The Autofab panel mount grommet is 1.5" in diameter. The shoulder of the fender mount fits into a 3/4" hole of the fiberglass panel. The fender mount accepts a 5/16" stainless steel flat head bolt. The mounting tab is 060 steel, 3" x 1.5", and has a 3/8" hole for the stainless steel nilox nut which secures the assembly. As the assembly is tightened the shoulder expands and tensions against the fiberglass panel.