A threaded coupler includes a plurality of circumferentially distributed internally threaded segments arranged to create a bore for inserting an externally threaded member. A casing surrounds the threaded segments. The casing includes a first inclined surface of revolution extending at an acute angle to an axis of the coupler. A spring resiliently urges the segments radially inward to the axis of the coupler. Each of the plurality of segments includes an inclined surface which defines a second inclined surface of revolution co-acting with the first inclined surface of revolution of the casing.