Joel -

I've read the TSB many times, examined both bolts and aside from the increased machining on the old style bolt can find no possible reason for the problem other than the one given in the TSB and backed up by what you've stated. There have been many theories expressed, but none that seem correct to me other than improper torque application. But the problem could be compounded by bolt design.

The only other things that I would be suspicious about:

1) machining of the fat bolt to create the undercut for the threads might weaken the bolt at the transition line from threads to bearing surface

- but more likely -

2) the above plus over-torqueing where the fat part doesn't give any yield twist and sudden pressure is applied to the transition/bearing area as the threads elongate and weaken the transition area. With vibration over time, the break could occur.

However, both of those suppositions would result in break/shearing at/near the transition point and while some do break there what we often hear is that the bolt simply backs out. The bolt could break from a wrapped belt snatching a loose pulley against it - but here again, the remaining portion of many of the snapped bolts have been difficult to remove, indicating some elongation of the threads may have occurred.

So - perhaps there could be two separate problems with installation - over and under torque plus faulty design. It will likely always be a mystery.

Frank