I am well familiar with the concept of
gasket. From my non-mechanical engineer perspective, I can understand how a standard rubber gasket works in various plumbing and automotive applications (such as O rings etc). However, working on both auto engines and residential plumbing over years I have noticed some fittings forming a sealed metal-to-metal connection without using a rubber gaskets. Some examples are galvanized or copper unions that look like this:
In metal-to-metal connections like this that require a high pressure seal, what insures that small cracks in the connection (that a rubber O ring would be conforming to due to its soft texture) do not leak?