Are the bores glazed ? Like mirrors?

How's the wall thickness of that block by opening it up another .030

A machine shop not familiar with the line 2 line coating probably measured the piston over the coating. Added the recommended clearance from the piston manufacture and you ended up with a ton of clearance once the coating wore in. It probably just beat itself to death in the bore.

Not at all, in fact more scratches than I would want. I found some pics

Spoke with the shop and the old setup had over .008 clearance with the coating so over 10 thousands with no coating and caused a lot of piston rock and horrible ring seal. It does look like the block was machined too big to the coating and not the actual piston skirt size. This will not happen this time. Lesson learned and opportunity for more power!

Why did only two cylinders have a problem

I dont think that clearance caused the problem. Sounds to me like the pistons got hot

I agree, looks like the coating has clumped up on the skirt on one piston

I never had any overheating issues with the boat. I never saw the water go over 120 degrees and mostly never even registered due to the cold lake I boat on and oil temp never went above 205. If it did have any heating issues it was on the dyno and I was not there.

I dont' mean overheating water temp. I mean overheating the piston from a lean fuel mix, timing issues, etc. When you heat the piston, it grows, and grows fast.

the scuff marks go all the way up to nearly the deck , and are along the piston pin side . If it was excessive piston rock, you wouldn't see the marks on the pin side of the bores . At least I wouldn't think so.

Did you tune this on the water ? We're u using a wideband or reading plugs ?