This is not 8mm outer diameter and 4mm inner diameter. They have an outer diameter of 4 mm. They are also not 20mm long, but just over 17mm long.
Linear Clamping Support Bearing by Sydien
4 Review
π Wheels Manufacturing Ball Retainer Bag: Convenient Storage and Protection for Your Ball Retainers
6 Review
FKG 99502HNR Bearing Width Pre Lubricated
5 Review
Enhanced Performance Sydien SCS12UU Bearing Bushing Diameter for Precision Machinery
4 Review