The Robert Tucci RT-64 is the classic F tuba mouthpiece that has stood the test of time.  The 64 works well on small, large, rotary, or piston F tubas.  It really is the best "starting point" for tubists who are looking for the all around F tuba mouthpiece.  This product provides moderate resistance which helps with quick response and a wide array of tonal colors. 
 
                                                  
Specifications:
Cup Diameter:  32mm/1.26in, shallow, bowl shaped                                             
Rim:  8mm/0.31in, slightly rounded                                                                       
Bore:  7.8mm/0.31in