HELP

remove the woodruff key and clean out the slot area for it.

then put it in, if its a tight fit, you might need to tap it gently down with a hammer or something. Sometimes, they will not go down and seat all the way.

I like to take a marker and mark on the end of the crank shaft the same spot / placement. that is inline with the wodo ruff. this also helps when I line it up to slide on the flywheel.

Sporty
 
If the key is the correct one, make sure that the key fits well into the crankshaft, it should not be especially tight but just snug and moveable by pressing with your finger.

If you have to hammer it in, then you should remove some of the key by gently rubbing it on fine abrasive paper.

Insert the key into the slot and gently press the end nearest to the threads so that it is a little lower to allow the flywheel to ride over it.

The key will straighten up as the flywheel slips on.

They are designed with a rounded bottom to allow the key to shift into position when tightening up the nut.
 
a flywheel puller... it applies even pressure to all points of the magneeto to pop it off as to what blaaster said absolutely 100% agree
 
It takes Practice and patients, I learned through experience! Lining the flywheel cut out up is the key!