The flank is the side of the thread. It connects the root and crest and becomes the mating surface on an assembled thread. There are two flanks on a thread, the leading flank and the following flank. The leading flank is the one that goes into the threaded hole first.

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Allowance is the difference between the maximum size of the thread and the basic size. It is usually a gap that functions as clearance but can be interference (as with NC thread forms).

It is possible to have more than one thread on a threaded part. These multiple, individual threads are referred to as “starts.”

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Lead is the distance a thread will move axially in one turn. On threads with one start it is equal to the pitch. See illustration at lead angle. See also starts.

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Chamfer is a conical surface a the end of a thread. The chamfer allows easy engagement of threaded parts and prevents the formation of a sharp edge on the first thread.

The flank angle is the angle formed between the the flank and an axis perpendicular to the axis of the thread. On symmetrical threads the flank angle is equal to half of the thread angle.

The pitch diameter is the diameter of an imaginary cylinder that exactly divides the thread and groove equally. The pitch diameter is an important dimension for mating threads.

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The basic thread profile is the boundary between the geometry that defines the internal thread and the external thread. It is a theoretically perfect form that establishes both tolerance zones. See illustration at allowance.

The major diameter is the diameter of the bounding cylinder of the largest dimension of the thread. It is the diameter of the crests of the threads on an external thread. The basic major diameter is the diameter of the basic thread form profile. See illustration at allowance.

The pitch is the distance between corresponding points on adjacent threads. It is the distance from one thread to the next. On a thread with one start, the pitch equals the lead.

The minor diameter is the diameter of the cylinder bounded by the roots of the threads of an external thread or the crests of an internal thread.