The tap drill for an M20 x 2.5 thread is a 17.5 mm drill (0.6890 in), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 17.56 mm, and 17.5 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 18.38 mm | 18.5 mm | 18.50 mm / 0.7283 in | 46% |
| 60% | 18.05 mm | 18 mm | 18.00 mm / 0.7087 in | 62% |
| 65% | 17.89 mm | 18 mm | 18.00 mm / 0.7087 in | 62% |
| 70% | 17.73 mm | 17.5 mm | 17.50 mm / 0.6890 in | 77% |
| 75% | 17.56 mm | 17.5 mm | 17.50 mm / 0.6890 in | 77% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 15 points of thread apart. 17.5 mm (0.6890 in) gives 77% and a stronger but harder-to-tap thread; 18 mm (0.7087 in) gives 62% and less tapping torque. On M20 x 2.5 the choice is mostly about tapping torque and machine power.
M20 fastens steel structures, large machinery, crane parts and foundation anchors. Its basic pitch diameter is 18.38 mm and its tensile stress area is 244.8 mm². A bolt of this size passes through a 22.00 mm normal-fit clearance hole (ISO 273).
Example: M20 x 2.5 at 75% thread
Result: 17.5 mm (0.6890 in), 77% thread
| Major (nominal) diameter | 20.00 mm / 0.7874 in |
|---|---|
| Pitch | 2.5 mm |
| Basic pitch diameter (d − 0.6495·P) | 18.38 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 17.29 mm |
| Tensile stress area | 244.8 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 coarse sizes:
Use 17.5 mm (0.6890 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 17.56 mm. For an easier-to-tap 50% thread, 18.5 mm (0.7283 in) gives 46%.
17.5 mm is 0.6890 in. The nearest inch drill to the 75% target is 11/16" (0.6875 in), which gives 78% thread.
A forming tap needs a bigger hole: 18.89 mm for 65% thread, so drill 19 mm (0.7480 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M20 x 2.5, 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 1.5 uses the same sizes.
244.8 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 2.5 takes 17.5 mm (0.6890 in); M20 x 1.5 takes 18.5 mm (0.7283 in). Check the pitch on the drawing before you drill.
COPLAIN AI PLATFORM
Put M20 tap drill, depth and torque values straight into a shop-floor job aid: Coplain turns your setup notes into clear, audit-ready work instructions.
Generate yours automatically with Coplain →Related Tools & Templates