This is the Mod-1 19T Pinion Gear from Castle Creations.            
                                                                               
FEATURES: Hardened steel construction                                          
          Designed for 1/8 scale cars                                          
                                                                               
INCLUDES: One Mod-1 19T Pinion Gear                                            
          One 4x4mm Set Screw                                                  
                                                                               
SPECS:    Outer Diameter: 21mm (0.83")                                         
          Inner Diameter: 5mm (0.20")                                          
          Width: 12mm (0.47")