I've concluded that my head is = leaking exhaust into the coolant. I still have our prior engine kicking = around with heads removed. I took a look to remind myself of how the = cylinder barrels seal to the head to prevent this kind of leakage. What I = found was a recess in the head the the barrel fits into, and a ridiculously = tiny insignificant little "o" ring around the barrel to seal it. Is = that it? There seems to be no sealant or gasket material at all where the = barrel seats against the head! Could my repair be as simple (!) as = removing the head and replacing this little "o" ring, or is it likely that I'm = actually looking at a cracked head? Do these "o"rings fail? This is = a rebuilt engine w/ about 20k on it.