BSW also is known as British Standard Whitworth. This gage checks the external parallel screw threads that have a 55 degree included angle and is of the Whitworth form. This is an older standard and this kind of thread form is mostly used for replacement parts.
WESTport provides a complete line of BSW (British Standard Whitworth) Thread Ring Gages. They are available in sizes from 1/8 inch to 6 inches. Free Technical support is available from our core of engineers to assist you with any questions you may have.