BTW, I'm thinking that the three of us may perhaps be talking about a different by-pass port.
The by-pass porting that I've been speaking of is an actual port that allows some coolant (aka sea water... river/lake/ocean) to escape around the thermostat.
Some of these ports are anywhere from 1/4" to 3/8".
Some of the automotive engine ports are even larger.
However, these automotive ports direct the coolant back to the engine circulating pump where BTU loss is minimal.
(see image below)
Whereas with the Raw Water cooled marine engine, any sea water lost via the by-pass port, is a total loss.
It does not return via the circulating pump. Instead, it's directed onto the exhaust system and out with the exhaust gasses.
As I mentioned, the thermostat's job is to hold back coolant until the temperature dictates that the thermostat begins to allow coolant to leave the engine block/heads.
IMO, if your by-pass porting is too large, you may not be able to increase engine temp with a thermostat temp range increase.
You may need to restrict the by-pass porting some.
Just a thought!
Here's an automotive closed cooling system T-stat housing. Note the rather large by-pass port and hose.
This directs the by-pass coolant to the Circ Pump.
If the above was applied to a Raw Water cooled Marine engine scenario, this loss of coolant may prevent the engine from reaching the correct temperature.
IOW, it may be as simple as too much coolant being directed past the thermostat.
Mark, does that make any sense to you?
I'd like to see a photo of your Marine engine T-stat housing.
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BTW, the GM SBC T-stat housing mounts to a horizontal intake manifold surface.......
..... whereas the 5.0L and 5.8L Ford T-stat housing mounts to a vertical intake manifold surface.
.