# How metal fittings seal connection without rubber gaskets?

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?

• I'm not an expert in pipe connection by any means, but typically a gasket material is used, such as teflon tape, for gas and water plumbing connections (gas piping in the US uses the component you are showing). I am unsure of what material or method would be used for high pressure (depends on the definition of high pressure, as well). – wwarriner Nov 28 '15 at 2:01
• O rings are often latex, I believe – amphibient Nov 28 '15 at 2:10
• Prior to the invention of telfon tape plumbers used to use a fibrous bristle like material to seal such joints. – Fred Nov 28 '15 at 3:37
• I think amphibient is asking about the seal between the two sides of the union, which is a metal to metal contact. They are machined surfaces so that the two sides meet with negligible clearance when the collar is tightened. Some deformation may occur, but I have not seen literature stating that. I don't think amphibient is asking about the seals achieved by the threaded pipe section. – JKEngineer Nov 30 '15 at 21:13