I've trashed a V6 before. My early years. Broken rings just like yours. No warning horn and engine was running normal temps on my gauge. My point is that if the cooling system is running normally, the overhot internal piston temps inside the combustion chamber won't always generate a hot horn in the control box. Meaning that a lean condtion (creates heat enough to fail a piston) may not go high enough on the cyl head to hit 212 degrees-the warning horn activation point. My engine failed due to a significant carbon buildup on the piston skirt. It became so thick, that it prevented the incoming fuel/oil from reaching the rings-to lube them and keep them cool. Resulted in a high speed overheat lockup-and cracked rings, broken block. So, the overheating is not just powerhead cooling, it's also the incoming shortage of cooling fuel/lube to cool the underside of the piston and ringsets. A primary reason then that only one piston fails is that it could be starving for fuel due to a fuel restriction in the carb. A classic example of why you do frequent de-carbs on crossflows. What do the pistons skirts look like? Had the jet backed out such that it restricted flow?
