68 slidably surround a like plurality of sleeves 70 in turn closely surrounding a like plurality 4 of bolts 74 extending between the outwardly disposed end of the head 32 and a guide plate 72 axially disposed downstream from the piston valve 54 one end of each bolt 74 is threadedly connected as at 76 with the head 32 and its other end portion is provided with threads 78 slidably received by the guide plate 72 nuts 80 on the bolts 74 abut the guide plate 72 against the adjacent ends of the sleeves 70 as shown by dotted lines fig5 the piston valve 54 may be moved axially outward by fluid pressure against its inward end toward the foraminated plate 52 during this movement the guide means 66 is maintained concentric with the valve seat 40 by the guides 68 sliding relative to the sleeves 70 the piston valve 54 is normally maintained seated with the valve seat 40 by an elongated rod herein called a pin as shown by the bold line 82 fig5 extending axially between the piston valve 54 and the guide plate 72 the cup shaped piston valve 54 is provided with a central boss 84 which is centrally bored for receiving a first pin bushing 84 39 surrounding one end portion of the pin 82 the guide plate 72 is centrally bored for threadedly receiving a pin support 86 in turn centrally bored to form a socket 88 similarly receiving a second pin bushing 88 39 surrounding the other end portion of the pin 82 the pin 82 is fabricated to yield bend in.