Core bending often complicates the injection molding of thin slender parts. Here, a fifth order nonlinear ordinary differential equation is derived for core deflection caused by a Newtonian liquid race tracking through the slit between the core and the rigid cavity wall. Solving this numerically, a universal graph is produced to help engineers predict core deflection. For small core deflections, explicit analytical solutions for these deflections and for the pressure developed in the race tracking fluid are also derived. We find that core deflection is governed by a single dimensionless group called core deflectability. Core deflection measurements agree closely with predictions of this fifth order nonlinear theory. (C) 2011 Elsevier B.V. All rights reserved.