Abstract: A security door replacement for increasing the security of homes or other structures includes a security door locking mechanism having a piston centrally located parallel to a longitudinal axis of the door. The piston has wedge shaped components on either end which interact with four locking arms. The security door replacement also includes a pinion gear connected to the door's doorknob which interacts with a rack gear integral to the central piston such that turning the doorknob causes the pinion gear to turn resulting in movement of the piston. This piston movement causes the locking arms to alternatively extend through and withdraw from the door frame and supporting structure thereby locking and unlocking the door.