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M18 x 2.5 tap drill size
Use a 15.5 mm drill to tap M18 x 2.5 (coarse pitch). The rule is major diameter minus pitch: 18 − 2.5 = 15.5 mm. Closest inch drill: 39/64 (15.48 mm).
M18 x 2.5 thread specifications
| Thread | M18 x 2.5 ISO metric coarse |
|---|---|
| Major (nominal) diameter | .7087 in / 18.00 mm |
| Pitch | 2.5 mm (≈ 10.2 threads per inch) |
| Recommended tap drill | 15.5 mm = .6102 in |
M18 clearance hole (ISO 273)
| Fit | Hole diameter |
|---|---|
| Fine (close) | 19 mm |
| Medium (normal) | 20 mm |
| Coarse (loose) | 21 mm |
A clearance hole lets the M18 bolt pass through freely. Drill the tap-drill size only in the part that gets threaded.
Tips for tapping M18 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 (18.0–27.0 mm) is enough for most steel parts. Softer materials need more.