The 160/180/195 thermostat rating refers to the degrees when it is fully open. It starts opening about 20 degrees earlier.
Best thermostat I found is the 'Mr. Gasket - high performance' type which is a better and smarter design which has least restriction when open.
It also stays open in case it ever gets stuck, unlike conventional types which would stay closed. Only a couple bucks more than standards.
Thermostats are there to provide the minimum temp the engine needs to run most efficiently with the least wear.
235 manuals recommend 180. Some think they run best around 200.
Once the coolant temp reaches 180 degrees, a 160 will be just as open as a 180 with the same flow - Meaning above 180 degrees a 160 will not cool the engine more than a 180 !
Especially in the colder months a 180 thermostat helps with warmup time and burning off harmful water vapor inside the crankcase when doing short runs.
If you get best results running straight water and no thermostat - your radiator or hoses are probably clogged (hopefully not the block) and it probably needs a recore.
Try first using a motor flush cleaner (or 4 gallons of vinegar from the 99c store soaking overnight) and then flush reverse from the petcock side with a water hose (on the drivers side next to the gas pedal linkage).
You can try flushing the radiator with DILUTED muriatic acid (or straight concrete cleaner from the hardware store).
But it's really ALWAYS best to get a recore or new radiator, if you have the funds.
Go for a 3 core type minimum and if funds are plenty also use Evans NPG+ coolant which is completely waterless and the best thing you can do for your cast iron engine, radiator and hoses in the long run.
Used or old radiators without known regular flushing history will almost always be clogged - water loves to corrode them sooner or later, even with 50/50 mix of glycol based antifreeze coolants. Regular flushing only slows down the corrosion.
Raw metals and water regardless of dilution or additives should never really be together. It's simply cheap and user friendly.
