The 3 inch Dual Action Backing Plate is designed for polishers with a 5/16 inch spindle. This includes the Porter Cable 7424XP. The 3 inch Dual Action Backing Plate features durable hook and loop backing. This backing plate is compatible with our 3.75 inch Redline Foam Pads. By equipping your dual action polisher with a 3 inch Dual Action Backing Plate, you greatly expand the capability of the machine by having the ability to polish small, intricate body panels.