So many sizes to fit different shafts and also different propellers. The best suggestion I can make is to first identify the make of the gear box and the propeller and then find out the output shaft diameter. If you bought you gearbox and motor at a local hobby shop go back to them for the right item. If not go to any of the online hobby dealers like www.hobby-lobby.com or www.towerhobbies.com and just put in the search "prop adapter" There must be at least 15 different ones to pick from. Knowing the gear ratio won't help, you need to know the mfr of the gearbox and the mfr of the propeller to make the right fit. Was there any instructions with the gearbox that told you what size adapter to use or specification describing the output shaft? Check that also. Best I can do for now. Good Luck One more thing, if you are a beginner flyer it would be suggested that you put the Dauntless on the shelf to admire for a while and find a slow flyer to learn to fly with first otherwise you will rekit the Dauntless on the first attempt to fly. It will take you about as much time to learn to fly as it did for you to walk after crawling as a baby.