According to the 26th edition of Machinery's Handbook, pages 1849-1850, and my Starrett tap drill chart, if you called out that you wanted the thread a 1/2-14 NPT on your print the shop should have done as follows:

Drill hole 23/32 in Diameter

Chamfer hole

Tap 1/2-14 NPT to a depth of .5343 or 17/32 (.53125)

In the diagram on page 1849 that depth is the "Handtight Engagement Length (L2=.320)" plus the "Wrench Makeup Length For Internal Thread (L3=.2143)"


Any reputable shop should know that if it is specified as an NPT thread it is because a seal is required at that joint. If you would like a copy of those pages from my Handbook if you don't have one, let me know. Also if you happen to be in the Chicago area and need any more of this work done again let me know.

As far as boring the hole out and pressing in an insert, I am assuming from what you said that this is for an intake manifold on an engine. If that is the case then you probably won't want to do that. Aside from the fact that there probably isn't enough wall stock to bore the hole larger, if you did that you would have to put an insert in that has the exact same expansion rates as the rest of the manifold, ie: Same material. Probably not the best idea. Go back to the shop and see what they say when you have proof, also review your print and make sure everything is called out right. Hope this helps.