KAG, IMO, the tiny holes in the thermostat flange aid in initial air purging. Once the system is purged of air, the tiny air bleed holes should be sufficient.
I have twin engines that are equipped with half closed systems (engine block/cylinder heads only).
The thermostats do have the tiny air bleed holes in them. The T-stat housings are not equipped with any by-pass porting.
I can assure you that the T-stat housing by-pass is not necessary for a half system.
However, when we bring the exhaust manifolds into the loop (such as your systems), the by-pass does help keep coolant to continually flow through the exhaust manifolds. In your case, the by-pass does make sense.
Your T-stat housings are double outlet.
They also offer the mixing chamber (unnecessary, btw) of which has the circ pump supply and the sea water pump inlet port both capped off.
(who knows what type of turbulence is being created within this unnecessary chamber)
Rather than mess around with drilling holes in the thermostat, and/or porting your already incorrect T-stat housings, why not replace these T-stat housings with a more correct Closed Cooling System T-stat housing.... such as the housing that I presented earlier?????
This one is already ported for a by-pass (clearly shown here). ....
and does NOT have the mixing chamber.
Also, get rid of those God Awful house style plumbing components
, all of the twists and bends
, and install a less restrictive coolant return system.
I'll be bold here and will suggest that you are attempting to put a Band-Aid on a system that is just not going to heal properly.
As you know, if you were to sustain severe over-heating, you'll be looking at an engine replacement.
Correcting/changing/fixing your existing system will be far less expensive than an engine replacement would be.
Hey, I'm in your corner, KAG........ I just hate to see you chase your tail here.
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