I have been having issues with air ingress into my cooling system. After numerous checks we were unable to determine the source of the air or combustion gas ingress. Today I removed the passenger side valve cover to replace a leaking gasket. I observed rust and dust particle on the rear cylinder. It appears there is a what I believe is a frost plug next to the spark plug and it may be leaking. I have attached pictures. I would appreciate and comments on if this is a frost plug, has anyone had a similar problem and how would this plug be replaced?
Keith



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