Classic/Antique Car Repair: 1947 Fleetline overheating, blown head gasket, degree thermostat

Question
Hello Brad,

I have a 1947 Heavy Fleetline. I bought it with a 235 engine out of a 1954 pickup. It had a blown head gasket and i figured i would just have the whole thing rebuilt. At the machine shop they found a crack in the block. I then found a 235 from a 1958 pickup, the only difference was it needed a different water pump. meanwhile this was getting done i took the radiator and carburetor in for service. the radiator was 90% blocked off so thats been repaired. New 160 degree thermostat and radiator cap. I dont think the carburetor was done properly, so i will just buy a new one. Anyhow my question is, Why or what else may cause me to over heat. Normal is 180 degrees but im running from 190-225. It idles at 185 and drives at 195 but after i shut off the motor it goes up to 225 and coolant goes flying out of the overflow. I check it with a heat gun to make sure the gauge inside is accurate. Everything has been gone through and rebuilt. the only none factory item is the radiator. It is from an 80's Mercedes. It worked for the other 235 so i figured it would for for this one. any thoughts or anything you have would be greatly appreciated. Thank you!

Answer
It sounds like the radiator cap is not holding pressure. The increase in temperature when you shut down is normal and is called heat soak. The radiator cap should hold pressure and prevent the boiling that you are getting. Also you should be running a mix of 50/50 anti freeze and water. That has a boiling point of 145 degrees F and with at least a 15 pound cap the boiling point should be well over 270 degrees. If all that is ok, then I would check for a leak in the head gasket.