Try having someone rotate the flywheel a little by hand as you try to push it on. This will help the splines align. If the lower unit is in gear, you can also turn the prop.
I changsd the impeller and I can't get the shaft to lign up to go back on. Is there a trick to this. This is the third time I've put an impleller on this old motor. I have trouble every time. Serial # 2937164.<br /><br />Thanks