The effects of the mouthpiece bore diameter on oscillation threshold pressure and tone quality are investigated by using an artificial blower and rapid prototyping. The bore diameter of the clarinet mouthpiece was reduced from 15.3 mm of original commercial resin mouthpiece to 11.3 mm, and we made those with acrylic resin using rapid prototyping. The artificial blowing system consists of a pressure chamber connected to a straight cylindrical resonator, an artificial lip with a force gauge, a pressure sensor, and pressure valves. The lip force was fixed at several values and the chamber pressure was systematically changed to explore the oscillation threshold pressure at each lip force. First, the prototyped mouthpiece with the original bore diameter was compared with the commercial mouthpiece, and results showed that the prototyped mouthpiece produced sounds with narrower pressure and lip force ranges than the commercial mouthpiece. This suggests the material properties and surface roughness influence the oscillation threshold. The comparison with different bore diameters demonstrated that the mouthpiece with the smaller bore diameter produced sounds with wider pressure ranges at the particular lip force, whereas the spectral centroids became higher which indicates the playability with brighter timbre.