A method of installing a filter unit includes aligning a filter unit into a filter head assembly. The filter unit includes a laterally extending key member and an engagement protrusion. The engagement protrusion includes a first portion that has a first radius of curvature and a second portion opposing the first portion that has a second radius of curvature larger than the first radius of curvature. The laterally extending key member is aligned into a key slot in the filter head assembly. An engagement surface is aligned on the filter unit into a complementary engagement surface on the filter head assembly. The filter unit is rotatingly inserted into the filter head assembly. The laterally extending key member shifts an interference member on the filter head assembly from an interference position, wherein the filter unit cannot sealingly engage the filter head assembly, to a non-interference position, wherein the filter unit can sealingly engage the filter head assembly.