Abstract: A heavy duty rotating blowout preventor which has a main outer support housing by which it can be affixed to the uppermost end of a casing, or to the uppermost end of a stack of tools affixed to the upper end of a casing. A rotating seal assembly includes a kelly drive bushing affixed to the uppermost end portion thereof, and a lower marginal end portion of the rotating seal assembly is removably received within the upper marginal end portion of the main support housing. The exposed upper marginal end portion of the rotating seal assembly forms an isolated bearing chamber by which the forces imposed by the kelly onto the rotating seal assembly is transferred into the main support housing in a new and novel manner. A stripper rubber assembly forms the lowermost end portion of the rotating seal assembly, and is separated from a nonrotating part of the rotating seal assembly by a stripper seal means which leads to a passageway underlying the bearing chamber.