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 (D maj in the diagram), and its pitch, P.iso metric threads consist of a symmetric V-shaped thread.
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The thread can be external, such as on a bolt or screw, or internal, such as inside a nut. The most common thread types used in the manufacturing industry today come in two varieties: spaced, designed to form threads within a pre-cut hole, and machine screw, designed to fit a pre-formed thread in a nut or hole.
Thread classes refer to fitting how loosely or tightly the screw fits in the nut. The most common thread classes are 2A or 2B. "A" indicates an external thread, such as on a screw or bolt. B indicates an internal thread, such as on a nut. The 2 (or, far less commonly, 1 or 3) describes the tightness of the fit.
In one instance I had to have a double-threaded pin made to complete the assembly of a dismantled Alan because the seat stay would not screw onto the seat lug with the stock threads. It was a matter.
Thread Pitch. Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimeters (measured along the length of the fastener). For example a thread pitch of 1.5 means that the distance between one thread and the next is 1.5mm.
Bolt thread size is a the measure of the small threads that run in a spiral along the exterior of a bolt or other attachment device. Getting the correct measurements is critical for purchasing pieces for a construction project or to replace attachments in a preexisting mechanism.