1973 Mercury 850 Free Spinning Lower Unit Shift Shaft.

[SD_Redneck]

I have a 1973 Mercury 850 that I just go done replacing the water impeller and key on. When I went to put the lower unit back on, I had my shifter in neutral and my shift shaft position in neutral. When I started tightening the bolts to hold it together I was receiving a bit of resistance when I heard a metallic pop. After the POP, the bolts sucked up tight really easily with just the weight of the lower unit resisting. Once everything was buttoned up tight, I operated the shift lever into forward then back to neutral then reverse. When I tried moving it from reverse back into neutral, it absolutely would not move into neutral. So I dropped the lower unit again and when I turned the shift shaft, it will free spin in both directions. The prop will also spin in both directions when I put pressure on it with my finger.

Any ideas of what could be wrong with the gears or shaft would be greatly appreciated.