When a crankshaft is turned to an undersize, it can and does change the stroke and degrees apart of each rod journal. To index a crank, first the machinist has to know where the stroke and degrees (swing) are. Once this is established, with the right machine to start with, the crankshaft can be indexed for both swing and stroke. Crankshafts that are ground to within +or- .0001 (that's one ten thousands of an inch) on rods and mains gives you a crankshaft that will give you full bearing contact with a lot longer life. If you want more strength and longevity from your engine this is a must.