John, Here's how I do it. Flow solder onto all of the mating surfaces of the mono-block and the barrels, using an ACID flux to wet the steel. While still hot and solder is molten, brush away as much solder as possible, using a brass-bristle, wooden handled brush, one of those ones about the size of a toothbrush that you see for sale at all of the gun shows. Once things are cool, wash everything well with hot soapy water to remove the acid flux residue, else you risk after-rusting of the assembly. I get someone to help me with final assembly because it really needs four hands. Start by painting all of the pre-tinned mating surfaces with a liquid ROSIN flux. Mount the mono-block in a bench vise, holding it by the under-lugs. Heat the shank of one barrel and the mono-block until the solder starts to melt, start screwing barrel into the mono-block, and feed solder, and more rosin flux as needed, onto the barrel threads. Continue this process until you have the barrel screwed VERY TIGHT into the mono-block. I find it best to switch things around at this point, gripping the barrel in padded jaws of the vise and using an apropriately-sized open-end wrench on the barrel lugs, for extra leverage, in order to get things REALLY TIGHT. Repeat with the second barrel and you are done.