melting down a piston in a diesel is neally inpossibel, but breaking a piston is not. most big trucks have steel tops cast into the pistons. some yaers ago a lot of pistons were cast iron. the egt is to moniter turbo temps. they will go critical way before piston temps will. this is why the real diesel engines in big trucks have the egt installed either emidiataly before the turbo or at its exhaust. besides over fueling souped up 6.0s, the biggest cause in hi egts is not pulling with enough rpms. if the egt gets 1200, down shift or slow down. keep it 2500 rpm or more.