How far should the thermostat open to be fully open?

dferg

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I have a F-250 1978 Ford gasoline engine. I removed and tested the thermostat which seems to operate at the specified 191F. My question is how far should the brass cylinder move to be considered fully opened? Mine only moved about 1/4 inch and the valve seat about 3/16 inch. Is this good or do I need a new one?
Thanks
 
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