Metrics in Engineering - Metric Fasteners (Screws, Nuts, Washers, and Bolts)

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Metric Fasteners

Machine Screws

ISO metric machine screws are limited to four different head shapes: countersunk, raised countersunk, panhead, and cheesehead.

All head dimensions are directly related to the basic screw diameter as follows:

Size	Minimum Length (mm)
M1.6	15
M2	16
M2.5	18
M3	19
M4	22
M5	25
M6	28
M8	34
M10	40
M12	46
M16	58
M20	70

Socket Screws

ISO metric hexagon socket screws are limited to two different head shapes: cap screw, and countersunk socket screw.

All head dimensions are directly related to the basic screw diameter as follows:

Size	Minimum Length (mm)
M3	3
M4	10
M5	12
M6	16
M8	20
M10	25
M12	30

Nuts and Washers

The nut and washer dimensions are related to the basic screw diameter as follows:

Bolts

Bolt head dimensions are related to their diameter as follows: