Some cranks will have problems that go beyond what can simply be inspected with the naked eye. Out-of-round or tapered crankshaft journals will create uneven running clearances in an operational engine. GM recommendations are that out-of-round cannot exceed 0.0003 inch for main journals or 0.0004 for rod journals. Similarly, acceptable taper (change in journal diameter) across the width of the journal is well under a thousandth. Fortunately, journal grinding and/or polishing can help bring journal shape within spec, but there is only so much that can be done with severely damaged or out-of-shape journals. Your machine shop will have the final say as to whether a crank with such issues will need to be replaced.
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