I need to weld the valve, because it is not a simple valve, and also because it is a big valve..
In my setup, i will filter all not in mash pot, but in an another one.. so i need a valve with a properly size, that allow transfer of all grains too..
@squeegeethree the valve you posted me, doesn't work for my purpose.. plus, I'm from Italy and the cost for the shipmentshould be too high. anyway for all others usage i need, i obviously use weldless fittings.
In this case, i must weld a valve on bottom of the pot..
At the end I get the solution: they will cut the entire bottom of the pot, for the size of the esternal part (that is 46 cm instead of the 48cm of the kettle bottom), and make a new one totaly in Stainless Steel that they will weld using the 2cm of the bottom remaining.. so even if the bottom is really thin, on the back now they will find a Stainless Steel part and not aluminium, so they will weld properly..
Well, I hope so 
