Headgaskets Aluminum Heads

Rat Bastid said

I don’t bother. I back them off and torque them again. That is the correct way to do it.

Of course, you’ll say it’s wrong so it must be because you know everything, even if it goes against what Cometic says.

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You are right. When you "torque" a fastener all you measure is the friction that is present between the threads and the surface of the underside of the nut or bolt. If you try to check the torque on an already installed fastener, you will get a reading for static friction which means nothing. The proper method to retorque a fastener is to break it loose and then retorque it. This will not tell you what the tension was in the fastener before you loosened it, but it will be appropriately torqued. Of course the right way to "torque" fasteners is by the stretch method or the torque-angle method.