I know of three ways to deal with broken off hard bolts. I'll toss them
into the pot.
1. There is a machine called a "tap disintegrator" that uses an electrical
discharge through a consumable electrode to destroy a hard part in a hole.
Usually a stationary machine, but there are a few portable ones on the
market. Disintegrating a tap, or in this case a broken bolt, does not
damage the threads in the part. If you can find a shop that has a portable
rig, or if you want to disassemble the engine and take the crank in, this
is one way.
2. I've removed broken stuff but never from the crankshaft flange.
However, if I had to do it, I'd use heat. The bolt is hard as heck but the
crank is not heat treated so the crank is not going to be damaged. Take a
cutting torch and heat the bolt until it glows, and then let it cool. Do
this separately with each bolt, allowing the whole thing to dool between
bolts. The crankshaft body will absorb the heat and the bolt will cool
slowly and become soft. The challenge with this situation is that if you
have the aftermarket rear seal, you'll probably cook it. Therefore, I'd
take a garden hose and run water over the seal area. Then the bolt is soft
enough for a drill and an easy-out. By the way, wonder where they got that
name?
3. Leave the darned things there and redrill the crank. Yeah, I know --
you didn't need a dumb one like that..........
uncle jack
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