Extreme Max 3005.2181 Roller Guide-On System Includes one pair of Roller Guide-Ons perfect for small aluminum boats, duck boats, jon boats and other light craft Zinc-coated for freshwater corrosion-resistance, not intended for use in saltwater or brackish water Easy, bolt-on installation fits bunk or roller-style trailer frames up to 3" wide x 4.5" tall* The size and shape of trailer frames can vary drastically, please measure your boat on the trailer before ordering to ensure a proper fit * 7" bolt kits are available for trailer frames up to 5.5" tall, see Extreme Max 3005.4062