Metric coarse
Tap drill 17.5 mm for a M20 × 2.5 thread — major Ø 20 mm, pitch 2.5 mm (10.16 TPI).
Profile
| Designation | M20 × 2.5 |
|---|---|
| Major (outside) diameter | 20 mm · 0.7874" |
| Pitch | 2.5 mm · 10.16 TPI |
| Pitch diameter | 18.376 mm · 0.7235" |
| Minor diameter (external) | 16.933 mm · 0.6666" |
| Minor diameter (internal) | 17.294 mm · 0.6809" |
| Recommended tap drill | 17.5 mm · ≈77% thread |
| Clearance hole | Close 21 · Normal 22 · Loose 24 mm |
| Hex head (across flats) | 30 mm |
| Socket key (across flats) | 17 mm |
ISO 273 clearance holes (fine / medium / coarse).
Target preload is 75% of proof load; torque is estimated from T = K · F · d.
| Grade / class | Stress area | Preload | Torque |
|---|---|---|---|
| Class 8.8 | 244.8 mm² | 106.49 kN | 425.9 N·m |
| Class 10.9 | 244.8 mm² | 152.38 kN | 609.5 N·m |
| Class 12.9 | 244.8 mm² | 178.09 kN | 712.4 N·m |
For a M20 × 2.5 thread, drill the hole with a 17.5 mm tap drill (17.5 mm — the standard D − P rule) before tapping. That produces roughly 77% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 20 mm and a pitch of 2.5 mm (10.16 TPI).
To pass a M20 × 2.5 bolt through a part rather than thread it, use a clearance hole: Close 21 · Normal 22 · Loose 24 mm.
All values are generated from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables — not hand-entered. See the full reference.
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