I always pre-tin all mating surfaces of the barrel shanks and the monoblock with Hi-Force, using lots of flux and a small wire brush. Make sure that you have 100% coverage,as you will not achieve that if you try soldering things at final assembly, without any flux. Allow to cool, then scrub well in HOT water and soap to remove all traces of the very acidic flux. If you skip this step, you'll have problems with rust at the joints forever. Don't ask me how I know this!!!! You won't need flux for the final assembly, once everything is pre-tinned. A second pair of hands is invaluable here because it's kinda tricky trying to play the torch over the monoblock, and the barrel shanks, while trying to thread the barrels into the block at the same time. Get yourself an assortment of machinist's polishing stones in 6"x1/2"x1/4" size, in grits from 240 up to 600. They are great for removing the excess solder once things cool, without scarring the steel.