Sorry to say this but many years ago GM found that many bottom end engine failures were caused by 3M Scotchbrite residue from prior warranty repairs like resealing intakes or head gaskets. If you are using anything in a rotary tool, use a Bristle disc. They have plastic fingers that don't produce harmful grit. They come in different diameters and grits. Scrape the most of the gasket with a razor blade scraper.
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