Abstract: A plunger for intermittently delivering counterblow air to a gob glass at a formidable temperature in a blank mold of an I.S. glass container forming machine being operated on the blow and blow process. The plunger comprises a fixed annular member with an enlarged recess at an inlet end and a valve seat at an upper end. The plunger has a sliding valve member with an annulus of the annular member, and the valve member has a stem portion and an enlarged valve seat portion at a free end thereof. The plunger further has a collar threadably and adjustably secured to the stem portion of the sliding valve member, and the collar, which is slidable within the recess of the annular member, has a plurality of air flow passages extending therethrough. Pressurized air is intermittently delivered to the recess of the annular member through an inlet line, which as an on/off valve therein.