are npt and npt threads compatible?
While NPT and NPTF are both designed to screw together, NPT requires a seal to be leak proof while NPTF do not. The L1 gauge requirements for NPT and NPTF differ for threads more than ½ in. in size, and thus, are not interchangeable. The difference lies in the Plug Gage Major Diameter and Ring Gage Minor Diameter.