If someone machined the fins off of the flywheel using a milling machine with the flywheel backside against the table, the machined surface of the fins may not be perpendicular to the shaft due to the casting irregularitues on the backside of the flywhel. I rough the fins off on the mill, and then mount the flywheel on a sawed-off crankshaft and chuck it in the lathe. I do it this way because I have a very small lathe (7 x 12). If I had a bigger lathe I'd do it all on it. All weedie tapers for the Phelon and Walbro flywheels seem to be the same, so finding a crank donor that works is pretty easy. That said, if you've really got several mm of runout then you've got other problems....