Repeated overheating can aggravate the situation. My engine overheated from a broken water pump, and after troubleshooting for a bit of an afternoon, without even moving it in the parking lot, my V6 definitely blew something. The coolant came out clean when I changed the thermostat, but two loads of fresh water later, the fresh water was coming out black. That took only 2 refills of the cooling system to happen. This gentleman has been actually driving with this problem, and if you can go from cold engine to overheated engine in a couple of blocks, things are way more effed than hairline cracks, would you agree? Or are the hairline cracks that phenomenal?