An improved vertical shaft impactor rock crusher is provided. The improvement comprises an anvil having a front impact surface, a planar rear surface and a side surface extending between the front and rear surfaces. A shaft extends from the rear surface and can have a polygonal shaped cam lobe at the end of the shaft. When seated in the slot of a hanger bracket, the cam lobe keeps the anvil secured to the bracket while a side of the cam lobe rests on a seating block attached to the bracket to hold the anvil in an indexed position. When the impact surface becomes worn from use in the rock crusher, the anvil can be rotated within the bracket so that the cam lobe rests on an adjacent side thereby presenting an unworn surface to fracture feed rock in the rock crusher.