blowout preventer when installing the blowout preventer on a wellhead the bore 23 through the preventer is opened by retracting the seal assemblies 22 by means of the operating screws 41 which are rotated to move the ram plates 51 with the connected seals 50 outwardly into the two side chamber spaces 31 when closure of the bore through the blowout preventer is desired the ram screws 41 are rotated threading the screws inwardly forcing the seal assemblies 22 toward the center line of the blowout preventer body bore 23 if a pump polished rod is present through the bore 23 the ram seal assemblies moving from the two opposite chambers close around the polished rod with the seal recess 52a of each of the seals fitting around the polished rod and the seals coming together along the inside vertical faces 52 of the seals the two seal assemblies are driven together by the screws 41 to the maximum extent possible forcing the seals 50 to expand extruding the seal material filling all of the voids within the chambers 31 around the ram plates to effectively and completely seal all communication upwardly through the bore 23 below the engaged seal assemblies when a seal is no longer required through the bore 23 of the blowout preventer body the ram screws 41 are rotated in the opposite direction causing the screws to move outwardly in opposite directions retracting the seal assemblies 22 into the side chambers 31 until the socket portion 75 of each of the ram.