U-shaped bolts are needed to secure the Cat-Proofer existing fence extender to your round chain link fence posts. Most chain link fence posts are either 1 5/8" in outer diameter or 2" in outer diameter. Many chain link fences contain both having the 2" O.D. posts at gates and/or corners. We recommend that you measure your round metal posts and take a fence extender from your kit with you to a hardware store to aid in purchasing the proper sized u-bolts for your existing fence.