Plastic Gears in Futaba Gearsets. Click on the servo to see
a picture of at least the plastic gears that are in that servo
gear set. Each servo has 4 gears. If only one is pictured, then
the rest are metal.

Here's a chart that shows the overlapping use of the plastic gears.
Gears marked in green boxes are available separately in the online
store. Use the Search the Store feature and put HSP in the box.

Gears in green are available separately
 
Servo 9151 9202 9203 9204 9206 9251 9252 9253 9401 9402 9451
9151 1st x           x       x
9151 2nd x                    
9151 3rd x                    
9151 x           x x      
FP-S22-2-5   x             x    
FP-S28   x                  
S31   x             x    
S31-1   x             x    
1-1-9203     x                
9204 2nd     x x              
9204 1st       x              
9206A         x            
9206B         x            
9253 1st           x   x      
9253 2nd           x   x      
9253 3rd           x   x      
9252 2nd             x        
9252 3rd             x        
FP-S31H-2                 x    
9402 1stB                   x