Abstract: Clam shell-type blow mold apparatus has a pair of opposed upper mold halves that cooperate to define a partial blow cavity. A one-piece bottom mold portion that completes the blow cavity is locked up against the top mold portion by a locking assembly to prevent separation of the bottom mold portion from the top mold portion during the high pressure blow cycle. The locking assembly includes a support ring for the bottom mold portion that has a number of radially outwardly projecting male lugs adapted to fit into female receiving notches on opposed halves of a lock ring on the upper mold portion. Individual ledges at the bottoms of the notches underlie the lugs of the support ring when the mold halves are fully closed so as to preclude separation of the support ring, and thus the bottom mold portion, from the lock ring and the upper mold portion. The notches in the support ring are interposed between pairs of bolt holes in the lock ring.