In commercial production, NPT thread depth is measured with a gage that has
a ground notch. Unless otherwise specified on the blueprint, and with the
gage screwed in as far as it will readily go, the thread depth is correct if
the notch is within plus or minus one thread from the surface of the part.
There is no need to specify depth for a standard NPT thread.



The correct depth is plus or minus one thread from that notch,
"coolcamaro79" wrote in message
...

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.


--
coolcamaro79