Abstract: A self-centering blow mold for a rotary indexing blow molding machine has at the blow station a system for obtaining and keeping alignment among the chordally arranged blowing pins, neck rings, and blow molds. This self-centering system includes a toggle system which moves one set of the blow molds radially past alignment with the blowing pins to an overcenter position and a fluid operated drive means that thereafter moves the other of the set of blow molds radially in the opposite direction until it is in alignment with the first mentioned blow mold at the overcenter position and then continues moving to force both blow molds together, compress the toggle system, and to where the blow molds, neck rings and blowing pins are all aligned. There is also provided means for engaging the neck rings by the toggle system molds and fluid driven molds, respectively, so that alignment is achieved and maintained. The means for engaging includes biasing means consisting of a resilient member such as a spring.