One of the earliest attempts to standardize fasteners. The bolts had a 60° thread angle with flats on the root and crest. The width of the flats was ¹⁄₈ of the pitch. Bolt heads and nuts had a raised area 1/64 inch thick and the diameter of the flat-to-flat width. The length of the threaded portion of the bolt was 1.5 times its diameter.
|Pitch (threads per inch)||28||24||24||20||20||18||18||16||16||14||14|
|Width of nut and heads,
|Width of slot for screwdriver||¹⁄₁₆||¹⁄₁₆||³⁄₃₂||³⁄₃₂||³⁄₃₂||³⁄₃₂||³⁄₃₂||³⁄₃₂||³⁄₃₂||³⁄₃₂||³⁄₃₂|
|Thru hole for cotter pin||⁵⁄₆₄||⁵⁄₆₄||¹⁄₈||¹⁄₈||¹⁄₈||⁵⁄₃₂||⁵⁄₃₂||⁵⁄₃₂||⁵⁄₃₂||⁵⁄₃₂||⁵⁄₃₂|
|Thickness of plain nut||⁷⁄₃₂||¹⁷⁄₆₄||²¹⁄₆₄||³⁄₈||⁷⁄₁₆||³¹⁄₆₄||³⁵⁄₆₄||¹⁹⁄₃₂||²¹⁄₃₂||⁴⁹⁄₆₄||⁷⁄₈|
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Last revised: 1 January 2016.