The British Standard Fine Screw thread is known as a parallel screw thread of Whitworth form having a 55 degree included angle. With the standardization of Unified threads, the BSF Whitworth thread form is typically being utilized for replacement parts. Steel bolts of ¾ inch and below should not be made to close class limits and instead should be specified to medium class and free class limits. Close class threads should be applied to screw threads requiring a tight fit. The better and more practical class is a medium class of fit which provides a more functional interchangeable screw thread. Free Class bolts apply to the majority of bolts of commercial grade quality. Normal class nuts are applied to commercial quality nuts and are intended for use with medium or free class bolts. Thread Check Inc can supply BSF thread plug gages from 3/16 to 4 ¼ inches.
BSF Thread Ring Gage