EMPLOYEE, WITH THREE OTHER MECHANICS, WERE CONDUCTING PREVENTATIVE MAINTENANCE ON A CAT DOZER. ONE OF THE MAINTENANCE PERSONNEL HAD PLACED A SLATE BAR LEANING AGAINST THE LEFT SIDE PUSH ARM. H E WENT TO PICKUP TO MOVE A WOODEN CRIB BLOCK. ASHE RAISED BLOCK, HE BUMPED THE BAR CAUSING IT TO FALL. IT STRUCK TOP OF HIS LEFT THUMB, CRUSHINGTHUMB.