm | 19.00 mm / 0.7480 in | 51% |
| 60% | 18.83 mm | 19 mm | 19.00 mm / 0.7480 in | 51% |
| 65% | 18.73 mm | 18.5 mm | 18.50 mm / 0.7283 in | 77% |
| 70% | 18.64 mm | 18.5 mm | 18.50 mm / 0.7283 in | 77% |
| 75% | 18.54 mm | 18.5 mm | 18.50 mm / 0.7283 in | 77% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 26 points of thread apart. 18.5 mm (0.7283 in) gives 77% and a stronger but harder-to-tap thread; 19 mm (0.7480 in) gives 51% and less tapping torque. On M20 x 1.5 the choice is mostly about tapping torque and machine power.
M20 x 1.5 is used on cable glands, hydraulic adapters and large-bore fittings. Its basic pitch diameter is 19.03 mm and its tensile stress area is 271.5 mm². A bolt of this size passes through a 22.00 mm normal-fit clearance hole (ISO 273).
Example: M20 x 1.5 at 75% thread
Result: 18.5 mm (0.7283 in), 77% thread
| Major (nominal) diameter | 20.00 mm / 0.7874 in |
|---|---|
| Pitch | 1.5 mm |
| Basic pitch diameter (d − 0.6495·P) | 19.03 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 18.38 mm |
| Tensile stress area | 271.5 mm² |
| Fit | Hole diameter |
|---|---|
| Close | 21.00 mm / 0.8268 in |
| Normal | 22.00 mm / 0.8661 in |
| Loose | 24.00 mm / 0.9449 in |
All Metric fine sizes:
Use 18.5 mm (0.7283 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 18.54 mm. For an easier-to-tap 50% thread, 19 mm (0.7480 in) gives 51%.
18.5 mm is 0.7283 in. The nearest inch drill to the 75% target is 47/64" (0.7344 in), which gives 69% thread.
A forming tap needs a bigger hole: 19.34 mm for 65% thread, so drill 19.5 mm (0.7677 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M20 x 1.5, ISO 273 gives close fit 21.00 mm, normal fit 22.00 mm and loose fit 24.00 mm. The hole depends only on the diameter, so M20 x 2.5 uses the same sizes.
271.5 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. M20 x 1.5 takes 18.5 mm (0.7283 in); M20 x 2.5 takes 17.5 mm (0.6890 in). Check the pitch on the drawing before you drill.
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