On my '83 peanut head there is a port on the very back of the head, right behind the #6 exhaust port. Is this an exhaust or simply heat passage? and what do we do about blocking it off? There is a threaded 1/4" or 5/16 hole right beside it and the port itself is not threaded.

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Sounds like the air injection port. The threaded hole held down the air injection tube. I would see if it's a common size hole and tap in a freeze plug if possible. Otherwise you can make a little metal tab to bolt down. If the head isn't on an engine you can drill and tap the hole for a plug.

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Joshua

See this post

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一期一会
Too many people who were born on third base actually believe they've hit a triple.