Exhaust Manifold Thread Size?

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mazda 2, auto.
I had this exhaust leak at the manifold gasket for over year. I am thinking of taking and replace with a new gasket. My guessed is that the studs is rusted out and any attempt to remove the nut will break the studs.
Before even making any attempt, I should clean out the rusted thread with a die assuming I can get enough clearance.
Any other suggestions?