This replacement bolt on 4 hole rear spring hanger for boat trailers is made out of hot dipped galvanized steel. This hanger will work on a double eye spring, on a single axle trailer. It simply bolts to the frame of your trailer using 3/8” Trailer Bolts. This spring hanger is used on 1 ¾” wide Double Eye spring. The bolts to attach the double eye spring to this hanger would be a 9/16” Leaf Spring Mounting Bolt. This hanger is skinnier than the front hanger to allow for the spring shackles to attach on the outside of the hanger. You would not be using this hanger on a tandem axle trailer, only a single axle trailer. The leaf spring shackles would mount to the outside of this hanger.