Prime-Line 11/16 in. D X 2.25 in. L Plastic/Steel Guide Roller 2 pk

This is a spring-loaded, 11/16 in. diameter nylon roller guide in a 7/16 in. plastic base. It mounts on the top edge of a wood bi-fold door that is 1 in. to 1-3/8 in. thick. Commonly used by Cox as well as a variety of other bi-fold door manufacturers.