They're plastic sealed ball bearings, which offer smooth rotation, great precision of attachment and also simplified maintenance.
• 840 Sealed Ball Bearings x2
• Applications include TA07 chassis pulleys, and M-chassis countertop gear area.
• Using ball bearings as opposed to plastic and steel bearings provides better components fit and much more efficient rotation.
• These ball bearings use rubber seals in the place of metal people – this allows one to open them around neat and work with them.
• Low-friction oil can be used to lubricate the internal workings.