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:

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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?

  • 1
    $\begingroup$ 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). $\endgroup$ Nov 28, 2015 at 2:01
  • $\begingroup$ O rings are often latex, I believe $\endgroup$
    – amphibient
    Nov 28, 2015 at 2:10
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    $\begingroup$ Prior to the invention of telfon tape plumbers used to use a fibrous bristle like material to seal such joints. $\endgroup$
    – Fred
    Nov 28, 2015 at 3:37
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    $\begingroup$ 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. $\endgroup$
    – JKEngineer
    Nov 30, 2015 at 21:13

2 Answers 2


The common pipe threads that are used in buildings for water and gas are tapered threads. The thread is cut on the cone rather than cylinder. In the US, these threads are called National Pipe Thread Tapered (or NPTT1, or simply NPT). They don't seal metal-to-metal. Thread seal tape is wound onto the male thread before it's screwed in. When the tapered threads bottom-out, they compress the tape, and it fills the gaps. Unlike a gasket or O-ring, the thread seal tape is good for only one mate. If the thread needs to be unscrewed, the old tape is cleaned out, and fresh tape is applied.

1 There is also National Pipe Thread Straight (NPTS), which uses a flat gasket.

2 There is a dozen major families of pipe threads. Some of them have a polymer gasket or O-ring, others don't. The trade-offs are between cost, ease of cutting thread along with other installation procedures, pressure and aggressiveness of the fluid in the pipe.

3 There are pipe connection systems that seal metal cone to metal cone. Swagelok, for example.


In addition to tapered threads for creating seals, there are Compression Fittings which use a matching male and female mating low angle taper to create a sealing surface. When properly torqued, they rely on elastic deformation to "squish" the mating surfaces together to create a seal. These are re-usable.

Crush washers are used in a similar fashion to create a seal. Instead of a taper, a softer metallic washer(usually aluminum or copper) is used between the male and female threads. When correctly torqued, the crush washer will plastically deform to create a seal between the fittings. When the connection needs to be serviced, usually the crush washers are replaced or annealed before re-assembly.

  • $\begingroup$ In the UK "crush washers" are known as olives, and are commonly found in copper and brass. In domestic plumbing, when undoing a connection, the olive will often have been mechanically connected to the pipe, and can be reused (in that place on that pipe). $\endgroup$
    – AndyT
    Dec 1, 2015 at 17:15

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