SBC!!!! looking into the root cause. Looks like the cams walking too :-(
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Potential causes...
Major valve float, piston meet valve head...
Valve guide tolerances too tight...
Hokey valve train geometry.
Coil bind.
Rotator contact onto valve guide.
Major guide plate misalignment..
Major hydraulic... Though that would be also apparent in the "other" rods as well...

Good luck.
GT