How open 160 Thermostat

I've been having problems with my 02 196 Ford GT40 running at 180.  I bought the boat in the spring and the old T-stat was rusted out so I ordered a new one from Nautique parts.  My boat has been running at 180 ever since and most suggested there was an air leak somewhere.  I checked all hoses, etc.. and couldn't really find anything.  Yesterday the impeller burnt up and while removing all the pieces I decided to pull the t-stat and check it.  I dropped it in 160 degree water and it didn't open.  I bumped it to 180 and it barely opened, I then went to 200 and it fully opened.  My question is how much should it have been open at 160?  I've emailed Nautique parts with a pic to see if maybe they screwed up or I just got a bad t-stat.

Yes my thermometer is correct. I would post pic but don’t appear to be smart enough!

Right at the top in the forum general discussion section there's a sticky post by Keith. 

Tbarile wrote: I should have been more clear.  I can figure it out on my laptop but my phone (where the pic is) doesn't show the same options for some reason.  The entire line starting with the scissors doesn't exist and I was too lazy to email to myself, save, etc...

Well I bought a new t-stat and did a side by side test with my old t-stat and it appears that a 160 degree thermostat barely opens at 180.  The old and new t-stat were identical and only opened maybe an 1/8'.