any time you grind a crank all sets ( mains or rods) should be ground the same more for uniformity and consistant clearance. If you grind a singular journal it could get wasted because the oil clearances in the uncut ones may be looser allowing oil pressure to bleed off through them and starving the newly ground one. If you spin a bearing, rod or main, most likely the main or rod caps are too loose, that's why the bearing spins instead of hammering.Many times people say a bearing has spun before they disassemble an engine, when it's typical bearing failure from lack of oil.
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