This 4.0" diameter x .5" thick round strike plate, armature plate or keeper plate is made of 1215 CRS and is Electroless Nickel plated. When selecting a strike plate, armature plate or keeper plate select a plate equal or greater than the diameter of your electromagnet. In applications where alignment may not be optimal it is always recommended that you select a strike plate, armature plate or keeper plate size that is larger than the diameter of the electromagnet. Be sure to select our mounting screws that allow your plate to articulate which assists in alignment and flush contact with your electromagnet for maximum holding forces.