L-730 is an Open Miniature Ball Bearing, L-730 Bearing has an inner diameter of 3mm, L-730 small Bearing outer diameter is 7mm and L-730 Bearing width is 2mm, L-730 Bearing is made of Chrome Steel, 52100 is a high carbon, chromium containing low alloy steel, heat treatment and hardening of 52100 alloy steels will get you 60 Rockwell hardness and very common in bearings.