Ball bearing supported spool bushing, compatible to 20639, to remove friction from spool spin. Ball bearing inner race supports spool cog side, while it is floating from spool shaft. With this bearing bushing, you may not much tighten the spool tension by cap, as bearing works against. Therefore, this is only recommended when you want to reduce minimal amount of friction from spool spin, and it is recommended to cast heavy rigs the longest. ABU4500CS, 5500CS, 6500C, 6500CS and etc, with 20639 brass bushing.