LM4UU Linear Bearing is 4mm bore miniature linear motion bearing works with 4mm smooth rod or shaft.  The inner Diameter is 4mm, the outer Diameter is 8mm and the Length is 12mm. Item: LM4UU Bearing Type: Linear Motion Number of Ball Circuit: 4 Size: 4mm x 8mm x 12mm Material: ST14 Frame and Gcr15 Ball Bearing Quality Linear Bearing