[Walt Fricke]

A dropped valve once ended up with a cylinder fracturing and the rod sawing a slot in the case at the spigot in an engine of mine (over rev caused valve issue). The connecting rod cap was distorted so that only the outer ends of the cap were still touching the main part of the rod. So the high strength aftermarket rod bolts were stretched. And bent. And you could see where they were narrowed where they stretched. But they didn't break.

ARP will let you know if any non-ARP bolts were involved. Though given how gently you say this engine was used, I'd have thought stock bolts would have worked, at least until you started putting the pedal to the metal.

Modern engines are able to use higher compression because of better spark plug placement, the use of knock sensors, and the ability of the ECU to change things on the fly based on what sensors are telling it.

Did the shop doing the disassembly by any chance have access to the bolt stretch figures used in assembly? If so, did they check the stretch? This could tell if other rods had overstressed bolts. But since A did the bottom end, and the shop you are using now did not, I don't suppose this was done? Though you could still test for this - measure the unbroken bolts, reinstall them and stretch to spec. Then loosen the nuts and remeasure to see if any of them were stretched beyond their elastic range. This should not happen, so if it does it suggests at least some of these bolts were bad/wrong.

Those main bearings look great. How were the rod bearings? What did the bearings which must have fallen out of the rod with the broken bolts look like? Was the broken rod discolored, as in a lubrication failure or some other issue like wrong dimensions somewhere?