To determine the proper nominal diameter size of a screw, you will need a caliper. Some calipers have LED read-outs in decimals. The\se decimal numbers will need to be converted into a whole number to determine what the diameter of the machine screw is. Check the chart below for easy decimal to diameter conversions for machine screws.


Diameters 1/4 inch and larger are converted from decimals to fractions in 1/16 inch increments.

a) MAJOR DIAMETER: The largest diameter of a screw thread; measured from the crests on external threads and the roots on internal threads.

b) PITCH DIAMETER: The imaginary diameter equal distance between the major and minor diameters.

c) MINOR DIAMETER: The smaller diameter of a screw thread; measured from the roots on external threads and the crests on internal threads.

d) THREAD PITCH: The distance between a given point on a thread and a corresponding point on the next thread. Also referred to as the "lead" which is the distance a screw will advance during one turn.

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