This is a replacement Bearing Retainer for Q3D TwoUp (and OneUp) LM8LUU as the melamine ones tend to break or fail. As there should be printed with a think shell as well as about a 30% in fill which will make them rather solid. As the ABS (the preferred material) tends to spread at the bottom layers, you will likely need to clean out the ridges inside with a hobby knife and bearing should then “snap” (i.e. make a snapping sound as they engage) in holder. Also when tightening down, would also suggest using a cross pattern being top left, to bottom right, back to bottom left and finishing at top right so pressure is applied evenly to the holder.