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M8 x 1.25 tap drill size
Use a 6.8 mm drill to tap M8 x 1.25 (coarse pitch). The rule is major diameter minus pitch: 8 − 1.25 = 6.75 mm, rounded to the nearest standard drill (DIN 336). Closest inch drill: H (6.76 mm).
M8 x 1.25 thread specifications
| Thread | M8 x 1.25 ISO metric coarse |
|---|---|
| Major (nominal) diameter | .3150 in / 8.00 mm |
| Pitch | 1.25 mm (≈ 20.3 threads per inch) |
| Recommended tap drill | 6.8 mm = .2677 in |
M8 clearance hole (ISO 273)
| Fit | Hole diameter |
|---|---|
| Fine (close) | 8.4 mm |
| Medium (normal) | 9 mm |
| Coarse (loose) | 10 mm |
A clearance hole lets the M8 bolt pass through freely. Drill the tap-drill size only in the part that gets threaded.
Tips for tapping M8 x 1.25
- 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 (5.0 mm here).
- Thread engagement of 1–1.5 × the diameter (8.0–12.0 mm) is enough for most steel parts. Softer materials need more.