Motor Gear Pinion for Electric are available in a wide range of Modulus and Teeth counts for a variety of Model RC Helicopters and gear ratios. Modulus .5 for 450 size Helicopters (e.g. Trex-450, NEM-450SE, Mini Titan and etc..) Modulus .6 for 500 size R/C Helicopters (e..g. 3DX-500, Trex-500, Hirobo Lepton and etc..) Modulus .7 for 600 size / 50 class Helicopters (e.g. Trex-600, Hurrican 50, X-treme 50, X50-2009 and etc..)