Unique nylon roller designed to rotate freely on 2mm dia. shafts is 18mm in outer diameter with removable hard black rubber outer ring which make it about 20mm in actual outer diameter. Width of roller is about 8mm with inner mounting parts extending 4mm on each side therefore center area is 16mm long. If you remove the rubber outer ring the roller can be used as a pulley. Great for small robots and mechanical devices. Sold in a package of 4.