Does anyone have a definitive reference for the required tap depth for a 3/8" NPT port. I understand from reading on here the tap depth is not usually called out, but if anybody has a standard or a guide that I could check our test parts against, I would much appreciate it./filter

I have a bonnet with 4 measurable ports. On three of them the depth of full thredas range from .330 to .350 and the 4th one is .470. This variation of almost 3 threads seem excessive. All the ports gaged within 1/2 turn of each other and the drill diameters are all within .003, so i ar struggling to figure out how these port differ so much in the threading.

The extra tap thread depth is not necessary but probably just a precaution by the machine operator.

This is not so difficult.  To properly tighten (seal) taper threads, the external thread length must be equal to or greater than the internal thread length.

Info for 3/8” NPT from ANSI/ASME B1.20.1-1983, R1992 in Machinery's Handbook, 25th Edn, pp. 1774-1775 (1996):

Internal thread length
L₁ (handtight engagement) + L₃ (wrench makeup length)
= 0.240 + 0.1667 = 0.4067”

Effective Thread, External
L₂ (also length of plug gage) = 0.4078”

Overall Length, External Threads
L₄ = L₂ + V (Imperfect Threads due to Chamfer on Die)
0.6006 = 0.4078 + V, so V = 0.1928

Your internal threads should be 0.4067” long, and external threads at least 0.4078” long.  
Shorter threads will not provide maximum strength, although sealing is not a problem per se.
The one port with 0.470” thread depth is out-of-spec.  You must have correspondingly external threads in order to seal.  The only apparent harm of longer threads is wasted material, as the thinnest sections of the coupling do not contribute strength.  Taken further, with enough wall thickness on the port, you could cut the threads long enough that the pipe 0.675” OD fits within the port.

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