All tolerances are exact and ball bearings need to be press fit in with a vise or equivalent device. If plastic cracks, increase stl model by 1-2 percent and print again. Standard 608ZZ Shielded 8x22x7 Miniature Ball Bearings used in this model. The center Bearing can be switched for a 608ZZ/C Ceramic Ball Bearing for increase spin time.
Spin on!