Fig. 385.
The Franklin Institute or United States Standard for the dimensions of bolt heads and nuts is as follows. In [Fig. 385], d represents the diameter of the bolt, j represents the short diameter or width across flats of the bolt head or of the nut, being equal to one and a half times the diameter of the bolt, plus 1⁄16 inch for finished heads or nuts, and plus 1⁄8 inch for rough or unfinished heads or nuts. k represents the depth or thickness of the head or nut, which in finished heads or nuts equals the diameter of the bolt minus 1⁄16 inch, and in rough heads equals one half the distance between the parallel sides of the head, or in other words one half the width across the flats of the head.
h represents the thickness or depth of the nut, which for finished nuts is made equal to the diameter of the bolt less 1⁄16 inch, and therefore the same thickness as the finished bolt head, while for rough or unfinished nuts it is made equal to the diameter of the bolt or the same as the rough bolt head. i represents the long diameter or diameter across corners, which, however, is a dimension not used to work to, and is inserted in the following tables merely for reference:—
TABLE OF THE FRANKLIN INSTITUTE STANDARD
DIMENSIONS FOR THE HEADS OF BOLTS AND
FOR THEIR NUTS, WHEN BOTH HEADS AND NUTS
ARE OF HEXAGON FORM, AND ARE POLISHED
OR FINISHED.
| Diameter at top of Thread. | Diameter at bottom of Thread. | Number of Threads per inch. | Diameter across Flats, or short diameter. | Thickness or Depth. | ||||
| 1⁄4 | .185 | 20 | 7⁄16 | 3⁄16 | ||||
| 5⁄16 | .240 | 18 | 17⁄32 | 1⁄4 | ||||
| 3⁄8 | .294 | 16 | 5⁄8 | 5⁄16 | ||||
| 7⁄16 | .345 | 14 | 23⁄32 | 3⁄8 | ||||
| 1⁄2 | .400 | 13 | 13⁄16 | 7⁄16 | ||||
| 9⁄16 | .454 | 12 | 29⁄32 | 1⁄2 | ||||
| 5⁄8 | .507 | 11 | 1 | 9⁄16 | ||||
| 3⁄4 | .620 | 10 | 1 | 3⁄16 | 11⁄16 | |||
| 7⁄8 | .731 | 9 | 1 | 3⁄8 | 13⁄16 | |||
| 1 | .837 | 8 | 1 | 9⁄16 | 15⁄16 | |||
| 1 | 1⁄8 | .940 | 7 | 1 | 3⁄4 | 1 | 1⁄16 | |
| 1 | 1⁄4 | 1.065 | 7 | 1 | 15⁄16 | 1 | 3⁄16 | |
| 1 | 3⁄8 | 1.160 | 6 | 2 | 1⁄8 | 1 | 5⁄16 | |
| 1 | 1⁄2 | 1.284 | 6 | 2 | 5⁄16 | 1 | 7⁄16 | |
| 1 | 5⁄8 | 1.389 | 5 | 1⁄2 | 2 | 1⁄2 | 1 | 9⁄16 |
| 1 | 3⁄4 | 1.491 | 5 | 2 | 11⁄16 | 1 | 11⁄16 | |
| 1 | 7⁄8 | 1.616 | 5 | 2 | 7⁄8 | 1 | 13⁄16 | |
| 2 | 1.712 | 4 | 1⁄2 | 3 | 1⁄16 | 1 | 15⁄16 | |
| 2 | 1⁄4 | 1.962 | 4 | 1⁄2 | 3 | 7⁄16 | 2 | 3⁄16 |
| 2 | 1⁄2 | 2.176 | 4 | 3 | 13⁄16 | 2 | 7⁄16 | |
| 2 | 3⁄4 | 2.426 | 4 | 4 | 3⁄16 | 2 | 11⁄16 | |
| 3 | 2.629 | 3 | 1⁄2 | 4 | 9⁄16 | 2 | 15⁄16 | |
| 3 | 1⁄4 | 2.879 | 3 | 1⁄2 | 4 | 15⁄16 | 3 | 3⁄16 |
| 3 | 1⁄2 | 3.100 | 3 | 1⁄4 | 5 | 5⁄16 | 3 | 7⁄16 |
| 3 | 3⁄4 | 3.377 | 3 | 5 | 11⁄16 | 3 | 13⁄16 | |
| 4 | 3.567 | 3 | 6 | 1⁄16 | 3 | 15⁄16 | ||
| 4 | 1⁄4 | 3.798 | 2 | 7⁄8 | 6 | 7⁄16 | 4 | 3⁄16 |
| 4 | 1⁄2 | 4.028 | 2 | 7⁄8 | 6 | 13⁄16 | 4 | 7⁄16 |
| 4 | 3⁄4 | 4.256 | 2 | 5⁄8 | 7 | 3⁄16 | 4 | 11⁄16 |
| 5 | 4.480 | 2 | 1⁄2 | 7 | 9⁄16 | 4 | 15⁄16 | |
| 5 | 1⁄4 | 4.730 | 2 | 1⁄2 | 7 | 15⁄16 | 5 | 3⁄16 |
| 5 | 1⁄2 | 4.953 | 2 | 3⁄8 | 8 | 5⁄16 | 5 | 7⁄16 |
| 5 | 3⁄4 | 5.203 | 2 | 3⁄8 | 8 | 11⁄16 | 5 | 11⁄16 |
| 6 | 5.423 | 2 | 1⁄4 | 9 | 1⁄16 | 5 | 15⁄16 | |
Note that square heads are supposed to be always unfinished, hence there is no standard for their sizes if finished.
