【with Mounting Bolt】Included mounting bolts; if ours mounting bolts don't fit your trailer's holes, need to purchase additional bolts. 【Modular Design】Allows for quick installation and disassembly, saving time and labor costs; compatible with 4" rollers. 【Durable】Ensure slide smoothly during towing, reducing friction and wear between the vessel and the trailer, and extending the service life of both the vessel and the trailer. 【Premium Material】Made of high-quality steel(heavy duty galvanized Bracket) and high quality rubber, with high strength and durability. 【Widely Used】Fit for small yachts, fishing boats, vessels, etc. If any questions, please feel free to contact us anytime. 4" Keel Roller Assembly with Roller Shaft and Bracket for Boat Trailer