The invention relates to a sealing piston, which is designed for use in a hydraulic expansion chucking device. The sealing piston comprises a pin that extends in an axial direction (A), and a seal that adjoins the pin and likewise extends in said axial direction (A). The seal comprises a sealing shaft that adjoins the pin and has a shaft radius (Rs). The seal further comprises a sealing head that adjoins the sealing shaft, wherein the sealing head comprises a circumferential sealing lip, which has a sealing radius (Rd) that is greater than the shaft radius (Rs). The sealing lip forms a contact surface in radial direction (R) for abutment and sealing against an inner wall of a bore in the expansion chucking device, and, measured in axial direction (A), has a sealing length (Ld) that is greater than the difference between the sealing radius (Rd) and the shaft radius (Rs). The invention further relates to a corresponding expansion chucking device.