The fracture surface has been filed so we can't be sure what shape it was intially but it looks as if it may have been conical. That is characteristic of a tensile failure, which means it may be easy to fix. However you do have to drill accurately in the center, parallel to the bolt's axis. I've done a fair number of such extractions, though probably fewer than Bluegmhtmonaro, and in most of them I was off center. However, I claim to be getting better, slowly. Strangely enough, I'm gradually homing in on the way the old guys did it when I was a kid. Put the engine up on a milling machine or a large bench drill, use a level on the mounting face to shim it up square to the drill, bolt the engine down rigidly to the machine's table, use a centerdrill at first to dimple the bolt stump lightly, inspect the position of the dimple and move the table if necessary until the second, third, or tenth try gets the dimple dead in the center of the bolt, replace the center drill with a short twist drill-bit or much better, a slot drill, then drill the full length of the broken bolt. If you've got one, use a left-hand drill bit or slot drill, and run the machine backwards so the stump of the bolt is trying to screw itself out, not in. Then keep the table and engine in that position while retracting the drill spindle so you can screw in an ezy-out to see if it is going to be an easy extraction. Change to a proper stud extractor if that doesn't work. When you've gone through the whole process, if you haven't succeeded, use a series of progressively larger drill bits to open up the hole to the root diameter of the thread, just leaving the spiral of steel engaged in thread in the aluminium casting. Then put a center or center-drill in the chuck and engage it in the center dimple in the back of a tap of the correct thread. That allows you to start the tap exactly straight in the thread. With a light pressure pushing the drill spindle downward, rotate the tap with a spanner on its square end, until it just starts to cut (less than half a turn). Then retract the spindle, remove the tap, and see if the thread you are cutting is correctly timed to match the existing thread. If you've got it right, the spiral of steel which is all that is left of the old bolt, will be pushed out of the thread in the aluminium casting. Use a scriber to pull more of the spiral out of the thread, removing as much as you can. If you can get at least a couple of turns of it out before the spiral breaks, you can put the tap back in and push some more of it out of the thread and repeat the process.
That's what I try to do anyway. Sometimes I succeed.
