How Do Metric Screw Sizes Work?


Quick Answer

Diameter, pitch and length in millimeters define the size of a metric screw. Metric screws have pitches of coarse, fine or extra fine. Carriage and stove screws are coarse threaded. Machine screws are either coarse threaded or fine threaded.

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Full Answer

Consider a screw size of M14-1.0 x 20. M indicates the metric thread designator, the value 14 indicates the nominal diameter in millimeters, the value 1.0 indicates the pitch, which is the distance from thread to thread in millimeters, and 20 indicates the length in millimeters. A coarse-threaded metric screw measurement may only specify the diameter and length.

The size of metric screws determines their applications. Coarse-threaded carriage bolts having a round head and square collar find use in furniture work. Stove bolts are flat, rounded or oval with a slotted head and are used in stoves and other fastening jobs. Screw head types are defined by the German Institute for Standardization and the well-known International Organization for Standardization in a categorization system that places metric screws alongside metric nuts, washers and other fasteners.

Coarse-threaded metric screws tend to be less coarse than non-metric screws of the same diameter and pitch. However, non-metric fine screws may be finer than comparable metric screws.

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