Bulk material is the water jacket temperatures. Those were the old days of 160F coolant temps as per the somewhat amusing artwork. My modern car has pressuriz

Bulk material is the water jacket temperatures. Those were the old days of 160F coolant temps as per the somewhat amusing artwork. My modern car has pressurized coolant and the thermostat doesn't even start to open until 180F. You are correct about combustion temps, but bulk materials like the cylinder walls and pistons run much cooler than combustion temps, yet bulk material temps generally only increase over time.