VC Victa, I'm not entirely clear on what is happening here. Are you saying that when you loosen the center bolt that holds the blade plate to the end of the crankshaft, holding the blade plate to keep the entire plate and crankshaft from rotating, once the blade plate comes loose, continuing to turn the bolt rotates the crankshaft? That would mean the bolt was tight in the thread in the crankshaft, which shouldn't happen unless perhaps it is rusted.
This sounds a bit tricky to deal with because the blade plate will get in the way. If you can, try holding the crankshaft to keep it from turning (i.e., apply a Stillson to the outside of the blade plate boss on the crankshaft. Do not hold the crankshaft itself - you'll mess it up so it will make it difficult to replace the lower crankshaft seal). If you can't do that, a rattle gun (impact wrench) will do the job - you may be able to borrow one, or take the mower and a suitable socket to the local tyre store for the ten-second job of removing the screw.
