Historically, it wasn't really necessary, an iron motor lasted long enough, especially in fresh water. But basically it made em cheaper. Downside is less than optimal running temp, bigger bore clearances and ring end gaps, plus the accumulation of crap in the block over time.
As TD says, all the big stuff is closed cooled now. Ya still have to winterise them though.
With Ali, it's not just the internals, but external bits too that fuzz up. I've just ordered some heads and have had em anodised, but I bet it'll be a constant battle.
Running a hotter thermostat is better for emissions, better for the motor if built for it, not really any downside i can think of other than cost and more plumbing.
The arrow motor will be closed cooled.
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