X-Series Hyperion 2-pole, high-rpm motors were produced in 2004. We suggest that they should be used as upgrade replacements for models which originally used brushed "300" and "400" class motors, such as GWS-style gearbox-equipped models, model cars, and boats. Choose Kv closest to the original motor. Also good for small fast airplanes on 2S lipo packs. Efficiency is much better than a brushed motor, and the prices can't be beat, so simply check motor temps during initial flights (or laps) to be sure the model is properly geared or propped. Temperature should not exceed 75C on ouside of the motor can. The "S" motors have outside diameter to match Brushed "300" motors and 2.0mm shafts, and the "L" motor match brushed "speed 400" motor size with 2.3mm shafts, so they fit perfectly in gearboxes, edf units, and models made for speed 300 or 400 motors originally. HP-X22L-3900-G is X22L-3900Kv Motor with 16T pinion for 4.5:1 gear ratio Gearbox.