What do you mean the whole thing unscrewed from the tank? Did you force the spigot that the cap screws into, out of the tank? If so, be prepared to solder it back on and repaint the tank.
Oh and make sure you get someone who knows what they are doing before they perform the welding on the GAS tank.....
I can't imagine how the cap would seize to the fill spigot other than through varnish build up or maybe some rust but....
Got any pics?
