Metric fine
Tap drill 18.5 mm for a M20 × 1.5 thread — major Ø 20 mm, pitch 1.5 mm (16.93 TPI).
Profile
| Designation | M20 × 1.5 |
|---|---|
| Major (outside) diameter | 20 mm · 0.7874" |
| Pitch | 1.5 mm · 16.93 TPI |
| Pitch diameter | 19.026 mm · 0.749" |
| Minor diameter (external) | 18.16 mm · 0.7149" |
| Minor diameter (internal) | 18.376 mm · 0.7235" |
| Recommended tap drill | 18.5 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 | 271.5 mm² | 118.1 kN | 472.4 N·m |
| Class 10.9 | 271.5 mm² | 169.01 kN | 676 N·m |
| Class 12.9 | 271.5 mm² | 197.52 kN | 790.1 N·m |
For a M20 × 1.5 thread, drill the hole with a 18.5 mm tap drill (18.5 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 1.5 mm (16.93 TPI).
To pass a M20 × 1.5 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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