you need thermostats, or at least a restrictor in it place. You will not fill the block all the way and you'll have hot spots. If your Merc overheated, odds are its the water pump impeller. They have a poor deign water pump, I recommend a new one every year, if you haven't replaced the upper housing, replace that while you're at it. Check the lower housing, if teh seals look bad or it appears to have gotten hot, replace that as well. At the very least, check teh gasket under the lower housing. On the high performance stuff, we would remove the thermostats and run restrictors, usually a quarter with a hole drilled in it. It takes a while to figure out exactly what size hole to drill, lots of trial and error. Basic merc cooling system, water pump feeds water to the powerhead, once you get above 2000 rpm or so, you build too much pressure and the poppet valve drops off water pressure before it gets to the block, the thermostats open once you get the block up to tempo, btw, those mercs don't pee till the thermostat opens.
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