633 is an Open Miniature Ball Bearing, 633 Bearing has an inner diameter of 3mm, 633 small Bearing outer diameter is 13mm and 633 Bearing width is 5mm, metric 3x13x5 mm, 633 Bearing is made of Chrome Steel grade 52100, 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.

• Item: 633 Bearing
• Load Type: Radial Bearing and can take minimal axial loads
• Design Type: Single row deep groove ball bearings
• Size Type: Miniature Bearing
• Material: 52100 Chrome Steel
• Closures: Open
• 633 Dimensions: 3mm x 13mm x 5mm
• Size: 3 x 13 x 5 mm
• Standard: Metric
• Bearing Inner Diameter: 3mm
• Bearing Outer Diameter: 13mm
• Bearing Width: 5mm
• Quality: Standard Precision
• Quantity: One Bearing

633 is categorized as a miniature deep groove ball bearing, have single row of balls with a deep groove raceway that enables the bearings to take radial loads and axial loads in both directions. 633 is commonly used in high speed applications, and of course they are designed to be mounted on rods (round shafts).