A stainless steel butt weld cap is a type of pipe fitting that covers the end of a pipe. Since it can have female threads, it can screw on to the male end of the pipe. It can also be welded on to close the end of the pipe. On a weld, if it is a temporary close, or the contractor wants to add to the piping system in the future, he or she should allow additional pipe before closing so the pipe cap can be cut off and the pipe system extended as needed. This way you will not have less pipe than you need and the new fitting can be attached correctly.Standard Specifications Butt weld caps meet the ASTM A403 standard specification with dimensional tolerances in accordance with MSS SP-43 for schedules 5s and 10s, and with ANSI B16.9 for schedules heavier than 10s. View the standard specifications chart.