55-65 foot pounds. I believe the factory originals where supplied with a pre-included loctite, and if you're not using OEM bolts, you want to be sure to use loctite on them (I use blue, red is forever stuff). Sounds like those three bolts were severely over-tightened to the point that they were weakened, but not to the point that they broke under the heavy wrench -- eventually breaking under use. That locking stuff HD uses is tough stuff and I've sometimes wondered if I wasn't going to bust a bolt trying to unloosen them.

PS: The factory says don't reuse those bolts, so if your already replacing three, you might as well replace the other two as well.