I kept blowing rear heater cores and front heater cores in my 93 Burban. The thing has a new radiator, water pump and thermostat. I had the heads off and replaced the head gaskets with fel-pro's. I never retorqued them though. On my LO3 heads the rear coolant passages are .25" away from the chamber. The truck would get really hot pulling hills too. You guys think combustion gas may have been getting into the cooling system causing extreme pressure? The oil wasn't black and it wasn't losing coolant before the cores went out. Test of system pressure revealed operating psi at idle of about 13-14psi. I have the heads off to do the vortecs, the head gasket tore and I couldn't see what may have been going on.