1979 mercury 20hp not going into gear

[Ryryridd]

I have a 1979 mercury 20hp. The prop will spend by hand in any gear when not running. Any suggestions? Could it be the little shift cam at the back of the prop shaft assembly? Some has recently been inside the prop shaft attempting to change the seals, so thinking this could be it?

 

[racerone]

Remove lower unit.---Check splines on driveshaft and splines in crankshaft.