Need help on exhaust manifolds, bolt size help

[weider1717]

On the drivers side I have everything loose but the air line that runs into the manifold and there is a small bolt in the middle of the manifold that is smaller than a 5/16 and slighty larger than a 8mm??

the 8mm feels about right but it slipped twice so I just stopped and a 7mm won't fit so I'm clueless?

I took my time with kroil and all and everything so far was easy nothing broke yet and I want to keep it that way!

also what size wrench do I need where that air line screws into the manifold it's an odd size too!

thanks for any help