A retract servo is a very specialized servo. It is non-proportional, going 180°, "lock to lock". It is specifically made for Mechanical Retracts to lock the gear in the up and down positions, and DOES NOT have any means of end point adjustment and cannot be adjusted by ATV on a transmitter. It is only able to go 180° and provide full maximum available torque at the two end points. This is its whole purpose and intentional design, but if someone has a unique design situation where they need other than 180° available, they must use a proportional servo, not a Retract Servo. Retract Servos are designed to withstand constant holding loads in the up and down positions, unique to the use of Mechanical Retracts. Pneumatic retracts do not need a retract servo. A proportional standard or mini servo is used to move the air valve back and forth. Two ball bearings. Metal gear set. 3-pole motor. Compact Size. Travels 180° - full-lock to full-lock -- to hold your gear in position securely. Not proportional. Perfect for mechanical retract installation. One-Year Warranty. One S136G Compact Retract Servo w/ J Connector is included.