Here's a copy / paste version of an earlier reply. It pertains to a thermostat housing bolt in an aluminum intake manifold, but the procedure is the same:
I had a broken bolt exactly like yours on my '88 Grand Caravan a few years ago. It created the perfect "teachable moment" for my students and especially the kid who felt bad for breaking it. You'll need a wire feed welder and an acetylene torch. Propane torch won't work. The flame is too large and not hot enough. Grab a nut with a center hole slightly larger than the bolt diameter. Center the nut over the broken bolt. Use a small torch tip with a nice blue flame with a sharp point. The tip of the flame is the hottest part of the flame. Touch that tip to the center of the bolt to warm it up. The goal is to get the bolt red hot but it never will get that hot because the heat is being sucked away by the intake manifold. Nevertheless, heat the bolt for at least a couple of minutes. Stay away from the nut as much as possible. You want it to stay cool. If you're careful and quick, you can heat the bolt first, then set the nut in place with a pliers just when you're ready to weld. When the bolt is as hot as it's going to get, hand the torch off to a helper and immediately grab the welder. Feed the wire onto the end of the bolt and start to build it up. Stay away from the nut as long as possible. Don't stop welding because you don't want the bolt to have a chance to cool down. As the weld builds, it will fill the hole in the nut and eventually you will also be welding TO the nut. The nut will turn orange. That's ok, but stop welding before the sides of the nut start to melt. You need those sides to be in good shape so a socket will fit on the nut.
Welding works, not by "sticking" to the metal, but by melting the surfaces of the two pieces of metal with a filler metal in between. That's called "penetration". If you don't preheat the bolt, the heat from the welder will be sucked away before the metal of the bolt melts. You'll end up building up the weld until it melts to the nut but it won't have penetrated the bolt. That would be like putting glue on the piece of wood you're using to build a bird house, then assembling the pieces after the glue dries. The preheating gets the bolt up to its melting temperature sooner. It has to reach that temperature from welding before the weld builds up to the nut.
Once the nut is welded to the bolt, let it cool by itself for, ... Oh, ... About ten seconds, then dribble a little water on the nut. Don't pour so much that it floods the surrounding area. You want to shrink the bolt but leave the intake manifold hot. The shock from the water will help break the bond between the bolt and intake manifold. Use a six point socket, ratchet, and extension on the nut to turn the bolt out. If the weld didn't stick to the bolt and you twist the nut off, just grab another nut and try again. I've already had to resort to as many as six attempts before this worked. Sometimes this works better with two people, one to run the torch and one to be standing ready with the welder. Use a high setting on the welder to insure good penetration into the bolt before the weld builds up to the nut.
If that doesn't work, you can also drill out the bolt, even alongside a broken Easy-Out, then install a Heli-Coil insert. If you've never done one or seen one, any auto parts store can show you what to do. The Easy-Out really should be removed to insure there won't be a leak later. You should be able to remove it once a hole is drilled beside it. You'll need to drill or grind the mating hole in the thermostat housing off-center to line up with the new bolt hole. I did this a couple of months ago for one of the bolts for the distributor cap on my '88 Caravan. On mine, the hole went all the way through the mounting flange but yours is a "blind" hole. That means it doesn't go all the way through. You can reach your finger through the thermostat hole to gauge how thick the metal is you're drilling into. Don't panic if you drill all the way through. That just means you'll need some gasket sealer to be sure it doesn't leak, but it will make getting the metal chips out easier. Use compressed air, a magnet, or a little grease on the end of a stick or Q-Tip to remove the metal chips. If there's too many chips left in there after drilling, the tap will but t up against them and won't cut threads all the way to the bottom of the hole. That COULD cause the new bolt to bind at the bottom of the hole and make it appear tight when it isn't really drawn all the way down yet. (To be safe, use a new bolt that is shorter than the hole). Once the hole is drilled with the drill bit that comes with the Heli-Coil kit, you use the supplied tap to make new threads.
Once the tap has gone in a few turns, back it off a quarter turn to break of any chips, then go another quarter to half turn. Keep doing this until you feel the resistance suddenly increase a lot. That means you're but ting up against the chips that fell to the bottom of the hole. Unscrew the tap and clean the chips out again, then run the tap down once more. You'll feel the tap get tight just like a bolt gets tight. Don't force it. I can't remember if your intake manifold is made from aluminum or cast iron. If it's aluminum, it will tap very easily, but it will be easy to peel the new threads off too if you force the tap once it becomes tight. Blow the chips out again so the bolt won't but t up against them.
So, blow the metal chips out after drilling so the tap will go in all the way, and blow them out again after tapping so the bolt can go in all the way. The insert is a wound-up stainless steel spring. The outer part forms threads that match the threads you just cut. The inner part forms threads that match the new bolt. There is a plastic tool in the kit that threads onto the insert, then you use that to wind the insert into the hole. The insert will shrink as you wind it in since it is just a spring. You'll be able to do that with just two fingers. There are two important things to watch for. First, you must wind it all the way in so no part of it sticks up above the surface of the intake manifold. If it sticks up, the thermostat housing will sit on top of it and be held up. There's a chance the gasket won't be thick enough to seal the gap. If you can't get it to screw in far enough, either the hole must be drilled deeper or you can try again with a second insert. Use an air cutoff tool to cut off one or two coils at the top of the insert. What's left will be plenty sufficient to do the job. To remove the first insert, grab the end of the coil with a needle nose pliers and twist it counter-clockwise. If it refuses to unscrew, which sometimes happens, twist and tug on it at the same time to uncoil it.
You can also use a cutoff tool to carefully grind down any part of the insert that is exposed above the surface of the intake manifold.
If you drilled all the way through to the cooling system, wash the threaded hole with brake parts cleaner or carburetor cleaner, then wipe a little gasket sealer inside the hole and on the outside of the insert. Use more sealer on the bolt threads. Stick everything together before the sealant sets up, typically 10 15 minutes. If you didn't drill all the way through, no sealer is necessary, but you might consider coating the bolt with a tiny dab of anti-seize compound to prevent more trouble in the future. Coat the other bolt too after cleaning the threads with a wire brush. I don't like grease on the bolts because the heat causes it to lose its lubricating properties and it can migrate away from the bolt over time. Anti-seize compound stays put. Be sure to blow out any rust or corrosion from the other bolt hole.
Friday, February 11th, 2011 AT 12:00 AM