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I am about to reinstall my exhaust manifold on a V6 Tacoma and I was contemplating the need for gasket sealant. When the old one came off, it didn't look like it had any sealant so my guess is no. The only reason I could see it as useful is to keep it put over the studs on the cylinder head and from sliding off but I can really mount it on the exhaust itself. Understanding that gasket sealant here may be optional, I was also wondering if it ever came be undesirable and detrimental.

The manifolds also have a considerable amount of rust on them. Should I sand off or somehow else clean at least the mating surfaces?

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You shouldn't need to use sealant on the gaskets. Most exhaust manifold gaskets either come with their own sealant (like Fel-Pro's do with the silver looking stuff), or they are metal and don't require it either. I believe sealants (such as high temp Permatex Copper) will just burn off anyway, because the heat at the head/header interface is far beyond what they can take.

You should only have to worry about the mating surface of the manifold as far as rust goes. The rest can still have the rust on it without any detriment to anything, including the manifold itself. If there is a buildup of rust on the mating surface, take a piece of fine (300 or finer grit), put it on the broad side of a scrap piece of 2x4 board about 4" to 6" long, and lightly sand the area. You don't want to stay in one area too long. You are just trying to knock down any major areas. When you get done with it, clean it off with some of that brake clean I saw in your other picture, wipe it down with a cloth, and you should be golden. The reason to use a flat piece of board is so that your mating surface is maintained as flat. If you don't, you could have slight low spots in your manifold which could allow for exhaust leakage.

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