-there are other people who should be much more up on aluminum hardness than me--but I can say for sure that on SBC's between the two center cylinders is a major place where annealing occurs because of the concentrated heat.
-an example is: we had a really nice set of 1970's era Brownfields, guy was running'm at Bonneville and bought a late Corvette water pump that actually was for a reverse rotation multigroove belt system---so the pump was rotating the wrong direction when used with V-belts.
-all it took was just a few runs to soften the center of the heads--and the metal deformed at the gasket just like this photo of an SB2 head--between the bolt hole and the torched out area----and this SB2 has two coolant passages right there thru the torched spot.
-I fixed the engine---got the right water pump on it etc---ran the Brownfields several more times---pulled the heads of after each event----about 6 runs caused about .010" worth of deformation--the heads had been angle milled so far over the years we eventually ran out of material to mill--so the heads died a slow death.
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