a catalytic converter is constructed by measuring a substrate to be placed within the outer shell or can , wrapping the substrate in a selected mat and loading the package into the can . the can is larger than it will be at completion of manufacture to render such loading easier . subsequent to loading , the measurement of the substrate is used to direct the degree to which the can is reduced in outside dimension such that a selected annulus is created between the substrate and can , said annulus being occupied by said mat .