That's the reason for a pressure cap- - -every pound of pressure raises the boiling point 3 degrees. Was the engine built with modern aluminum pistons? They're manufactured with expansion-limiting struts to keep them from tightening up and dragging at higher temps. Does the engine start when it's hot without the cranking speed being abnormally slow? If so, you don't have an overheat problem.
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!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!