from the gas inlet 314 the first internal ports 412 are of different dimensions the actuated coupler 42 has a guide slot 420 extending axially the positioning member 32 is secured to the housing 31 and is configured to guard against axial displacement of the valve body 41 the positioning member 32 includes a rear end wall 326 which confronts the actuated coupler 42 and a front end wall 327 which is opposite to the rear end wall 326 along the axis and which has an axial hole 320 that extends through the rear end wall 326 as shown in fig3 the positioning member 32 includes a plurality of positioning recesses 324 which are formed in the rear end wall 326 and which are angularly displaced from one another about the axis and a guiding block 325 which extends rearwardly of the rear end wall 326 and which is inserted into the insertion bore 301 to be in frictional engagement with the front inner peripheral end 316 so as to facilitate mounting of the positioning member 32 to the housing 31 the actuator 5 extends through the axial hole 320 and is rotatable about the axis relative to the positioning member 32 the actuator 5 includes.