A dual fuel common rail system includes first and second common rails fluidly connected to a fuel injector by a co-axial quill assembly. Distillate diesel fuel at a first pressure moves from the first common rail through a first fuel passage defined by a quill, through an inner tube and into the fuel injector. Liquid natural gas a second lower pressure moves from the second common rail through a second fuel passage defined by the quill, through a space between the outer tube and the inner tube, and finally into the fuel injector. The quill is partially positioned in a block. First and second compression load adjusters are threadably attached to the block to adjust a compression load on the inner tube and outer tube to inhibit leakage of fuel from the tubes.