Why is one 340 connecting rod bronze color?

lilcuda said

This is correct. If it the big end was too big, they would just dip 'em real quick and re-hone them.

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Except that there is no sign of the copper where the bearing rides only the exterior surface. That lends credence to the use of copper plating as an inhibitor of the carburizing process so that only the bearing area gets hardened.