The major diameter is the outer diameter at the top of the thread crests. Threads are designated or named by two qualities: the external thread major diameter and the pitch measurement. Right: right-hand threads on a right pedal. Notice the slope of the threads in the pedals threads below. Left-hand threads on bicycles are seen on the drive side of bottom bracket and the left pedal. The left-hand screw tightens counter-clockwise (to the left). The right-hand screw tightens clockwise (to the right). For external left-hand threads, the threads slope up to the left, while the internal left-hand threads slope up to the right. Most screw threads used on a bicycle use a 60-degree thread angle.įor external threads (bolts), a right-hand thread slopes up to the right, but the internal right-hand thread slopes up to the left. The angle of this “V” is called the thread angle, and is determined by fastener engineers. The thread forms a “V” shape between crests. The angle must be sloped, either upward to the right (for right-hand threaded screws) or upward to the left (for left-hand threaded screws). Threads are set at an angle to the axis of the bolt or nut. Between each crest is a space, called the root. A thread is a continuous helical ridge formed on the inside (nut) or outside (screw) of a cylinder. To effectively service fasteners and threaded components, it is important to have a working knowledge of threads.

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