How did this alternator bolt fail?

[JINGA]

Okay. I heard a nasty clacking noise and when I finally found what it was, I had a busted alternator pivot bolt!

Here's a write up of how I got the bolt stub out of the head. Busted Bolt (http://www.geocities.com/wwwjeffdotcom/altbolt/)

Here's some pics of the busted bolt. What was the mechanism of failure? How can I avoid this happening again? I think the PO had it happen too!
At a little bit of an angle: http://www.geocities.com/wwwjeffdotcom/altbolt/bolt1low.jpg
Pretty much right on:
http://www.geocities.com/wwwjeffdotcom/altbolt/bolt2low.jpg

It's not precisely a 90degree fracture, but pretty close. I think it's a grade 2 bolt. Jeep sells the bolt for about $7 (yeah, right!) but doesn't say if it's a grade 5 or an 8 or what.

So. How do you think it failed and what should be done to avoid failure again?

Thanks for the discussion,
Jeff

[ibnfe]

go to hardware store and get a matching grade 8 bolt. It will have 6 (or 8?) lines on the head of the bolt. grade 5 hardware will have 3 lines on the head. Grade 8 hardware should not fail under cyclic or torsional stress. I've used them on tractor PTO attachments as a shear bolt with no problems! :D

[Spinner]

Over-torqued -> hairline fracture + vibration = busted bolt ????? :confused: