If you've got an R bike from the '80s or '90s, chances are good you've doused yourself with gasoline more than once while fighting the flapper that lies inside the fuel filler neck. In the photo above, the flap has been removed, leaving an opening wide enough to admit even the crudest filling device (sometimes found at points south of the border and other likely adventure destinations). The procedure isn't hard, but does require a steady hand and a little nerve. One slip, and its new tank time.
Start by draining the gas from the tank and removing it from the bike. Remove the petcocks to make it easier to get the last bits of gas out. Wash out the tank with plenty of water to remove any traces of fuel. The last thing you want is an explosion from the sparks you'll be generating as the neck is cut off. Set the tank back on the bike so it doesn't move around while you work on it.