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[Rig Wrench]

I've seen em go both ways, both cat and cummins go a million plus, others go just a fraction of that. Pulled apart a 3406E not long ago with 6 broken rings, poor bearings ect ect. Most cats I deal with come apart sooner due to blowby and head gasket issues. The best engine I personally deal with are cummins 870's. I see them go more miles than any other not needing to be opened up. Yes they tend to go through turbos and egr parts, but seldom do I need to get into the bottom end of one.
Simply put, there's good and bad in any mass produced engine