A captive seal is made using metal to metal angled surfaces, or a combination of metal to metal with an o-ring. Female swivel BSPP (British Standard Pipe Parallel) has a 

tapered nose flareless swivel where the seal occurs on the cone seat of the male connector. Similar but not interchangeable with NPSM/NPT since most sizes 

will have a different thread pitch and OD and thread forms are close, but not the same. Thread sealant is recommended when securing this connection style.

 BSPT (British Standard Pipe Tapered) male adapters are compatible with both BSPP and BSPT female ports.

Thread sizes are expressed as fractional dimensions. "R" represents a tapered thread, for example BSPT 1/4-19 can be designated by R 1/4.