Quote:

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The gauge temperature and thermostat level don't tell what's happening in the head's water jacket around the exhaust valve and spark plug.
As water temp goes up, the thermo seals the engine system (more or less) until its cracking temp is reached. That pressure has been keeping the water in the exhaust area from boiling, and when the thermo opens that water boils immediately (your gauge still shows 180° etc. but at the exhaust it's 230°).
The only thing that keeps it liquid is cap pressure.

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Almost all the time that surface temp is much higher
than that but in a minor distance its cooled back
down... the gauge is showing the average of the engine
and rad itself
EDIT
once the stat opens