76 the connecting rod of which is forked to form two ears 78a and 78b best shown on fig2 connecting rod ears 78a and 78b are provided with holes through which pin 80 projects pin 80 is integral with the cross bar 82 and the ends of cross bar 82 are welded to the side walls 84a and 84b fig5 and 6 of the extendible box formed by the side walls 84a and 84b and integral top and bottom walls 86a and 86b the extendible box formed by the integral side walls 84 and top and bottom walls 86 is movable longitudinally within the boom assembly under the influence of the hydraulic cylinder 76 the extendible box formed by the integral side walls 84 and top and bottom walls 86 is held in place in the frame of the boom assembly by the combined action of the cross bar 68 which is integral with the side plate 66a and 66b such that it is contained within the slot 87a and 87b in the side walls 84a and 84b of the extendible box and the runners 88a and 88b and plate 90 both of which are also integral with the side plates 66a and 66b of the frame of the boom assembly 20 additional rigidity is provided by the.