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Ancient Sunray Boiler Question - Leak
which can be removed and using a sealant put back in place.
The removal is the tough part, you need to drain the system down and let it cool. Then load up the plug with WD-40 and let it soak in. Then take a torch and heat all around the out side of the plug. Some of the old timers had a tool that fit into those and with suffcient torque they could be removed. The key thing is to not cause the boiler itself to crack. It takes some work but it can be done.