Torque specs are 300FT lbs IIRC with a 90 degree torque 2nd stage. So yeah, they're torqued on pretty good. Alec, I'd bet on your car the PO thought he could put the crank bolt on with an impact and that would be sufficient enough. That's why the bolt backed out.

Crank pulleys rarely go bad, so I'd get one from a yard that doesn't look like it had some major failure.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k