The spark plug hole threading on an automotive alu head that I'm working on is not consistently conducting/grounding spark energy back to the battery negative terminal. There is high resistance and/or open circuit between the all 4 thread holes, and other parts of the head (which is well-grounded to the battery negative terminal), whereas all other parts of the head show virtually no resistance (between two probe points on the head and/or to the battery negative terminal). The threads appear undamaged, never repaired, reasonably clean, but have a goldish hue to them and appear 'hardened' (for lack of a better description). The plugs seatwell on the head - no blow-by - and other ignition components and wiring have been ruled out as best as possible.
I'm not sure if correlation will extend to cause and effect of a particular driveability/misfire problem I'm trying to sort out, but is theresomething that could cause the alu to 'harden' and to change/impede, at least at the thread surface, its electrical conductivity properties?