Abstract: The adapter assembly is used on a vessel having an opening defined by the rim of an externally threaded neck. PTFE inserts with different internal bore configurations are used. The inserts include a lower portion adapted to be received in the vessel opening and an upper portion adapted to extend above the rim. The portions are separated by the flange with the circumferential groove which rests on the rim. An O-ring is received in the groove. An internally threaded PBTP cap with a central opening fits over the insert and engages the threads of the neck. The cap has an internal wall which cooperates with the O-ring to seal the cap to the neck. The upper portion of the insert extends through the cap opening. The internal bore may be tapered to receive an inner taper joint or threaded to receive an externally threaded connector. An insert with multiple bores is capable of receiving several components.