Metric coarse
Tap drill 15.5 mm for a M18 × 2.5 thread — major Ø 18 mm, pitch 2.5 mm (10.16 TPI).
Profile
| Designation | M18 × 2.5 |
|---|---|
| Major (outside) diameter | 18 mm · 0.7087" |
| Pitch | 2.5 mm · 10.16 TPI |
| Pitch diameter | 16.376 mm · 0.6447" |
| Minor diameter (external) | 14.933 mm · 0.5879" |
| Minor diameter (internal) | 15.294 mm · 0.6021" |
| Recommended tap drill | 15.5 mm · ≈77% thread |
| Clearance hole | Close 19 · Normal 20 · Loose 21 mm |
| Hex head (across flats) | 27 mm |
| Socket key (across flats) | 14 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 | 192.5 mm² | 83.73 kN | 301.4 N·m |
| Class 10.9 | 192.5 mm² | 119.81 kN | 431.3 N·m |
| Class 12.9 | 192.5 mm² | 140.02 kN | 504.1 N·m |
For a M18 × 2.5 thread, drill the hole with a 15.5 mm tap drill (15.5 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 18 mm and a pitch of 2.5 mm (10.16 TPI).
To pass a M18 × 2.5 bolt through a part rather than thread it, use a clearance hole: Close 19 · Normal 20 · Loose 21 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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