Sparker it broke the original keyway so I'm thinking taper problem. Before you fit the keyway, fit the flywheel and see if it grabs on the taper and if there is enough thread left so the nut is not bottoming out on the end of the thread. If it is you will need a washer under the nut. Also check that the underside on the flywheel is not bottoming out on a shoulder on the taper.
