In addition to coarse and fine threads, there is another division of extra fine, or “superfine” threads, with a very fine pitch thread. The terms “coarse” and “fine” have (in this context) no relation to the manufacturing quality of the thread. 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 “coarse” pitch is the commonly used default pitch for a given diameter. ISO 262 selected sizes for screws, bolts and nuts ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261. ISO 261 specifies a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads. Internal threads are designated by upper case letters, G or H. If, for instance, only M20 is given then it is coarse pitch thread.Įxternal threads are designated by lowercase letter, g or h. Tolerance classes defined in ISO 965-1 can be appended to these designations, if required (e.g., M500– 6g in external threads). If the pitch is the normally used 'coarse' pitch listed in ISO 261 or ISO 262, it can be omitted (e.g., M8). The relationship between the height H and the pitch P is found using the following equation where θ DesignationĪ metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D ( D maj in the diagram above) and the pitch P, both expressed in millimetres and separated by the multiplication sign, × (e.g., M8×1.25). The outermost 1⁄ 8 and the innermost 1⁄ 4 of the height H of the V-shape are cut off from the profile. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other. ISO metric threads consist of a symmetric V-shaped thread. Each thread is characterized by its major diameter, D ( D maj in the diagram), and its pitch, P.

The design principles of ISO general-purpose metric screw threads ('M' series threads) are defined in international standard ISO 68-1.

Bs 3643 iso metric screw threads