Just put a good used lower unit on my 87 johnson 150. put new water pump on it.got it all back together including the contrary shift shaft and discovered that I forgot to put the o-ring on the upper drive shaft. what part does the o-ring play? there wasn't one on the old shaft when i took it off, forgot to look and see if it was in the motor. Do I need to do it all over again? Not to much fun for a 71 year old man. thanks