I have modified regular nylon servos and noticed that the variable resistor from many tutorials have changed from an insert knob to a fixed metal knob. You have to drill into the gears with a 1.5mm drill and now the gear has too much play and the gears block because of the play the gears grind at each other in an angle. So the sensible idea here was to make little M1.5 spacers / washer from LED / resistor legs.
This is not something you want to repeat for all the servos of your robot army! So just buy continuous servos and be happy that you saved an hour by paying a dollar more!
Since the knob rod is fixed, it's tough to even remove the resistor from the controller - so I just soldered directly to the motor and ended up with a geared 6V motor. You can buy these on your own, too! Look for 6V N20 motors.