Cylinder block and cylinder sleeve, method of producing cylinder block and cylinder sleeve by friction stir welding, and friction stir welding method

A cylinder sleeve for being inserted in a cylinder bore defined in a block body of a cylinder block for an internal combustion engine. The cylinder bore includes a first annular recess and a second annular recess, and projecting walls. The cylinder sleeve comprises a hollow cylindrical body that is inserted into the cylinder block with at least part of an outer circumferential wall thereof adjacent to an inner circumferential wall of the projecting walls of the cylinder block, a larger-diameter portion projecting diametrally outwardly from an upper end of the outer circumferential wall of the hollow cylindrical body, a step disposed on an outer circumferential wall of said larger-diameter portion, by which the larger-diameter portions of adjacent cylinder sleeves are stacked, and a reduced-diameter portion provided by reducing a diameter of an inner circumferential wall of said hollow cylindrical body. The reduced-diameter portion is located at a position adjacent to the projecting walls of the cylinder block and is removed after the cylinder sleeve inserts into the cylinder bore. A method for inserting the cylinder sleeve into the cylinder bore with a friction stir welding is also disclosed.