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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.
| Diameter, inches +0; −0.001 |
¼ | ⁵⁄₁₆ | ³⁄₈ | ⁷⁄₁₆ | ½ | ⁹⁄₁₆ | ⁵⁄₈ | ¹¹⁄₁₆ | ¾ | ⁷⁄₈ | 1 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Pitch (threads per inch) | 28 | 24 | 24 | 20 | 20 | 18 | 18 | 16 | 16 | 14 | 14 |
| Width of nut and heads, flat-to-flat |
³⁄₈ | ½ | ⁹⁄₁₆ | ¹¹⁄₁₆ | ¾ | ⁷⁄₈ | ¹⁵⁄₁₆ | 1 | 1¹⁄₈ | 1¼ | 1⁷⁄₁₆ |
| 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.