api5000API has specified weld necks for many 6BX flanges, and not done so for many others, but API has provided the basis for determining the dimensions of a 6BX integral flange that the manufacturer may attach to aAPI 5K API 5K means an API-defined BOP confuration with a rated working pressure of 5,000 psi and as described in API RP 53, adopted by reference at 20 AAC 25.527(c).