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In combination with a propeller having blades mounted for pitch changing rotational movements, a hollow drive shaft supporting and driving said propeller, a hydraulic motor carried, by said drive shaft and rotatable therewith operatively connected with said blades to rotate the same for pitch adjustment, a piston in said motor, a relatively stationary source of hydraulic fluid under pressure and a relatively stationary drain sump, a two part valve, a connexion between said course and said valve, a connexion between said sump and said valve and a pair of connexions leading from said valve through said hollow drive shaft to opposite sides of said piston, registerable ports in the two parts of said valve for controlling the connexions between said source and said sump and the opposite sides of said piston, manually operable means for moving one of said valve parts to control said connexions to move said motor piston in a selected direction, and piston actuated means for moving the other valve part to control said connexions to discontinue movement of said piston, when said piston has moved a distance having a predetermined relation to the amount of movement of said manually movable valve part.