What Harley says is true and if it has lots and lots of miles on it, it may just crack again. Especially if it is in a part of the manifold that stays very hot, like in the center. The machine shop I use can plasma spray weld cast iron which is good if you need to save a special head or block. The minimum is $75. The part is heated in a special oven to about 1000deg first, then welded. Expensive, but it works.
What the other two replies have to say are basically true. The thermal spray is probably the best, however their are a couple of other options
if you have just a crack it can be arc welded with nickel rod (not low hydrogen)
you would want to drill a 1/16 hole at each end of the crack and intermitent or stitch weld aprox 1/2' stop and peen the weldment (ball peen hammer tapping on weld)
for about a minute. then let manifold cool to touch. start on the other end of crack and do the same thing, then go to middle and do the same thing always peening and letting get cool before going to the next weld.
your origional question was about brazing, I have successfully brazed manifolds before it is a little more difficult than arc welding
again if it is a crack drill your 1/16 holes on each end wire wheel or grind the surface where you want to braze set your torch to a slightly carburizing flame (a little less oxygen than a neutral flame)and braze as you would normal steel. I would use a 3/332 flux coated bronze. when you are done brazing it is imperitive that you peen for a verrrrrrrry long time
hopefully this helps you
the other guys are right it is probably cheaper and easier to replace it. I'm just hardheaded enough to prove I can do it
I agree with drilling the holes at each end of the crack and grinding a nice "V" the length of the crack.The shop I worked at would heat the manifold red and wire weld with stainless rod because it will flex when the manifolds heats and cools.We never had any to come back and seems like we always had a lot of repeat business.We could have it done in about a half an hour.Peening is good for it too.
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