I bought some Corona Metal Gear Servos and all of them had a plastic gear inside. The gear close to the motor works well in most of the cases, but could not take much effort when used in a tricopter.
I used two broken servos and used a second gear from one of them, To make a new gear and create a full metal gear servo.
After disassembling the entire servo, I positioned the sensor in the centrer position. as shown to the right picture.
A engrenagem de metal era uma pouco mais alta que a de plástico e em função disso, lixei a mesma com a Dremel até obter a altura desejada.
After that I put the gear in place and noticed that the engine did not run because we needed to seating the gear with its new position.
To solve, put some oil and went running gear with the hand until the engine started to spin by it self.
After that I assembled the Servo and used in my tricopter.
Link of the video