Abstract: This relates to the filling and forming of food product units such as sausage links, wherein a flowable fill is introduced into a shirred casing through a conventional filling head and wherein a controlled underfilling of the preselected circular cross section casing is automatically maintained by modifying the cross section of the casing from circular to oval. In this manner the required underfilling of the casing is automatically effected so that the necessary space for the product fill is automatically supplied to permit the necessary crimping and twisting of the casing to define links of uniform size and weight or other changes in volume due to processing. This abstract forms no part of the specification of this application and is not to be construed as limiting the claims of the application.