cast iron does have a higher melting point than steel, right? I'm running out of ideas on this.
Well, don't use that idea.
The melting point of cast iron is around 2100 F and carbon steel melts at around 2600 F.
We all know what a pain it is to cut cast with an oxy-acetylene rig, but that probably isn't the same type of cast iron that SBC heads are made of. With cast iron, the better the quality - the easier it is to cut with a flame. Lower grades of cast have a lot of contaminants that make it hard to get the iron to melt and flow.
I would take a deep breath and look at the drill-out option again.
The key to successfully drilling out a stud is locating the center. If you can get a center punch in (or very close to) the exact center of the bolt/stud, you have a real good chance of removing it without destroying the machine you are trying to repair. One huge obstacle however, is if the stud is sheared off below the surface (especially if it is not a clean flat break).
Read this, I wrote it for Saab's but it is the same - just skip the parts that don't apply to you:
Remove broken stud from cylinder head
This procedure is tough even when done on the bench.
Drilling is required to get those 2 broken studs out - to gain access, you will have to do one of the following:
1. Pull the head and get it on the bench.
2. Lift the engine/transmisson as a unit. Requires removal of the hood, motor mounts, axles, some engine wiring, hoses & tubing, etc.
3. Come in through the front. Need to remove everything between the radiator support and the engine. That means the heat exchanger stack (radiator, A/C condenser, intercooler, oil cooler, ATF cooler). Dump the A/C charge, pull the compressor.
Even then you might not have enough room to get a drill in there. I dont suggest using an angle drill. Also be aware that the downward angle of the of the head will make drilling difficult. The drilling steps must be done absolutely perpendicular to the head.
I'd pull the head. But whatever you decide here is a good method to extract sheared studs:
1. If the studs are sheared below the face of the cylinder head; forget about DYI, bring it to a machine shop.
2. If there is some stud still showing, it must be brought down to level with the face of the flange. Carefully file it till it's flush. Take care not to remove any aluminum from the head.
3. Center punch the stud exactly in the center. This is the most critical step in the procedure.
4. Drill the pilot hole. Use a small (1/16th" max) cobalt drill bit and plenty of cutting oil.
Note: the depth of the hole cannot exceed the depth of the
stud. If you drill deeper than the tapped hole there is a good chance
of hitting an internal coolant passage. Using an OEM replacement
stud determine a safe depth. Put a piece of tape on the bit to use
as a depth gauge. It is not necessary to get all the way to the
bottom of the stud but try to come close. This applies for all the
drilling steps.
5. Change to a slightly bigger bit (maybe 1/3rd of the stud diameter) and drill your next step.
6. Another hole that is maybe 75% of stud diameter.
7. Final hole should be close to the minor diameter of the stud. The minor diameter means the bottom of the thread cuts. Do not go out so far as to cut the tips off the threads in the tapped hole of the cylinder head.
At this point the stud should be pretty much just a fragile/hollow shell. There are two ways to get it out.
You could try an EZ-Out (I never have much luck with them) or take a file and tap the tang end down into the hole. While pushing down on the file use an adjustable wrench and try to screw the stud out.
The other way is to collapse the remanant of the stud in with a round nosed chisel.
Either way, if you drilled the stud out to the minor diameter, it will come out with some heat, penetrating oil and imagination.
Be sure to chase the threads in the cylinder head (when replace manifolds, good practice dictates chasing all of them anyway).
Good luck, man.
