I have run into the situation many times - most of the time I remove the engine and effect an extraction - sometimes I drill out the broken bolt and do a Heli-coil thread repair - in situations where the owner has tried to use an easy out an broken it off, I remove the head from the engine and use a solid carbide drill bit in a vertical mill to effect a repair - only problem with milling out the bolt is the time it takes to jig the head in preparation of the operation. It can cost $250 to repair a screwed up extraction by a monkey with a few hand tools who thinks he can fix it without taking it to a professional mechanic.
If you have the experience, the time, a right angle drill (if you are not going to remove the engine), a short 5, 5.5, 6, and 6.5 mm left twist drill bits along with a penchant for center punching the broken bolt DEAD center, you might remove the broken bolt without damaging the threads as pictured. I started removing this broken bolt from a TRX400EX head and as it started to spiral out of the hole, I thought I was removing a previously Heli-coiled hole, then as the remainder of the bolt unscrewed out of the hole, I realized it was just the corroded threads of the bolt that were spiraling out of the threads of the hole. When removed, I chased the threads with a tap and installed a new bolt.
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