brazen osco manifold

One Water intake pipe (on left) comes from the raw water pump....... (the engine has a closed, recirculating internal water system, with its own pump, sort of like a car) - except for this heat exchanger and the wet exhaust manifold/muffler system i.e instead of a radiator in a car, the internal, closed system engine water is cooled by dumping its heat into the cold raw water in the heat exchanger). They don't physically mix. The actual path of the raw water is through the raw water pump, then the transmission oil cooler, to the heat exchanger, into and back out of the manifolod, into the elbow at end of manifold, and through the wet exhaust chamber, and out the back of the boat.
SO, ANYWAY, I had to braze up a pair of cracks from some dumbass leaving water in the engine manifold in the winter (IT HAS AN EASY TO GET TO DRAIN PLUG!!!. The manifold is cast iron, so that means, choosing from the options of welding versus brazing.. "Welding" implies reaching the melting point and fusion of the two metal pieces being weld together (not quite crack patching, as I am doing, but parameters apply). Brazing is the joining of two metal pieces without fusion, but just simply the adhesion of a melted brazing rod of brass. Temperature wise, in the processes of joining metals, catagorized on the temperature at which the processes occur, brazing is above 800 degrees, while silver solder is below that, and "welding" starts at the melting point of the materials. The reason to braze is that cast iron is a dirty metal which does not weld well, BUT brazing rod has a melting point way below cast, so you can melt the brazing rod and depend on its adhesion without melting the cast. Because cast is brittle, you need to heat the whole piece slowly and uniformly, because heating one area could result in thermal shock - and the cracking of the piece from differential expansion. Brazing is all about achieveing the correct temperature on both pieces, or sides of a crack to be joined. When the materials are at a dull red the brazing rod should start to flow, assuming you have fluxed correctly. Flux is a chemical scavenger that cleans the surfaces. You can overheat the brazing rod, made of zinc and copper, and the zinc will fry out as white powder. That is not good.
So when everything is just right you get to see the brazing rod melt, and FLOW freely over the surfaces, and into the crack. Also, the exhaust water (originally the raw water), after it leaves the manifold chamber, flows briefly through a hose and is then reinjected into the elbow at the end of the manifold, cooling the exhaust gas and going as a mixture to the wet muffler, losing energy the whole way. - and creates a burp of water and gas, out the transom, in the process of turning exhaust pressure into mechanical energy, - and a pretty quiet one at that.