I'm fixing up an old spare WB I have and I successfully extracted what was left of 6 of the 8 exh. studs/bolts. Last 2 broke off, one flush and one with a stub. Working on the flush one, I able to drill in about 1/4" but no more. It is as if I have hit some harder steel. I've blunted about 7 drills, and even a brand new cobalt drill does not want to make any chips.

I'm puzzled as to why it all was going well until 1/4" in. This is with engine on an stand, I can't imagine the dismay and annoyance if I came across this kind of thing when trying to drill out a stud with engine installed.

alistair