valve may be of any standard type such as operator or pilot actuated wherein spool 11 has a plurality of metering slots lands or the like formed thereon to intercommunicate various passages in the valve although valve 10 is shown in the form of a two position valve wherein spool 11 is adapted to be moved from its first or centered position illustrated in fig1 to a second or detent held position in fig2 it should be understood that additional positions of the valve may be designed therein as required a main body 12 of spool 11 is reciprocally mounted in a housing 13 an elongated stem 14 of the spool extends from main body 12 into a separate casing 15 which is secured to housing 13 by a plurality of bolts 16 for discussion purposes herein attached housing 13 and casing 15 may be considered together as comprising a 34 housing 34 a biasing means or centering mechanism 17 is associated with spool 11 to move it towards it first or centered position illustrated in fig1 automatically upon release of the spool by a detent means or mechanism 18 centering mechanism 17 comprises a compression coil spring 19 which is disposed axially between first and second cup shaped retainers 20 and 21 each reciprocally mounted on stem 14 of spool 11 flanges 22 and 23 are provided on stem 14 to engage retainers 20 and 21 respectively detent mechanism 18 comprises a tubular retainer 24 having an annular flange 25 thereof clamped between housing 13 and casing 15 to precisely position an end surface 26 thereof at a set axial distance from casing 13 to provide a stop means 27 the function of which is hereinafter fully described a plurality of circumferentially spaced holes 28 are.