That's a "0.9 BAR" cap which converts to 13 PSI. The radiator is probably rated for that kind of pressure, but you would need to get an ORM heater recored with a modern replacement if you intend to run that much pressure. Every pound of pressure raises the boiling point of the coolant 3 degrees, so a 13# cap would let the coolant go to 251 degrees before it boils. A stovebolt engine doesn't need to run that hot unless you're running air conditioning in a traffic jam, and even then it's a bad idea. A 7# cap should be plenty, which raises the boiling point to 233 degrees.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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