Bolt Measuring Guide

Measuring

a Fastener

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Shank Diameter

Also called Major diameter. The diameter of a bolt is the Shank diameter, expressed in millimetres for Metric bolts. Because this is approximately the same as the Major or Thread diameter, the thread diameter measurement can be used for fully threaded bolts.

Shank Length
Fastener length is measured from where the material surface is assumed to be, to the end of the fastener. For fasteners where the head usually sits above the surface, the measurement is from directly under the head to the end of the fastener. For fasteners that are designed to be countersunk, the measurement is made from the point on the head where the surface of the material is, to the end of the fastener.

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 millimetres (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. In general smaller fasteners have finer thread so they have lower thread pitch.