Abstract: The tip of the blow core on a stretch blow molding machine is slightly smaller in diameter than the neck of the preform into which it is inserted such that during both insertion and withdrawal there is no physical contact between the core and the preform. However, when the core tip is in place within the preform and the blow surge commences, a resilient sealing ring around the core tip expands into radial contacting engagement with the preform to seal off what would otherwise be an air escape route. When the air surge terminates, the ring contracts into a dove tail gland around the core tip so as to be substantially flush with the surface of the tip, permitting the core to be withdrawn without contacting the preform.