Professional design: The bolt is hidden in the fastening groove, trolley will not be blocked in the channel. 4 Wheel Trolley Assembly With Silent WheelQuality materials:Strut trolley wheels made with quality stainless steelfor impact resistance corrosion resistance.Resulting in incredible strength and qualityVersatile:4 wheel trolley rollers widely used as barn door rollers, sliding door rollers replacement or garage door wheelsTrolley Assembly: fit 1-5/8 inches wide and all 1-5/8 inches or taller strut channels,M10 Bolt in the middle, 92mm (3.7inch) in length,2 nuts are include that is better for upside down used,which can help you to achieve work easily, and bring you lots of convenience for your lifeOperate Steadily:Beam trolleys standard deep groove ball bearing installation,is very smooth no noise to it when used. Bearing welding positioning,to ensure the stability of the work