[olddog]

A good engineer told me that a good rule of thumb is "the thread engagement needs to be at least the same length as the diameter of the bolt." In other words, a 1/2" bolt needs a 1/2" of threads engaged. Now that is for steel and course threads.

Aluminum is a very poor metal to put threads into because it slowly creeps over time. Lots of different grades, with some much better than others. The extremely long threads makes sense in aluminum. As the aluminum slowly moves, the load will eventually even out over all the threads.

This explains why the bolt torqued solid, and over time the threads pulled out. What is bothering me is "if one bolt pulled out what do the other ones look like?" Remember you said the other head had a slight leak at the gasket too. Has some of those bolts stretched the threads and are no longer at torque? Could all the holes have the threads stretched.

I hate to be spending your money, but you said you like to overkill everything. Should all the holes get heli-coils put in them?