Abstract: Apparatus and associated method for capping a blowing well, one embodiment of the apparatus including a hydraulically activated clamp for engaging the exposed well pipe stem, a hydraulically operated shut off valve having a tapered fitting and hydraulically activated pistons connecting the clamp and the valve for axially displacing the fitting into sealing engagement with the open end of the pipe stem. A preferred embodiment substitutes a hydraulic motor driven threaded coupling for the connecting pistons and includes thread-forming dies or taps on the tapered fitting.