NOTE: BSW is largely superseded by UNC and has been for most engineering and automotive uses since the 1950's.  The main difference between the threads is that a BSW  thread has a "V"  angle of 55 degrees and UNC threads have a "V" angle of 60 degrees, otherwise the pitch or TPI is the same for both UNC and BSW except at 1/2" where UNC is 13 TPI and BSW is 12 TPI.  For most practical purposes a UNC bolt will screw into a BSW female thread and vice versa. For a few critical applications this may not be acceptable and for threads with very close tolerances this may also produce a problem.