A blow out preventer comprises a valve casing assembly, a tubing cutter, a hydraulic cylinder and piston, and a linkage assembly. The valve casing assembly has a first port and a second port. The ports are in axial alignment along a longitudinal axis extending between the ports. The well tubing is run through the ports and transversely through the tubing cutter. The tubing cutter is rotatably positioned in the casing assembly and is rotatable from a first position to a second position. The tubing cutter has an axis of rotation which is transverse to the longitudinal axis of the valve casing assembly. The piston is annularly shaped and moves parallel to the longitudinal axis of the valve casing. Linkage between the piston and the tubing cutter converts the axial movement of the piston into rotational movement of the tubing cutter to shear the tubing when needed. When the tubing has been sheared, a seal is formed between a face of the tubing cutter and a valve seat in the valve casing assembly.