This is a GD-600 Electric Flight 2.5:1 ratio Gear Drive Unit for Electric R/C Airplanes Applications. FEATURES: Designed for electric aiplanes using reverse rotation 550-size motors Converts the rotational speed from the motor to the propeller, making the motor more versatile and efficient than a direct drive system. It is capable of delivering more thrust and longer flight times by swinging larger, more efficient propellers. Dual ball bearings and precision machined aluminum prop hub provide true, vibration-free rotation. Open housing for optimum cooling and easy pinion changes Compact, lightweight, and highly durable Extended prop shaft provides plenty of room to hold prop, washer, and nut. Includes a 2.5:1 steel pinion gear for 18-turn motors.(see COMMENTS) One year warranty INCLUDES: One ElectriFly GD-600 Electric Flight Gear Drive 2.5:1 One 2.5:1 (18 tooth) steel pinion gear (GPMG0851) One precision machined aluminum prop hub and washer (GPMG0855) One instruction sheet All mounting hardware and wrenches REQUIRES: One ElectriFly T-600R Reverse Rotation Ferrite Motor (GPMG0705) OR a similar 550 size motor. SPECS: Weight: 1.4oz (40g) Shaft Diameter: 4.9mm COMMENTS: Optional Pinion Gears: GPMG0851 18T for Gear Ratio 2.5:1 GPMG0852 15T for Gear Ratio 3.0:1 GPMG0853 12T for Gear Ratio 3.8:1 Replacement Main Gear: GPMG0854 JAL 6/22/00 ldt 12/11/01 DESIGNERNOTES: "Does the gearbox require a reverse direction motor?" Our motors are timed to provide a higher RPM. That is why we had to have a reverse motor. Some other motors will turn the same RPM forward or reverse. It just depends on what motor he is using whether he needs a reverse rotation motor. per MH, R&D 7/00 AMC shaft dia updt tjt 081705