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I'll second the restricted use of magic sealers and take it a step further. Once one of these products has been introduced in an engine -- it will always be there. So the disadvantages mentioned above can exist or surface for years after a fix (like replacing a head gasket) has been completed properly. You might be able to flush out the majority of the sealant (I like to use the analogy of sand circulating in your engine), you'll never get it all and it will slowly clog your engine's arteries and build up in heater, radiator, and block crevices with undetermined effects of limiting proper heat transfer.