Originally Posted by SloopJonB
I'm betting the shaft of that bolt is stuck really tight. I've found over the years that the single biggest reason for bolt heads failing is over tightening - someone leaned on the wrench until they felt the bolt head start to give and called it good enough.
Yes... overtightening a bolt can compromise the strength of the bolt but this is unlikely as Grade 8 bolts have a tensile strength of 150,000 psi... that's quite a bit for hand tightening... but if the bolt was over torqued I could see it failing... I believe it was more due to an overload condition on the engine causing the over torqued bolt head to break off... which is why you must change not only the two bolts shown on the photo but all bolts holding this engine... bolts are cheap... ruining your alignment is more costly... The preload, or thread tension in a fastener is higher when the type of steel being used is grade 8 in quality which if you can get aircraft bolts that would be the best overall grade.