The Franklin Institute standard dimensions for hexagon and square bolt heads and nuts when the same are left unfinished or rough, as forged, are as follows:—
| Bolt Diameter in Inches. | Diameter across corners, or long diameter of hexagon heads. | Diameter across corners or long diameter of square heads. | Short diameter, or diameter across flats for square or hexagon heads and nuts. | Thickness or depth for square or hexagon heads. | |||||
| Inch. | Inch. | Inch. | Inch. | ||||||
| 1⁄4 | 37⁄64 | 7⁄10 | 1⁄2 | 1⁄4 | |||||
| 5⁄16 | 11⁄16 | 10⁄12 | 19⁄32 | 19⁄64 | |||||
| 3⁄8 | 51⁄64 | 63⁄64 | 11⁄16 | 11⁄32 | |||||
| 7⁄16 | 9⁄10 | 1 | 7⁄64 | 25⁄32 | 25⁄64 | ||||
| 1⁄2 | 1 | 1 | 15⁄64 | 7⁄8 | 7⁄16 | ||||
| 9⁄16 | 1 | 1⁄8 | 1 | 23⁄64 | 31⁄32 | 31⁄64 | |||
| 5⁄8 | 1 | 7⁄32 | 1 | 1⁄2 | 1 | 1⁄16 | 17⁄32 | ||
| 3⁄4 | 1 | 7⁄16 | 1 | 49⁄64 | 1 | 1⁄4 | 5⁄8 | ||
| 7⁄8 | 1 | 21⁄32 | 2 | 1⁄32 | 1 | 7⁄16 | 23⁄32 | ||
| 1 | 1 | 7⁄8 | 2 | 19⁄64 | 1 | 5⁄8 | 13⁄16 | ||
| 1 | 1⁄8 | 2 | 2⁄32 | 2 | 9⁄16 | 1 | 13⁄16 | 29⁄32 | |
| 1 | 1⁄4 | 2 | 5⁄16 | 2 | 53⁄64 | 2 | 1 | ||
| 1 | 3⁄8 | 2 | 17⁄32 | 3 | 3⁄32 | 2 | 3⁄16 | 1 | 3⁄32 |
| 1 | 1⁄2 | 2 | 3⁄4 | 3 | 23⁄64 | 2 | 3⁄8 | 1 | 3⁄16 |
| 1 | 5⁄8 | 2 | 31⁄32 | 3 | 5⁄8 | 2 | 9⁄16 | 1 | 9⁄32 |
| 1 | 3⁄4 | 3 | 3⁄16 | 3 | 57⁄64 | 2 | 3⁄4 | 1 | 3⁄8 |
| 1 | 7⁄8 | 3 | 13⁄32 | 4 | 5⁄32 | 2 | 15⁄16 | 1 | 15⁄32 |
| 2 | 3 | 5⁄8 | 4 | 27⁄64 | 3 | 1⁄8 | 1 | 9⁄16 | |
| 2 | 1⁄4 | 4 | 1⁄16 | 4 | 61⁄64 | 3 | 1⁄2 | 1 | 3⁄4 |
| 2 | 1⁄2 | 4 | 1⁄2 | 5 | 31⁄64 | 3 | 7⁄8 | 1 | 15⁄16 |
| 2 | 3⁄4 | 4 | 29⁄32 | 6 | 4 | 1⁄4 | 2 | 1⁄8 | |
| 3 | 5 | 3⁄8 | 6 | 17⁄32 | 4 | 5⁄8 | 2 | 5⁄16 | |
| 3 | 1⁄4 | 5 | 13⁄16 | 7 | 1⁄16 | 5 | 2 | 1⁄2 | |
| 3 | 1⁄2 | 6 | 7⁄64 | 7 | 39⁄64 | 5 | 3⁄8 | 2 | 11⁄16 |
| 3 | 3⁄4 | 6 | 21⁄32 | 8 | 1⁄8 | 5 | 3⁄4 | 2 | 7⁄8 |
| 4 | 7 | 3⁄32 | 8 | 41⁄64 | 6 | 1⁄8 | 3 | 1⁄16 | |
| 4 | 1⁄4 | 7 | 9⁄16 | 9 | 3⁄16 | 6 | 1⁄2 | 3 | 1⁄4 |
| 4 | 1⁄2 | 7 | 31⁄32 | 9 | 3⁄4 | 6 | 7⁄8 | 3 | 7⁄16 |
| 4 | 3⁄4 | 8 | 13⁄32 | 10 | 1⁄4 | 7 | 1⁄4 | 3 | 5⁄8 |
| 5 | 8 | 27⁄32 | 10 | 49⁄64 | 7 | 5⁄8 | 3 | 13⁄16 | |
| 5 | 1⁄4 | 9 | 9⁄32 | 11 | 23⁄64 | 8 | 4 | ||
| 5 | 1⁄2 | 9 | 23⁄32 | 11 | 7⁄8 | 8 | 3⁄8 | 4 | 3⁄16 |
| 5 | 3⁄4 | 10 | 5⁄32 | 12 | 3⁄8 | 8 | 3⁄4 | 4 | 3⁄8 |
| 6 | 10 | 19⁄32 | 12 | 15⁄16 | 9 | 1⁄8 | 4 | 9⁄16 | |
The depth or thickness of both the hexagon and square nuts when left rough or unfinished is, according to the above standard, equal to the diameter of the bolt.