An apparatus for locking the position of the rotary valve, wherein the valve has a valve shaft adapted to have first and second opposed flatted formations. The apparatus includes first and second cam followers. The cam followers are reciprocally movable via the action of a cam assembly comprising first and second cams mounted on a rotatable cam stem. The cam followers can be simultaneously moved in opposite directions whereby engagement walls in the first and second cam followers can engage the first and second opposed flatted formations simultaneously.