Should the exhaust check balls fit snug, or slightly loose like mine?
What happens if they're loose? If they're not supposed to be that is.

I'm trying to figure out why one engine has 4# lower water pressure allowing it to heat up at WOT. The good engine has 8-12# @ WOT. Pressure is measured from the side of the block.
The only obvious differences between the two engine's water plumbing is that one has a long straight hose (1.25") feeding the strainer (U turn style) and the other has a short hose (1.25") with a sweeping 90* bend feeding the strainer (90* style). Staggered is why the different length hoses. The shorter side (rear engine) is the one that heats up.
The short side used to have a U turn strainer and two 90's bent into the hose which I changed trying to resolve this.
The PR dumps are in the strainers. One was bleeding pressure on the questionable side (witnessed on vid) so I changed it and no difference although I haven't watched it on vid to see what it's doing. I'm going to plug it next season and see what happens. That's not as scary as leaving a GoPro back there again.
I have the Teague adjustable PU's.
What's the likelihood of air being pulled in at the O-ring? And actually causing a problem?

pull the block plugs and check for mud,sand,sludge build up..

I do every year. They're clean.

I've also tried other raw water pumps. The SS Hardin unit does NOT work. I've polished/deburred the inside of the plastic pumps and no luck.

Even 8-12psi at WOT is pretty low.

I try not to think about it. Once I can maintain 100 for more than a minute we'll see what happens.