Abstract: A coaxial cylinder rheometer having a very small (less than 0.020 inches [0.0508 cm]) gap between the concentric cylinders and being capable of charging highly viscous (greater than 100,000 poise) and/or thermally sensitive materials, e.g., Polyvinyl Chloride, Polyurethane, etc., into this gap for testing. The inner cylinder, rotor, comprises a single element which can be driven axially and rotationally. The outer cylinder, stator, comprises an upper part having a projection-like ring on its lower surface and a lower part having a trough-like ring in its upper surface. The two parts are fastened together with mating screw threads on each part.