There is a pipe dope used by gas installers POS grey or some thing like it. Go to a furnace installer and ask for it. It is lifetime non hardening properties. Supposed to seal all low (5 psi and lower) type leaks. If you dont have a thermostat like my 60, drop in a teaspoon of ground ginger seasoning and run the tractor for a hour or two. The ginger trick also works well with small steam boilers with leaky flues.