An I.S. machine wherein a parison is blown into a bottle in a blow mold. The parison is initially suspended from the neck ring mechanism and the blow molds are displaced to close around the parison below the finish. The neck ring mechanism is displaced away from the mold and a blow head is located on top of the mold to provide final blow. A heat sensor senses when the parison, during reheat has reached the bottom of the mold and supplies a signal to a controller which starts final blow based on the time when this signal was supplied.