This retainer saves time and eliminates frustration when mounting Hiwin MGN-12C/MGN-12H carriages back onto the linear rails. These 12mm linear rails are typically used in Kossel based printers and although the carriages have a wire retainer it does not do a good job in preventing the balls from escaping. Note: The no-name carriages vary wildly in dimensions (I found a difference of almost 0.5mm between two carriages in the same set) so you might have to scale before print. This part is designed to have 9.25mm clearance between the retainer wires.