I have a 50 Chevy and 216. I installed a new copper line to oil pressure gauge in new original style dash cluster. I tightened it well and can’t seem to keep it from leaking. It is 1/8” cooper with a brass fitting. It doesn’t look like a usual compression fitting with a ring. And the tube doesn’t fit through the fitting to flare it. Anyone have any suggestions or have a similar issue? I’d hate to go nylon or something.