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.