BSW (British Standard Whitworth) Chrome Alloy Hand Taps 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. The thread angle for BSW is 55 as opposed to 60 in Metric and Unified threads. 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.