Hydraulic flange connectors connect two flange components together, and hydraulic pressure applied to ports in connector holes of one of the flanges tension the flange studs and crush the gasket between the flange components. Each connector has a piston that disposes in the connector hole and can move therein when hydraulic pressure is applied through the port by a pump. A first stud connected to the piston extends beyond one open end of the hole for connection to a first nut, while a second stud connected to the piston extending beyond another open end of the to the second flange component for connection to a second nut. A retainer having seals threads in this other open end of the hole and seals between the hole and the second stud.