ISO 261:1998 (EN) General purpose metric screw threads. General plan

ISO 261:1998 was created by Technical Committee: ISO/TC 1 Screw Threads. It replaced and canceled the earlier version ISO 261:1973
ISO 261:1998 specifies general purpose metric screw threads (M) having a basic profile according to ISO 68-1. Basic dimensions are mentioned in ISO 724 & tolerances are mentioned in ISO 965-1
ISO 261 shares a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads. ISO 262 specifies a shorter list of thread dimensions, it’s a subset of ISO 26
A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimeters and separated by the multiply sign, × (e.g., M8×1.25). If the pitch is the normally used “coarse” pitch listed in ISO 261 or ISO 262, it can be omitted
The length of a machine screw or bolt is indicated by a following × and the length expressed in millimeters
Tolerance classes defined in ISO 965-1 can be appended to these designations, if required (e.g., M500– 6g in external threads). External threads are designated by lowercase letter, g or h. Internal threads are designated by capital case letters, G or H.
The design principles of ISO general-purpose metric screw threads (“M” series threads) are defined in ISO 68-1 standard. 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 thread depth is 0.54125 × pitch. The outermost 1⁄8 and the innermost 1⁄4 of the height H of the V-shape are cut off from the profile.
ISO 262:1998(EN) Selected metric-screw-thread sizes for screws, bolts, and nuts
ISO 262:1998 defines selected sizes for screws, bolts and nuts in the diameter range from 1 mm to 64 mm of ISO general purpose metric screw threads (M) having basic profile according to ISO 68-1.
The selected sizes are recommended also for general engineering use. The screw threads are selected from ISO 261. Thread designations are explained in ISO 965-1
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