How long should I go on engines?

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AND, I had/have a crack in my case, and was talking to someone a few months ago, that got a REMAN engine, had less than 300 hours on it, and developed big cracks.

Case cracks, at least on the 2 that I am aware of aren't catastrophic if caught in time, but I can just see the whole engine come appart on you. it would be a really bad time.