Sirs,
I would assume 90 degrees, once all bolts are torqued in stages to 45Nm, is demanded because how accurate would, say, 45 degrees X4 be? Hard to measure.
My concern is the sudden demand placed on the bolt and the warping effect of a 90 degree rotation on the head...after all, only one bolt at a time can be rotated, unless one had 18 assistants.
A 90 degree rotation exceeds the elastic limits of the steel, and flows into the plastic limit, as the bolt is engineered to do...but wouldn't a more gradual torquing, with appropriate rest periods, give the crystal lattice time to realign and 'heal" somewhat?
Besides, bolts, like people, are not identical...some will deform while others will resist deformation. Torquing, vis a vis degrees of rotation, would sense the behavior of individual bolts and compensate accordingly...not to mention manufacturing fluctuations in the head gasket from front to back.
The head, upon installation, is being warped...just warped in a manner that minimizes built-in stresses.
Just pondering...
Parenthetically, I think the bolt will heat up from the friction, and cool down from the stretch...with a net rise in temp, therefore length, momentarily.





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