Thread stress of pressurized valve cap

[wolfours]

Hi guys.
I need help approaching this problem.
Hope you like my paint illustration.

What you see is a valve body made of aluminium. This is a small part of the valve but I'm right now focusing on the structural integrity of the brass cap/aluminium body threads. My goals is to investigate the stress induced on the threads and calculate a roughly approximated safety factor.
For a given thread size, pre-torque and internal pressure, what stress can be found in the threads of the cap?
I'm asking for help with the METHOD of calculating it by hand, therefore I have not included all data. I'm just out for estimates.

 

[jmec87]

This is pretty basic. How do you think you should calculate it?

 

[Doug Hunter]

By what method is the plug secured? Is it torqued to load it in compression at the conical surfaces?