The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter D and its pitch P. ISO metric threads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other. The outermost 1/8 and the innermost 1/4 of the height H of the V-shape are cut off from the profile.
The height H and the pitch P are related through the 60° angle via
In an external (male) thread (e.g., on a bolt), the major diameter Dmaj and the minor diameter Dmin define maximum dimensions of the thread. This means that the external thread must end flat at Dmaj, but can be rounded out below the minor diameter Dmin. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the thread profile must end flat at Dmin but may be rounded out beyond Dmaj.
The minor diameter Dmin and effective pitch diameter Dp are derived from the major diameter and pitch as
If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it is omitted.
Tolerance classes defined in ISO 965-1 can be appended to these designations, if required.
Example: M10 – 6g in external threads.
International standard ISO 261 defines a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads. ISO 262 defines the following shorter list of thread dimensions – a subset of the combinations listed in ISO 261 – which are selected sizes for screws, bolts and nuts:
|10||1.5||1.25 or 1|
|12||1.75||1.5 or 1.25|
|18||2.5||2 or 1.5|
|20||2.5||2 or 1.5|
|22||2.5||2 or 1.5|
The "coarse" pitch is the commonly-used default pitch for a given diameter. In addition, one or two smaller "fine" pitches are defined, for use in applications where the height of the normal "coarse" pitch would be unsuitable (e.g., threads in thin-walled pipes). The terms "coarse" and "fine" have in this context no relation to the manufacturing quality of the thread.
Equivalent national standards: