So I found a crack in my exhaust manifold where one of the runners connects to the main tube. I'm going to weld it up, but was thinking of putting extra weld around all the other runners at the same connection. Do you guys think I'd be causing more harm than good?
I've welded up a few, The stock exhaust manifold is an alloy (almost stainless but not quite stainless), First thing is find something strong and immovable (I use an old "I" beam) and clamp the mounting surface flat (at numerous spots) before welding.
I use a combination of oxy/acetylene and MIG depending on the location of the crack, both work. Heat control is somewhat important, if you get it way too hot it may melt in globs, to cold and the weld isn't going to take. Cleanliness of the surface is always important for a good weld, bare shiny metal. Let it cool slowly, don't quench it, a soft finished product (in this case) is preferable to a hard, heat stressed and hardened finished product.
The first one I did (without clamping it down) flexed and I had some issues getting the bolt holes and alignment pins to line up. The third one I did ended up with a slight bow in the mounting plate, nothing drastic but it was there. Sometimes there is no telling how the heat is going to stress the metal. I don't have an oven big enough to stick one in to anneal/relax it.
One thing you might want to think about is welding a brace between the tubes just above the collector. My repairs usually last from 3-5 years, the longest lasting repair is still going after 7. The seven year repair was the one where I welded the cross braces between the tubes 1 1/2- 2 inches above the collector. Where that one cracked, it looked like the tubes were fighting each other (unequal length and heat) at the collector and that caused the cracks.
If the welded repair does fail, it is usually right next to the weld, which leads me to beleive, slow cooling (soft) is better than fast cooling (hard) The metal is likely to be harder at the weld edge from the heat leeching away faster at the weld edge. This is where an oxy acetylene torch may be better, you can cool it down slowly and gradually. The down side to a torch is it heats up a larger area and may add to heat flexing.
