This is a roller only and is used on bottom mount installations. It is a 1 in. high density polyethylene roller with a 3/8 in. axle hole. The 1/4-20 x 3/4 in. carriage bolt inserts into the 3/8 in. Dia high density polyethylene bushing which allows the roller to turn on the bushing/axle smoothly. The carriage bolt is then secured in the bottom rail with a nut.