I don't think the IP can bend rods, a lot of fuel to hydrolock, ... exhaust-valve crash bending rod? It would have to happen at the least-clearance moment, when the exhaust is at least partially open, which would bend the crap out of the valve stem and leave an imprint on the piston before bending a rod.

My guess is simply bad rods. Being in the casting/forging business for part of my career, there are a lot of things you can do wrong making an I-beam (or any other cross-section) that can make one side weaker than the other, a post-mortem on the rod would tell a lot. Could even be in the heat-treating.

As far as symmetrical loads on the rod, always the compressive load is symmetrical provided that both journals are centered and there is no significant friction in said bearings, the inertial force on the other hand can cause the bend to favor one side if it is close to the yeild strenght already (a long shot), ... probably the trailing side on the compression stroke as it accellerates laterally, but mostly the bend will compress wherever there is a weakness such as porosity, poor tempering, etc. in the metal.

If this is the case, and the rod is near yield somewhere over its length, eventually the plastic deformation cycles will cause it to weaken further, eventually failing. No cure for this, no prevention. Everything has a maximum life and usually the harder it's run the faster it'll fail.

Why it manifests itself more on the 3.5 is anyone's guess, I'd say since the rods are unique to the engine, it probably has nothing to do with rod integrity in the other engines of the same family or even the increased power of the engine. Same metal with different flow in a die can create totally different finished product. Proper / upgraded replacement rods will probably out-last the block.

Bad rods, nothing more, and it likely was corrected later in production or in replacement parts.

Just my opinion.

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Gone to the dark side

- Jeff