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My stock exhaust manifolds have two 3/8-16 threads on output flange and the other has a 3/16 stud on one side and a 1/2-13 thread on other. Should I assume someone drilled and tapped the one bigger or is the normal.
 

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Yes, someone has drilled & tapped for larger stud, that's very odd. Get to a frustrating point with the one manifold, feel free & post which side, & heck, even the casting date. Am organizing parts I've hauled in & have a bunch of clean late '71 & '72 production log manifolds, and am cheap on any of them.
 
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