comprises a support web 33 welded to the cylindrical bar 22 substantially centrally and projecting thereabove a support sleeve 34 is welded to the top of the web 33 and has its axis parallel to the top plate 16 and transversely of the boom axis a wear plate expander bar 35 having a fixed sleeve extension 36 on one end thereof has its opposite end directly abutting the interior side of one wear pad retainer 28 the sleeve extension 36 engages telescopically within the support sleeve 34 an adjusting sleeve 37 of the expander means is telescoped into the sleeve extension 36 movably and has a nut element 38 welded to its outer end receiving an adjusting screw 39 secured by a lock nut 40 the head of screw 39 directly engages the interior side of the other wear pad retainer 28 as best shown in fig2 an expander coil spring 41 is arranged within the chamber formed by the interfitting sleeve components 36 and 37 and this spring may have its tension adjusted as required for best results by the adjusting screw 39 this same screw also serves to facilitate installing and removing the wear pads by relieving the spring force thereon at required times during normal operation of the.