Big Block 454 MPI Magnum. Cruising along, temperature spiked. checked drive pickup, no restrictions. Ran compression test, two cylinders low. Pulled thermostat and ran for another few days and water intruded # 5 cylinder, and bent a pushrod. Disassembled engine, assumed head gasket(s) blown. Could not find anything wrong with head gaskets, no visible cracks in head nor block. Also water in # 4. No water in oil pan, but top end milky. What else could it be? Risers and manflds are only a few years old.