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Rebuild about done

Look at the pictures of the pistons. Are they upside-down? I'm sure someone would have mentioned it if they were. The reason I ask is there are cut outs in the pistons. I thought those were for the valves? The one valve that hit on each piston is above the cut out and to the left.
Just checked the compression. The results are as follows: 1-390, 2-485, 3-430, 4-500, 5-380, 6-490, 7-395, 8-470. All were checked with 8 cranks. Additionally, if you look back at my picture with the deteriorated glow plug, only 1 and 3 are deteriorated. All of the others are like brand new. I will post a picture of them. It looks like the issue is on the driver side. I'm thinking an issue with the heads since the pistons are all moving to their correct positions when turning the engine. What is the next step?
Ether (starting fluid)does it because it burns too hot. Too hot is the key,
Advanced timing does it also. So does nitrous and nitropropane, and propane can in some conditions. Having a valve open can do it also - which imo is what we have here.

If a compression check had been done prior we could compare for variation.
The wavered

What to do from this point-
Removal and compete inspection of both heads and all the parts for damage and straightness replacing as needed. Rocker arms, valve guides, valves, springs, retainers, push rods. All are suspect.

Now to the lower half:
Straight edge the head surfaces, inspecting cylinder walls for impact/scoring. the two pistons that were hit and any others that are suspect of being hit, I would replace. A gamble here of running them can be decided upon. Time, money, effort are personal impacting values.
Sometimes valves hit pistons hard enough to damage them like done here and the engine can run a few hundred thousand miles. Sometimes they make it less than a thousand. Did the piston skirts smack the cylinder walls possibly damaging cylinders and or pistons? What is for sure is the compression will be ever so slightly lower.
Compare quantity of material removed for the valve reliefs Chris cuts into his 18:1 pistons vs his 18.5:1 pistons. Contrast that to the material difference of the damaged pistons. The compression changes would be very minor.

The top of the piston gets screaming hot. The aluminum has been impacted hard enough to deform it. Will the structural integrity have been altered? Yes. Better or worse- we need an engineer to cover all the basis. We can ponder them but in the end is a guess. Depends how long you really expect this engine to run, and what happens if the pistons fly apart and destroy the rest of the engine wether or not the additional labor, expense, and time is worth the risk. Times in my life I would just run it and see, other times I would tear it all down and replace everything in question.

Beyond the pistons- the rods that hold the damaged pistons could be bent.
Disassembly is best for verification. If you are leaning to risking running the pistons- then a way to verify rod damage:
Dial indicator on each piston at bottom of stroke and top of stroke to ensure equal stroke among all eight. Obviously a spot on the piston that wasn’t hit is important. Then measure protrusion height on all eight.

Remove, disassemble and inspect lifters involved. Look at cam lobes for obvious signs. Bearings- this part is low risk but I mention all the parts of the sky that I might see as fall potential- haha. Bearings can get smushed as my nephew described things. Really low potential here, was hesitant to even say it even.
Less and less likely as the list continues becomes cam or crank damage. If damaged rods or lifters were found checking run out of these two would be done. But if lifters or rods weren’t damaged then definitely not enough energy could have been transferred to the crank or cam.

What ya say guys- did I miss anything to check rocker arm shaft maybe?
Piston return springs?
@Rodd so have you made any progress ?
No. My son has been doing most of the work. The problem is he just moved to Idaho last week so I'm back to working on it solo. I am thinking about getting a military optimizer take out or if I'm lucky a p400 if they're taking them out yet. Then I'll try and get this one dialed in and put it in my suburban.
My son got the engine pulled yesterday and the pistons out today. He checked the rings and the gaps and they are all within spec. The cylinder walls don't have any scratches / scoring. I had him take them to the machine shop just to inspect them and see if they can see any issues with them.