Following behind MMMRS, I thought I'd post a few images and say a few words about how I approach the bolts that do break. To those of you reading this: Good news! Extracting the broken bolt ends is easily performed and can be done safely without ruining the block. There is a tool set needed though. Read on for the details.
I broke most of the head bolts on my recent disassembly: 9 1/2 out of the fourteen bolts to be exact. In attempting removal, I would whack the bolt twice, followed by an attempt at loosening until I felt the stress building up in the bolt, then strike the bolt again, repeatedly. I gave the bolts that broke about 6 tries each to un-stretch and loosen. They just don't want to cooperate.
Here's the oddball bolt that both broke and came loose, making the 1/2 bolt statistic.
For the removal of broken bolts left remaining in the block, first clean out any residual oil from engine disassembly. A dry recess area will facilitate drill chip evacuation.
From the company seen on the business card, get yourself this drill kit, with drop-in drill guides, a right hand drill, extractor and bottom tap (to be used for assembly prep). I have no affiliation, I represent the company by being a satisfied customer only.
Fit the proper guide over the broken bolt, and drill down just deep enough for the extractor to bite, about 5-7 mm. Remove the guide and clear the chips. Fit the extractor into the drilled hole and give it a light tap to set the flutes. The really good news is that the bolts un-thread easily and do not require any effort to turn.
Rick