Features: BSW - British Standard Whitworth; The angle of thread is 55 degrees as opposed to 60 degrees in Metric and Unified threads. ; This thread form was developed by Sir Joseph Whitworth in 1841. ; The thread form has rounded roots and crests and is specified in BS84:1956. ; BOTTOMING TAPS have a lead of two threads and are used to ensure minimum thread run-out when tapping to the bottom of a blind hole. Bottoming Taps are typically preceded by the use of a Taper or Intermediate Tap. ; Specifications:Diameter5/16"Pitch18 tpiLeadBottomingOverall Length72 mmTapping Drill6.5 mm