Thanks dave. Could the staining be oil from a leaky o ring seal, burning itself on to the plug? The seals look a little glossy. I've also got to replace the cam cover gasket as there's a slight leak from the same side. Which side is it referred to as? I've seen them referred to as left and right but since the engine is from an st220 and mounted 90 degrees differently, I'm unsure which way would be glassed as left and right.
