No, this isn't a secret answer and I'll share it with all of the other frequentors of this Forum.
A few days ago, Nathan sent me a Private Message asking me whether I had some kind of document stating the fabrication tolerances of pressure vessels. As the question came via a Private Message, I thought I should also answer via Private Message, and that's what I did.
But, as Fortysix believes I should share the information with the rest of the Forum, I'll do that.
Very few fabrication tolerances are stated on ASME VIII Code. The rest (and they're quite a few) are stated, as Joey clearly said, on Engineering Standards issued by engineering companies or corporations that are users of pressure vessels (Esso, Dupont, Monsanto, Dow Chemical etc. etc.). I've got with me, since my days of erector engineer, the fabrication tolerances issued by an engineering company called The Power Gas Corporation, from London, England. This company doesn't exist any more, so if I show their standard to someone else, I'm not infringing any patent nor am I revealing any confidential information that belongs to a company that doesn't exist.
So, I scanned the standard (it was originally a printed copy) and sent it to Nathan. If someone else wishes to receive it, please ask, mentioning his (her) webmail address. This is what Joey suggested, isn't it?
Giovanni S. Crisi