LFR50/5-4NPP 5mm ID x 4mm U-Groove Track Roller BearingLFR50 5 4NPP U Groove Guide Bearing 4mm ID x 16mm OD Track Roller for Linear Motion The LFR50 5 4NPP is a compact U groove track roller bearing made for guiding systems that ride along round shafts or tracks. With a 4mm inner diameter and 16mm outer diameter, it's a popular choice for small linear systems, 3D printers, and automated setups needing smooth motion in tight spots. 1 U groove guide bearing per pack Inner diameter (dw): 4mm U groove