A plastic keg includes a liner including a neck portion and a body portion. A lid having an opening is disposed at least partially over the liner. A retainer is secured to the neck portion of the liner and aligned with the opening in the lid, the retainer including external threads. A locking ring having internal threads is secured to the external threads of the retainer. The internal threads of the locking ring and the external threads of the retainer include a ratcheting mechanism permitting attachment of the locking ring to the retainer and inhibiting removal of the locking ring from the retainer, the locking ring contacting the lid. A beer keg valve assembly includes a port having a head portion having a convex upper surface and at least one leg extending downwardly from a lower surface. The leg includes a stop surface projecting radially outward further than the head portion. A piston defines an opening. A spring biases the port against a periphery of the opening defined by the piston to seal the convex upper surface of the head portion over the opening.