NBrink, thank you. I think you're correct. Now that I see those two paragraphs I wonder how I missed them before. The depth of the threaded holes is obviously limited, but with a 2.25" thick flat head and 1/2" studs, it leaves plenty of wall thickness.
A vessel design has a 20" diameter head (flat, 2.25" thick) and it requires a 2" opening through which a shaft will pass through. From what I can gather in the code, this opening size is considered insignificant and does not need require reinforcement or additional thickness added to the head...
