Before you start heating things up....there are several ways to get broken bolts out of holes.
If the engine is still in, it will be a pain, but it's doable.
You'll need an angle head drill motor. If you have an air compressor, you can get a cheap harbor freight unit for about $30.
First, center punch the bolt and use a left handed carbide drill bit at low speed (400 rpm or less), it may catch and turn the remnant out of the hole.
If that doesn't work, drill all the way thru the bolt and use a screw extractor. Be careful not to break the extractor by applying too much torque.
There are a variety of extractors out there, the most common ones look like a really dull tapered drill bit, try that one first.
If the bolt is stuck good, I'd suggest using an​​​​ 1/8" carbide burr and cut grooves 90* apart, and use a square extractor.
Be sure to squirt plenty of penetrating oil (kroil or pb blaster) thru the hole, so it is also penetrating from the inside also.
Worse case scenario, after doing all of this and the bolt is still in the hole, get a 1/4" carbide burr, and cut the grooves deeper, almost to the crank threads, then go back to the 1/8" burr and pick 2 of the grooves 180* apart, and cut into the crank threads. About half way through the crank threads should be good. Then, using a thin sharp punch ground to the shape of a thick knife point, fold the bolt remnant over on itself. Once it is out, clean up the threads with a thread chaser. Try to avoid a tap, cause you don't want to remove any more material than you have to.

See, easy as punch.

Seriously, though, getting broken bolts out is a real PITA, I've pulled a lot of them, and the corroded/ pulled thread ones are the most challenging.
I've had the pleasure of removing high pressure fuel pump bolts out of Paccar engine blocks, where failure to save the hole means a new block, cause there ain't no room for a helicoil.

Good luck, Jeff

 

Unless there is a problem with the threads, all of the torque on this bolt was in the area between the remaining broken piece and the bolt head. I have found many times, after a bolt has broken off in a hole, there is almost no resistance holding the remaining piece and they come out quite easy.
Again, this only applies if the threads have no damage.
Good luck, Greg

 

After rereading your question, a possible solution similar to getting broken/ rusted in place Cat exhaust manifold bolts out.
If you have access to a lathe, you could turn a pilot with an od to match the hole in the crank, then bore a hole thru the center of the pilot, slip the cylinder you created in the crank to center drill a pilot hole. Then bore out the center of the stuck bolt to the minimum thread diameter, using the pilot hole to guide the larger bit, and clear the threads with a tap.
Or instead of turning a round, center up a bolt in a lathe as before, the same size as the crank bolt and bore a hole in it, and use it as a guide.
Sounds more complicated than it is.
Anything to avoid heat.
I suppose a question that needs answered is, what caused the bolt to break in the first place. Over torque and pull a thread, cross threads, rust ? Pulled / crossed threads are a real biotch .
If that remnant is seized in place, from pulled / crossed threads, no amount of heat short of burning it out will help. In that situation, the pilot hole, drill to tap size and cutting what's left out with a tap is about the only real option.
As before, good luck.
Jeff

 

I had the same thing happen using a "quality" made in Taiwan harmonic balancer installation tool. Like yours the bolt snapped off inside.

The fan and A/C compressor were already off so I had a clear shot to drill a hole in center of the bolt. I put a mirror down so I could see the broken bolt. I put a fender cover and leaned over the fender and carefully started drilling a hole in the center of the broken installation tool bolt. I drilled the bolt using a cordless drill. After I got the pilot hole drilled I used a standard easy out and the broken piece backed out easy, no problem. Drilling that hole from the position I was in was not easy. It may have been easier had I removed the radiator too.

I think the broken piece of stud came out easy because the threads were oiled, clean, no rust and it was on the installation that it broke.

I went and bought a US made installation kit and everything went back together without a hitch.

The tip above from Jeffwebley is outstanding. Takes the worry out of drilling the hole offset. Wish I would have thought of that to get the hole started. Once you get the hole started in the right place you re good to go.
Best of luck.

 

One negative possibility. Loctite. If loctited. You will need heat. Not glowing red with oxy-acetylene, 400 degrees or so to get loctite to release. Then carb cleaner. Penetrates better than WD. If not loctited, you can probably knock it out with a punch or small chisel. Put punch in at an angle so you are turning bolt counter-clockwise.

 

Go to a craft / hobby store and get brass tubing. (Some Ace hardware have K&S brass, they come in 1/32 and 1/16 increments) Make a guide by using 3 or 4 pieces of the tubing to keep the drill bit in the center of the hole.

Use a reverse rotation drill bit, it may just pull the remains of the bolt out. Get a good one like Milwaukee not Harbor Freight. I start small and work up.

If you can get an old soldering iron - you can heat the bolt by putting the tip of the soldering iron into the hole. I've done it , it works.

Yes it's a pain in the but = but can be done.

 

Been there, done that in a 70 1/2 Z/28 back in 1972. Done in January of that year in the driveway in below freezing weather. Not fun, but doable. Follow suggestions here but don't use heat unless nothing else works. The reverse drill bits are your best bet. Take your time and make sure to center punch right in the center. Off a little and you're screwed. My balancer launched at around 6500 rpm and went into a cut corn field that had about a foot of snow cover. Never did find the balancer. Good luck.
Duane

 

Many years ago, wife had a '91 Miata, same thing, but the crank for some stupid reason was only into the balancer by ~1.5 inches or so, not the full depth, so all of the above, and yes I used heat from my small torch used for plumbing soldering work, just not a whole damn LOT of it.....like about ~400 degrees or so....

I had to get an American tap in there , tons of fun then a larger hardened bolt....got lucky managed to do it in the car.......should have never sold that car....damnit....

 

I have performed the crank snout/keyway repair on these.........it is not that the balancer is not on all of the way....it is that it had a 3/4" long key for a 1 1/2" slot.....this knocks the crank timing gear off and will make the car extremely sluggish. The accepted repair on the car is a Permatex metal repair for the crank, a 1 1/2" key, and some green loctite. Believe it or not...it works.
I had a chance to by a two top, 80,000 mile, all original red 91' Miata for $3800...in excellent condition....that chance is gone......yes it was the car in the repair. It would have looked cool sitting next to the 72' !

As far as the OP is concerned....try putting a left hand drill bit on an electric impact gun....soon as the drill "chips" the impact will rattle and help shake out the offending broken bolt.

Jebby

 

The only way I'd use heat would be if I knew there was some kind of thread locker used on the harmonic balancer bolt when it was installed.

Good Luck!

 

Like the idea of a guide tube to save the threads. I too lost a bolt using a cheap balancer removal tool. It was on a members car here too,,,,oops!

 

I noted the length of the snout was shorter than the depth of the balancer, by over 1/2 inch , I don't remember much about the keyway itself, I may well have put in a longer key, been over ten years now....paid a grand even for the car, ~4 years later sold for 1700 today be worth over 4k hard to believe, it was an automatic, so just purrfect for the wife to drive....