Abstract: A sealable water heater manifold door is formed by adhering a gasket to a manifold door base plate. The manifold door base plate is sized and shaped to cover an opening in a water heater combustion chamber. The gasket is formed from a sealing material that is cut into multiple partial gaskets. The arrangement of multiple partial gaskets on the sealing material reduces excess and unusable sealing material. Each partial gasket has opposed ends that engage opposed ends of another partial gasket to form the gasket. A pair of partial gaskets is applied to an assembly device for accurate and quick placement of the pair of partial gaskets with the manifold door base plate. Door guides on the assembly device are used to align the base plate with the gasket, and the base plate is adhered to the gasket by an acrylic material that is applied to the gasket.