For the steampipes I tend to use brass bar and just drill it out. I suppose it depends on how long the pipe is? You could always use ordinary brass tube (or copper even) and silver solder a thicker ferrule on the end that needs to be threaded. I do that for blower stays in boilers.
Most of the usual ME suppliers stock suitable O rings.
I go with John and say solder a piece of bronze bar to the end of the tube, suitably threaded and drilled accordingly. Make it over size, then after silver soldering turn on the tube, collet chuck if you have one, 4-jaw if not.
Got: 3½" Lilla, 5" Lion, 2½" Annie Body.
Building : 5" Edward Thomas/Peter Sam, 7¼" Jubilee1897
In planning : 4" Burrell Road loco, 5" Standard 4 tank
Would like to build : 5" Duchess, NGG16 Garrett
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I generally use GLR Kennions for my Silicon 'O' rings, as they have worked well in the past and also they have a guide against each size for groove width and depth. I have not had any problems... so far. Donald
3 1/2" Rob Roy 2" Durham and North Yorks Traction Engine now stripped and being cleaned up ready for a new paint job Rebuilt bought 13xx with Valve gear and many other problems. 5" gauge Class2T 84029 passed Steam test 18/10/17 and run. Galloways Non Dead Centre Engine which is next project.