U.S. Standard Hexagonal Bolt Head
An illustration of the hexagonal bolt head showing measurements. The measurement of diameter, D, is used to calculate the dimensions. The diameter of the head and the nut, H, is 1 ½D ₌ ⅛, and the distance of T is half the diameter.
Anthony, Gardner C. Elements of Mechanical Drawing (Boston, MA: D. C. Heath & Co., 1904)
Courtesy the private collection of Roy Winkelman