Abstract: Hydraulic bolt tightening means comprises a body, a piston mounted in the body, the piston and the body being of generally annular configuration to allow the device to be assembled in encircling fashion to a bolt or like components, and an annular chamber between the body and the piston for the admission of fluid under pressure to effect relative axial displacement of the body and the piston whereby the axial dimension is increased, the piston comprising an inner part having a pressure area which is subject to the pressure in said chamber and an outer part to which the fluid pressure is transmitted via said inner part and an interposed mass of solid, yielding and substantially incompressible material whose effective pressure area is less than that of said inner piston part whereby, in use, said mass is caused to expand radially so as to seal against leakage of fluid from the chamber while at the same time transmitting the pressure to the outer piston part.