B model radiator - pressurized vs non pressurized

Does anyone have any information regarding when AC started putting pressurized radiators on the model B?  I'd be interested in knowing at approximately what serial number the change was made.  I need to replace the radiator on my 1940 B and believe it was originally non pressurized.  The only sources I've been able to find for new radiators are all pressurized.  Just want to find out if my serial number would be correct with the pressurized version.

Thanks Bob.  I understand the physical difference between the two radiators and their caps.  The radiator I have is unusable.  My question is whether either would be correct for a 1940 B.  I bellive it should be the non pressurized type, but the only new radiators I can find for sale are the pressurized versons.Regards, Jim

Dick, Now I understand.  So it's the cap that makes the difference, pressurized or not, not the radiator.  I assume any new radiator I buy will accept either cap.

Its conceivable the tanks and tubes for a pressurized system are made of heavier metal to stand the pressure.

Its sure that with a 180 degree thermostat you need a pressurized system to keep it from boiling.

Its sure that with a 180 degree thermostat you can't use an alcohol based antifreeze, you need a permanent (glycol based) antifreeze that also raises the coolant boiling point. Much of the antifreeze in the unpressurized tractor era was not called permanent.

Thermodynamics indicate that an engine with a 180 degree thermostat is a bit more efficient because with a higher temperature cooling system less combustion heat is lost to the coolant. There are limits set by survival of valves, head and especially cylinder wall lubrication which still prevent going to higher coolant temperatures.

It may be possible to find a none pressurized or very low pressure cap like 1 or 2 pounds to fit the replacement radiator. I'm sure a drop of solder in the right place or a bit of picture hanging wire or a 1/32" drilled hole can convert a 15 psi cap to nonpressurized. Just it won't look like a 1938 radiator cap.

Gerald J.