A swivel coupling used to connect metal tubes to externally threaded fittings in hydraulic systems includes a swivel nut which is attached to a tail piece by deforming a collar portion of the swivel nut into a locking relationship with the tail piece. This is accomplished by having facing and aligned shoulders on the swivel nut and on the tail piece which prevent the swivel nut from axially disengaging from the tail piece when the swivel nut is rotated to thread onto the threaded fitting. The tail piece has a flat end face which engages a flat end face on the fitting so that a leak-proof joint is formed between the tube and the fitting. The nut includes a frusto-conical portion that is engaged by a frusto-conical die surface on a split die ring in order to cam the collar portion toward the tail piece and align the respective shoulders thereof.