I think it is a stress crack from the bolts tightening down on the head but not from over torquing them and that there may have been a fault in the manufacturing of the head. By the looks of it the machining of the inside of the head may of had a little too much meat taken out so that when the two parts are brought together they don't sit flush and hence when they are placed on top of the cylinder there would be a small gap of a tenth of a millimetre between the cap and chamber where the blots go through. Then when the bolts are torqued up the gap would be closed but with alot of stress focused on these points and then with the added stressed of the engine running it leads to the top cap stress cracking around the bolts.
I'd be looking into how well both parts sit together and if they marry cleanly.
Just a theory.
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