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M20 x 2.5 tap drill size
Use a 17.5 mm drill to tap M20 x 2.5 (coarse pitch). The rule is major diameter minus pitch: 20 − 2.5 = 17.5 mm. Closest inch drill: 11/16 (17.46 mm).
M20 x 2.5 thread specifications
| Thread | M20 x 2.5 ISO metric coarse |
|---|---|
| Major (nominal) diameter | .7874 in / 20.00 mm |
| Pitch | 2.5 mm (≈ 10.2 threads per inch) |
| Recommended tap drill | 17.5 mm = .6890 in |
M20 clearance hole (ISO 273)
| Fit | Hole diameter |
|---|---|
| Fine (close) | 21 mm |
| Medium (normal) | 22 mm |
| Coarse (loose) | 24 mm |
A clearance hole lets the M20 bolt pass through freely. Drill the tap-drill size only in the part that gets threaded.
Tips for tapping M20 x 2.5
- Use cutting fluid, and with a hand tap back off about half a turn every turn or two to break the chip. Aluminium and soft materials are the exception: they tap cleanly with spiral-point taps.
- For blind holes, drill deeper than the thread you need. Allow at least 3–4 pitches beyond the last full thread for a plug tap (10.0 mm here).
- Thread engagement of 1–1.5 × the diameter (20.0–30.0 mm) is enough for most steel parts. Softer materials need more.