Re: petcock size
Stock I believe is a straight pipe thread. However 1/8" NPT (National pipe taper) threads have allways fit in my tanks without any problems (And I use heavy brass petcocks), just use some teflon tape over the petcock threads first.
Some people mention chasing it with a 1/8" npt tap, but I havent really seen the need. Likey would be a little stronger...
However i'll note that on the tanks I have been getting, the braze on the bung ALLWAYS cracks... Especialy when I go over any washboard terrian with my heavy brass petcock. (And sometimes additional brass hardware depending on what im testing)
My solution? Add another 1/4lb worth of brass braze to the tank around the bung. I have gotten pertty good at it, leaves a nice smooth but large fillet.
(Subnote: My MUCH MUCH older kits tank is 2x as thick metal, and the braze fillet on the bung is like 1/4" radius as opposed to the newer kits 1/16" radius fillet, That tank has never leaked.)