Those pistons were def swelling up. One reason I preach oversize piston oilers / coolers to wick away as much heat as possible. However, in your case main issue looks to be as stated.

Here is simple "open loop" setup that came on my first Dmax boat, and actually worked ok. Never an issue holding at least150F in the coldest water. This is base on running factory circulation pump, and the factory thermostat housing and bypass tube. On these engines GM ran a 1/2" tube from under the thermostats back into the top of the gear driven circulation pump. So when thermostats are closed, a small percentage of coolant flow returns to the water pump. When this engine was marinized, a singe stage Merc seawater pump was added. Which routed to charge cooler, oil cooler, then into the factory circulation pump which feeds flow to back of the block.

However, they added a 1/2" or 3/4" bypass line right when the main 1-1/4" line enters the circulation pump. This bypass line then routed up to the main manifold that dumped in post turbo. So when thermostats were closed, the coolant flow was very similar to factory, with all water being forced to recirculate thru main pump. Seawater still flowed thru charge and oil cooler, then dumped straight to exhaust. Once thermostats opened, full flow thru the engine. Not your typical Merc layout, but it seemed to work fine..

I found an off the shelf unit good for 900hp block only, no exhaust or charge cooler, dimensions are MRM-5322 cools engines producing 700-900 HP. Heat exchanger is 5 1/8 x 30 mrm-5321-off-engine-high-performance-500-700-hp.html
I understand that you are running 1200per, still, the size should be proportionally larger.

don't know if your engines have recirculating pumps or not, but i do remember your out of room on the front
of the engines for pulley driven accy's.

the old mercury 4 hose thermostat housing comes to mind, 2 hoses constantly bypass to risers
and 2 go to the manifolds when the thermostat opens up and lets water into the engine.

a same function thermostat housing but with 2 hoses, (since your inline),1 bypass overboard so you have
constant flow thru the coolers,
and 1 overboard for when the thermostat opens up might be what your after.
did you make the bypass assembly you posted on the previous page ? is it a remote unit ?

the link, at the page bottom "warm manifold system", just something to look at.

https://www.perfprotech.com/blog/tag...aust-manifolds

piston clearance issue or pistons getting hot / if block heats bore will shrink

W.T.F.

That goes against everything I know about thermal expansion. Would you care to document that?

I'll just put this right here...



Ball and ring; Plate with hole