The tool is called a Swage. There are threads on here, about how to Soldering SS that explain it in detail.
http://www.homebrewtalk.com/f51/soldering-stainless-steel-155782/ you might also search for "the tool" (aka)
I made my own tool from that thread and it makes gorgeous tight fittings for soldering or welding up. here's a couple pics of the tool and some ports on my Kettles.. So in a nut shell what you do is use a step bit to drill a hole in the steel the same diameter as the small end of the tapered fitting on the bolt. thread it thru and then thread on the nut and tighten until the tool pulls the tapered fitting through. It will mushroom out and now it will tightly hold a coupling, in my case all 1/2" SS. in these pics here, I used an acid flux and lead free plumbing solder to make an extremely strong watertight fitting in the wall of the vessel