M33 thread data
| Major diameter D | 33 mm |
| Pitch P | 3.5 mm · coarse pitch |
| Pitch diameter D2 | 30.727 mm |
| Minor diameter D1 (basic profile) | 29.211 mm |
| Thread engagement H1 | 1.895 mm |
| Cutting tap drill | 29.5 mm |
| Forming tap drill, indicative range | 31.09 – 31.28 mm |
| OSG 6HX | 31.09 – 31.28 mm |
Clearance hole and counterbore for an M33 screw
| Clearance hole close / medium / free | 34 / 36 / 38 mm |
The medium clearance hole (36 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: 29.5 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 31.09 mm up: 1.59 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 (31.09 mm) the thread is fuller, but the torque on the tap rises. Towards the maximum (31.28 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.
Large sizes
From M24 up the tapping torque gets high, for both cutting and forming. On machining centres and lathes with live tooling thread milling is often the better choice: one tool covers several diameters and there is no risk of a tap broken in the part.
Forming tap drills: indicative range from a single source: OSG, forming taps guide to tap drill sizes (class 6HX). Basic profile to ISO 68-1. Indicative values: the catalogue of the tap you use is what counts.