If you are going to sweat on a longer tube, be aware that heat will destroy the 'O'-rings inside the petcock.
This has already been reported in the forum.
So employing a proper procedure is imperative.
Jus sayin'
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You're correct, but I don't recall seeing anything posted with that problem. I sweated mine on using a soldering gun, not a flame. Only needed to get warm enough to melt the solder medium and the pickup tube below it near the body was quite cool. Just like me.

If you click on the link you'll see the extension tube doesn't go all the way to the bottom of the pickup tube. You just rough up the top 1/4" of the pickup tube and slide the extension over it. By using a soldering gun instead of a flame, the heat is localized instead of the entire tube being heated. Plus, I wrapped the bottom portion of the tube with some tinfoil held in place with forceps. It got a little warm but the tube was nearly room temp. I think using a gun instead of a flame was the difference.