Made by Boca Bearing, the ceramic 50mm axle bearing spins freer and basically lasts forever. Part number UC210C50MM NB2 non-removable seal.

Bearing info:

Bearing Material Chrome Steel 52100

Rolling Element Mat. Ceramic SI3N4

Dimension Type Metric ID (d) 50 OD (D) 90

Width Inner Ring 51.6

Width Outer Ring 22

Insert Type Spherical-Curved OD

Locking Type Set Screw

Enclosure # Two Seals

Enclosure Material Chrome Steel

Enclosure Type Contact Seal

Retainer Material Chrome Steel

ABEC/ISO Rating ABEC#1/ISOP0