A new flange inlet tube (number 2 in the photo) need to be inserted into the engine block's bore (cast iron). Is it okay to apply a thin layer of silicone grease (against rust) on the walls of the bore before inserting the metal tube?
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There is no problem with lubricating the tube prior to insertion. It will not create any issues for you.
Something to consider is, what are you actually sealing against? If you are trying to contain any kind of liquid such as oil or water, you might want to consider using a sealant. It will lubricate it during insertion (help smooth the way), will seal the part and block against corrosion, plus it will contain any liquids you might be trying to keep in the block (oil or coolant). Might be a better choice than just silicone grease.