Overheating of an aluminium head will cause it to loose hardness. A severely overheated head will be soft on the exhaust side of the head face where most of the heat is generated. When a head face becomes softer on one side than the other, an uneven clamping results, causing failure of the head gasket.All of the aluminium heads that come through our workshop are hardness tested on arrival and checked again after all work is completed to ensure that they do no fall below the recommended hardness levels.