Abstract: A block and locking mechanism and cutter bit for a stump grinding machine. The block having a shank receiving cavity being defined by a surrounding irregularly shaped inner wall that includes a pair of opposed end walls interconnected by a pair of opposed side walls with each side wall being tapered inwardly from the opposed end walls such that the cavity has a varying width from end wall to end wall. The cutter bit including a shank adapted to be received within the cavity of the block with the shank having a cross-section that generally conforms to the shape of the cavity such that the cutter bit is prevented from rotating when inserted within the cavity and held within the block.