CONCENTRIC SLAVE CYLINDER

PROBLEM TO BE SOLVED: To smoothen the motion of a piston 53 and a seal ring 57 while hardly separating the seal ring 57 from the piston 53 in a concentric slave cylinder 5 for actuating a friction type clutch 3 provided between an input shaft 1 of a driving source and an input shaft 2 of a transmission.SOLUTION: A release bearing 55 is attached to an outer diameter of a protrusion of the cylindrical piston 53 that is inserted slidably into a cylindrical hydraulic chamber 56 where a side of the transmission is closed and a side of the clutch 3 is opened. In the piston 53, the seal ring 57 is disposed not to be fixed at an inner end side in a shaft direction opposite to the protrusion. The seal ring 57 includes seal lips 57c, 57d that individually contact with an outer sidewall surface and an inner sidewall surface of the hydraulic chamber 57. In the outer sidewall surface, an enlarged diameter face 52c having a gradually increased inner dimension is provided in a region from an initial assembly position of the seal ring 57 to a predetermined position near an opening of the hydraulic chamber 56.