Fastener Terminology


Bolt:       An externally threaded, headed fastener.  Difference between a bolt and a screw is: a bolt is generally used through a hole with a mating nut; a screw is used in a threaded hole without a nut.

Class Of Thread:     A system used to distinguish thread tolerance differences.  Classes 1A, 2A, and 3A apply to external threads, and 1B, 2B, and 3B to internal threads. Classes 2 and 3 apply to both external and internal threads.

Crest:    The outermost tip of male thread from the fastener cylinder, or innermost tip of female thread.

Driving Recess:  A recess in the fastener head designed to accept a driving tool.

External Thread:  The male thread. Thread outside of the cylinder surface.

Internal Thread:  Female thread. Thread on the inside surface of a cylinder or  cone.

Left-Hand Thread:  Thread that winds counter-clockwise from the starting end of a  fastener.

Major Diameter: The largest diameter of a screw thread, on a straight thread it is the diameter of an imaginary cylinder which would pass through the crest of an external thread or the root of an internal thread.

Minor Diameter:  On a straight thread it is the diameter of an imaginary cylinder bounding the root of an external thread or the crest of an internal thread.

Nominal Diameter:  Basic diameter that determines the size and dimensional limits of fasteners.

Pitch: Distance from a point of a screw thread to a corresponding point on the next thread measured parallel to the axis.

Right-Hand Thread: A thread that winds clockwise from the starting end of a fastener when viewed from the starting end.

Screw:  A headed fastener which is externally threaded.

Sems®: A term applied to pre-assembled screw and washer assemblies. Washers are placed on screws before roll threading. Threads being of larger diameter than the washer hole thus prevent the washer from slipping off the screw.

Stud:  A threaded rod - at both ends or for entire the length.

Tapping Screw:      A hardened thread screw made to cut or form its own mating thread in an untapped hole.

Teks®:   A screw which drills its own hole and then threads in.

Thread Pitch:  The distance from the crest of one thread to crest of another.