M18 thread data
| Major diameter D | 18 mm |
| Pitch P | 2.5 mm · coarse pitch |
| Pitch diameter D2 | 16.376 mm |
| Minor diameter D1 (basic profile) | 15.294 mm |
| Thread engagement H1 | 1.353 mm |
| Cutting tap drill | 15.5 mm |
| Forming tap drill, indicative range | 16.66 – 16.85 mm |
| OSG 6HX | 16.66 – 16.81 mm |
| Emuge, recommended | 16.85 mm |
Clearance hole and counterbore for an M18 screw
| Clearance hole close / medium / free | 19 / 20 / 21 mm |
The medium clearance hole (20 mm) is the normal one; close is used when the screw must locate better, free when hole position tolerances are wide.
Clearance holes to ISO 273, heads to ISO 4762, counterbores to DIN 974-1. Values checked against several published tables.
Why a forming tap needs a larger hole
A cutting tap removes material as chips, so the hole starts close to the minor diameter: 15.5 mm here. A forming tap does not cut: it displaces the material and pushes it up to form the thread crest. That is why the hole is larger, from 16.66 mm up: 1.16 mm more than for cutting. With the cutting drill, a forming tap would jam or break.
Where to sit within the range
Towards the minimum (16.66 mm) the thread is fuller, but the torque on the tap rises. Towards the maximum (16.85 mm) the tap runs lighter, but the crest stays more open. The right point depends on the tap and the material and comes from the manufacturer's catalogue. With forming taps every hundredth counts: use a good drill and check the hole.
Same diameter, other pitch
What the makers say
For M18: OSG 16.66 – 16.81 mm, Emuge 16.85 mm. Each maker designs the tap profile its own way, so the range shifts by a few hundredths. The range at the top runs from the lowest to the highest of these values: if you use a tap from one brand, follow that brand's catalogue.
Note: here the sources do not agree, by 0.04 mm. Use the value in your tap's catalogue.
Forming tap drills: indicative range, from the lowest to the highest value given by the tap makers (DIN 13 / ISO 965-1): OSG, forming taps guide to tap drill sizes (class 6HX); Emuge, recommended tap drill sizes for cold-forming (DIN 13, tolerance 6H). Basic profile to ISO 68-1. Indicative values: the catalogue of the tap you use is what counts.