Deve, the NAPA thermostat appears to work just as you describe above. It didn't make sense to me that the open thermostat would allow some coolant to bypass the radiator and go directly back to the engine 'uncooled' via the bypass to the water pump.
That left me scratching my head. Then I saw the pix of the thermostat that Tim posted with the perforations in the side. In his pictures #8 and #9, it appears that there is a sleeve that slides down when the thermostat is open to block the perforations and thereby send all coolant to the radiator.
That got me to scratching my head harder and made me think that the NAPA thermostat might have a design flaw.
You posted a picture of a perforated thermostat that looks similar to Tim's but does not have what appears to be the sliding sleeve. It would operate as the NAPA one does and allow coolant to still go through the bypass to the engine when the thermostat is open.
http://devestechnet.com/Images/Projects/Venerable261/261Thermostat010lg.jpgTim, I empathize with you having to keep the house neat and tidy all the time when trying to sell. Been there, done that...too many times. Good luck. If you get a chance to throw the thermostat into hot water, it would be interesting to see if it closes off the bypass access when it opens.
Now wouldn't it be neat if we could all sit around a table with the various housings and thermostats we have collected to compare them in person?!!
Matt
PS--I checked with my parts friend at my local NAPA. He confirmed that the THM 155 is open stock and they are back in inventory at the Phoenix warehouse after I ran them out by buying two about a month ago. He confirmed that they should be available nationwide in the NAPA system even thought the website lists them as out of stock.
