When you have a stripped or broken stud in your exhaust manifold, the traditional fix is to heat the manifold flange with acetylene or MAP gas torch, and unscrew the stud with a Vice-Grip.

If the stud has been broken off near flush so you can't get a grip on it, then it might be drilled out. If you are very careful about drilling concentric to the threads, you might then be able to clean out the original threads with a tap. Otherwise you may need to drill to the major diameter and install a Heli-Coil for thread repair.

An alternative to drilling may be to burn the stud out with an ELOX machine (Electronic Discharge Machining). This device uses a carbon electrode with high frequency AC current and electric arc to burn away the metal (usually at about 0.001-0.005 inch per second). This is usually a specialty service, not the sort of equipment the average shade tree mechanic might have at home. This is more commonly used to remove a broken tap or a broken "easy-out" which is so hard that it cannot be drilled.

If someone has neglected to use the prescribed brace brackets between the header pipe and bellhousing, or may have abused a rusted stud during an impatient moment of disassembly, you might have a broken flange on the bottom end of the exhaust manifold where there should be a threaded stud. A squeaky tight enthusiast such as myself might have the urge to weld this back together. If you should mention this to a friend you may hear the immediate comment that you cannot weld cast iron, and especially a part that has been repeatedly heat cycled at high temperature and is likely to be quite brittle. My response to that is, I'm really glad no one told me that before I did it. I have done this a few times with good results. It is a bit time consuming, so you might consider the value of the part you are trying to save before you make the commitment. But if you want to try it, here's the skinny.

I don't really weld these parts back together, It's more like re-casting the part freehand in a local area. You can do this with an oxy-acetylene torch (not acetylene-air torch or MAP gas). The procedure is similar to gas welding, cooking up a molten puddle and adding filler rod. For filler you need iron material, not steel. Almost anything made of iron can be used. An old horse shoe might not be suitable, as vintage blacksmith wrought iron may have a lot of impurities. But a piece of black iron gas pipe or a rail road spike would do nicely.

Start by positioning the broken part with the area to be repaired on top within easy access and generally horizontal. Heat the repair area until the parent metal begins to melt and will make a puddle. At the same time also heat the filler material until it is white hot and about ready to drip. While keeping the parent metal in a state of molten puddle, heat the filler material to melt it and add the molten filler to the puddle. Do not drip molten filler onto any non-molten area of the parent part, because it will not stick and would only leave a very weak finished part. As the puddle grows you can allow it to cool and solidify at one side while you add metal where it's still molten. In this manner you can continue to add metal to build the part up to a shape which is roughly equal to the original shape, and a little oversize. At that time you may take a break and allow it to air cool. Do NOT dunk it or throw water on it, as the resulting thermal stress could easily make the part crack before full cooling.

After the part has cooled you can go at it with a power grinder to grind away any excess material until the part may be returned to its original desired profile. Once the original shape has been restored you can drill the part and tap the hole to accept the specified stud or bolt. The trick here is of course to get the new hole in the correct location, and straight, so the new stud will stand properly in line. Having the part set up in a drill press would be a big advantage, although I have never had such luxury at hand when needed. With enough finesse I have usually been able to eyeball and center punch and hand drill the hole close enough to be serviceable (although I may also have failed and started over on at least one occasion).

This is all jolly good fun as long as it's a hobby and you have plenty of time available. If the part is dearly valuable or otherwise irreplaceable the time may be of less concern. When it's finished and it works, you can have an ale and a big grin and bragging rights. And you just might want to have a camera handy for record, as the next dude you mention this to may not believe it. I'm sorry I have never kept the picture, but maybe next time.