Originally Posted by
awesomeame
I'm using OEM everything.
I had a bunch of bolts around the shop today (all shimano cranks) and was measuring them. The longer the bolts the less torque they could handle. The longest bolt broke before 80 inlb. The shortest bolts could handle the 125inlbs. That's what got me onto the bolts-are-stretching theory. We are talking ten-thousandths of inches here, not much. This could also just be from the manufacturing process, but it sure seemed to be a trend this afternoon.
As for precession, I'll buy a new bolt set from LBS and Loctite them in, maybe physically marking them as well. That'll show if precession is an issue. I'm not totally convinced, but it could be precession is putting a different load on the bolt as well, causing them to break when re-torqueing.
Matt
The stretching is weakening the bolts. A bolt can stretch under tension only so far before it breaks (the elongation at break). But the stretch you are observing isn't reversed when the tension is released. It is creep. You have a stretched bolt even at zero tension. So it takes less tension to stretch the bolt the rest of the way to break. It already has a head start.