Difference between Boss seal industrial vs military fittings SAE specs

[legopuffer]

TL;DR Summary

Issue sealing o-ring with military boss specs

Hi - attached is 3 pages from the PARKER o ring guide and I am having trouble with specs in the military straight thread tube fitting chart not fitting the designated o-ring. I can't find a different resource for something in my thread size needed - 5/8-18

Does anyone have an alternative reference, or can explain how the slope of the military boss seal is supposed to work? I have traditionally used the industrial chart and had no issues...

 

[Baluncore]

Does anyone have an alternative reference, or can explain how the slope of the military boss seal is supposed to work?

As the hex is screwed into the block, the O-ring is pushed down the taper into the block. The O-ring seals the joint in that situation, by being initially trapped in the cavity between three surfaces.
When subjected to internal pressure, with fluid moving up the thread, the O-ring is pressed against the taper in the block and the flat under-surface of the hex. The O-ring is then no longer pressed against the groove machined above the male thread, as that is filled with fluid under pressure.

The deck surface of the block is not part of the seal, but the gap between the bottom of the hex, and the potentially rough surface of the block, must be small enough to prevent extrusion of the O-ring.