M20 × 1.5 thread data
| Major diameter D | 20 mm |
| Pitch P | 1.5 mm · fine pitch |
| Pitch diameter D2 | 19.026 mm |
| Minor diameter D1 (basic profile) | 18.376 mm |
| Thread engagement H1 | 0.812 mm |
| Cutting tap drill | 18.5 mm |
| Forming tap drill, indicative range | 19.25 – 19.35 mm |
| OSG 6HX | 19.25 – 19.34 mm |
| Emuge, recommended | 19.35 mm |
Clearance hole and counterbore for an M20 screw
| Clearance hole close / medium / free | 21 / 22 / 24 mm |
| SHCS head ISO 4762: Ø × height | 30 × 20 mm |
| Counterbore (DIN 974-1): Ø × depth | 33 × 20.6 mm |
The medium clearance hole (22 mm) is the normal one; close is used when the screw must locate better, free when hole position tolerances are wide. The counterbore sinks the socket head below the surface: the depth shown is the head height plus a small allowance; add the washer thickness if one is used.
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: 18.5 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 19.25 mm up: 0.75 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 (19.25 mm) the thread is fuller, but the torque on the tap rises. Towards the maximum (19.35 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.
Fine pitch
A fine pitch has a shallower thread, so the drill gets closer to the nominal diameter: for M20 × 1.5 the cutting tap drill is 18.5 mm. It is used for fine adjustment, thin walls or better resistance to loosening. Always read the pitch on the drawing: the same diameter has different tap drills.
Same diameter, other pitch
What the makers say
For M20 × 1.5: OSG 19.25 – 19.34 mm, Emuge 19.35 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.
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.