welding cast manifold, pointers?

I might get rained out for work tomorrow, and I was going to try and repair the manifold for my D17.  I took it off the other day and the exhaust ports all look pretty thick and has quite some life left in it.  The problem with it is the previous owner must have driven under something low, and hooked the muffler and popped the casting out around the threaded outlet.  I have one of the broken pieces, and was going to find a cast fitting to cut another piece out for filling in the hole and then weld it all up and put a stub piece of pipe in the top and weld that in and just clamp the muffler over it.  I figured it would be a fun thing to try and it's broke already anyways so I'm not out anything if I mess it up anymore.  After doing some reading tonite about welding with Ni rod, cleaning and preheating sounds like the key.   If I do this, I'll bring my dad over for coaching as he was a welder but was looking for any other tips before I get into it.  Thanks.

Chuck, The guys at the shop always had a big chunk of insulation they wrapped around a casting after welding. I think cooling slow is almost as important as clean area and preheating. I had a bad manifold on my 45 years ago and a fellow said he would fix it for me. Every time he touched the rod to the part it turned to dust and crumbled to the floor. He was trying to weld around the bolt hole for the muffler. I found out a new one was a good investment. It's been on there for over 20 years now. Good luck and let us know how it turns out. Charlie

Ive always heard your supposed to bury it in warm sand. Same principle that Darrell mentioned. On smaller things I wrap them up with old rags, on your manifold if you bury it there may be air pockets inside still which may make it difficult. The stress cracks start popping up seconds after you break arc, so you may want to have things ready. If I can I try to have someone holding a torch on it behind my weld, but may not be necessary. Good luck

For post heating, a helper with a rosebud or cutting torch will be a big aid in avoiding stress cracking while it cools.  Practice, practice, practice (and with cast iron, it still may crack........good luck)

I heat castings in the wifes oven to 500 degrees, weld/braze and then put back in the oven and drop the temp 50 degrees every hour.

I don't care for arc welding as cracking even when doing it properly is a possibility.  You can buy cast iron rod for oxy/fuel welding, which is the cats meow.  Put nickel rod to shame in strength and appearance, but takes a special touch.

Brazing in most applications, if you have a very tight fit, actually has higher tensile strength than arc welding electrodes, but it has to have a very tight fit to get the strength, but it should be alright in this application.  Just remember to flux the hell out of it and clean the hell out of it.  Don't grind, as it just smears the carbon into the pores, sand blast