In our current project we are using Aeroquad SK3 2188 style motors. Unfortunately they have an in-built prop saver on the front housing that suits 3.9mm and 5.5mm prop hubs (8mm with a supplied collar). Since we want to use Mini-Quad style propellers we developed this adapter. Please note: You still have to use a O-ring to hold the prop on the saverPrint with at least 2 perimeters and 70% infill.