A control valve for a hydraulic adjuster for the camshaft of an internal combustion engine is provided, wherein an actuator acts on a pressure part (12) embedded in a control piston (5). In order to prevent the piston (5) from being blocked in the valve housing by the expansion thereof caused by the pressure part embedment, the pressure part (12) and/or the control piston (5) is/are provided with radial recesses (34) in the embedment area (13) which make it possible to limit joining forces and the resulting radial expansion of the piston (5). Alternatively or in addition, the external surface of the control valve (5) has a reduced cross-section in the embedment area (13).