A piston-combination comprising an elongate chamber bounded by an inner chamber wall, and comprising a piston sealingly movable in the chamber relative to the chamber wall at least between a first and a second longitudinal position of the chamber, said chamber having cross-sections of different cross-sectional areas at the first and second longitudinal positions, and at least substantially continuously differing cross-sectional areas at intermediate longitudinal positions between the first and second longitudinal positions, the cross-sectional area at said second longitudinal position being smaller than the cross-sectional area at the first longitudinal position, said piston changing dimensions and having different cross-sectional areas thereby adapting to different cross-sectional areas of the chamber during the relative movements of the piston between the first and second longitudinal positions through said intermediate longitudinal positions of the chamber, said combination engaging a rigid surface and being movable relative to said surface.