A preform locator and blow air valve apparatus, for use in a stretch pin assembly in a blow molding machine that produces hollow thermoplastic articles, which functions as a positive blow air shut-off valve means in the absence of a preform in a mold assembly. A tubular valve body is mounted in a stretch pin assembly, with a longitudinally movable blow air nozzle means slidably mounted in the valve body for carrying the preform and locating and sealing the preform against a shoulder in a mold assembly on a blow molding machine. A stretch pin is slidably mounted longitudinally through the blow air nozzle means for stretching a preform mounted on the nozzle means. A blow air passage means is operatively formed through the tubular valve body and blow air nozzle means for admitting blow air under pressure into a preform located in the mold assembly, by said blow air nozzle means, for blowing the preform into a finished article. The blow air nozzle means is moved by mechanical means into an initial position for loading a preform thereon. The blow air nozzle means is movable toward the mold assembly by either a mechanical means or a pneumatic means. The blow air nozzle means is moved a first distance toward the mold to locate and seal a preform in a mold assembly, and in the absence of a preform on the blow air nozzle means, the blow air nozzle means is moved a second distance, whereby it functions as a valve to close the blow air passage means to prevent loss of high pressure blow air and the resultant high noise levels.