i used these grommets to protect 5/16" ID (7/16" OD) beer lines as they pass through the walls of a cooler i've made into a jockey box. the openings on these grommets are just barely large enough to allow the beer line to slide through them, which is great because i wanted the external line length to be adjustable without changing the length of the beer line and affecting the line's resistance level. as the cooler walls are constructed of an inner and outer plastic shell with foam filling in between, i needed to use one grommet on the inside wall and another on the outside wall.