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When Hurley and I had the shop around the corner from American Turbo-Pak eons ago, and when I worked for RC Engineering, we saw a bit of this occurring when the pistons weren't sourced well. At RC, I had to fix a supercharged CBX that did this, the guy lived in Las Vegas, and also had a turbo CBX.
What to look for in a piston is one that has a very thin top ring, and the ring land be cut down farther from its top deck on the piston face, instead of at the regular point for a naturally aspirated engine.
Two factors are in play here, detonation/pinging, and heat. With the normally aspirated pistons, it doesn't take much more heat than made by the regular engine to induce detonation, with ring land to deck land failures, as seen in the pictures. Also, if you find a set, or have a set made that have the lowered ring grooves in them, have the piston guru cut the face area from the top of the upper ring groove to deck at a 1 degree inward angle.
What that cut will do is to allow more cooling mixture to get down to the top of the upper ring, and along the area between the upper face of the piston, and bore wall. An added benefit is that on pistons run with giant clearances to the bore walls, the upper edge of the piston doesn't scrape the bore wall. I do the 1 degree cut on just about every piston I install, including car, 4 and 2 stroke, and especially the triple stuff I do.
Ever see a piston from upper ring to deck have top to bottom scrape marks, along with the bore? this is because the piston, at rock over is hitting the bore, and that ain't good. The 1 degree cut eliminates that, along with helping the cooling, and mixture fill in the area. Also, keeping the piston to bore clearance low helps, too, from a lot less piston rock over.
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