Servo motors are designed to rotate a maximum of +/- 130 degrees. But they can be easily modified to make 360 degree turns. The hack is very well documented for various servo motor models. Here I use a Hitec HS-325HB servo purchased at ServoCity. This motor has a very good torque/size ratio and costs only around 10 dollars.
Step 1: Take the Motor Apart
Remove the screw holding the white wheel on the front of the motor.
Remove the four long screws on the back of the servo.
Remove the front cover of the servo (might require some pressure).
The gears are now exposed. Remove them, place them in a clean place making sure they don't get dusty.
Remove the back cover of the motor and save all the covers and screws in a safe place.
Step 2: Replace the Potentiometer
If you look on the back of the motor, you will see the PCB. Unsolder the two large metal tabs holding the PCB in place. Gently remove the PCB with pliers. This should not require force. If it does, make sure the metal tabas have been unsoldered properly and completely.
Unscrew the screw holding the potentiometer in place. Remove the potentiometer from the motor case (you might need to give it a little push from the front).
Cut the three wires between the PCB and the potentiometer (red, gree, yellow), but not too close to the PCB.
Get two resistors of 2.2K. tie a leg of one of them around a leg of the other, and put some solder to hold. Cut one of the legs you just twisted. You are left with a Y shape as seen below.
Strip the three wires you just cut off and insert small pieces of heat shrink tube in each one. Solder the center leg of the Y resistor ladder to the green wire, and the two others to the yellow and red wires. Heat the tubes over the joint to make sure the joint is secure.
Insert the PCB back into the servo cavity making sure the resistors don't short out any components on the PCB. Resolder the two tabs to secure the PCB.
Step 3: Cut the Stop
Now that you have replaced the potentiometer with a resistor ladder, you can cut the mechanical stop preventing the motor from rotating 360 degrees.
The biggest gear has a small rectangular plastic stop. Using an exacto knife, cut the stop off without damaging the gear teeth.
Return each gear to its place, screw the covers back on the motor. The servo can now rotate continuously.