According to what I read from Hendren, they recommended to not a thermostat on their 604's. Mine ran hot last season so I bought a GM 19" fan and hope that helps this year.
The GM fan made the biggest difference for me. I've removed the poppet from the thermostat to restrict water flow, run single and double pass radiators, run with and without fan shrouds... that fan is what really made it stay cool.
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