I don't know how many of these heads I've taken off. - not like I've done it for 20 years in a shop though.

Often I have experienced that the bolts have a wide range of "de-torque' values. this is a mixture of the headgasket collapsing and the bolts losing plasticity in that range.

Primarily, the rear passenger bolts on the head will rust due to a failed HG at some point which will not be perceivable by visual inspection or testing of the coolant. this reduces the holding power of the bolts.

Fast forward, you now an have an aluminium head prone to cracking which is held against the block by uneven forces. then add high combustion pressures and heat. . . . well, uneven distribution of forces and internal stresses in the metal seem the most likely candidates for a cracked head, in my humble opinion.

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