Abstract: Accurate gap adjustments in an optical encoder used in a magnetic disk drive are achieved within a vane encoder assembly which incorporates a four-bar linkage. The optical encoder includes a fixed reticle and movable vane disposed between a light source and light detector. The fixed reticle is coupled indirectly to a housing which has a prismatic cavity defined therethrough. The prismatic cavity is characterized by a prismatic axis which is generally perpendicular to the direction of gap adjustment between the reticle and movable vane of the optical encoder. The prismatic cavity further defines four generally parallel hinge lines within the housing in a direction parallel to the prismatic axis of the cavity. A single adjustment screw is coupled to the housing to apply a compressive force to the housing in the direction of the gap adjustment.