Decided to change my water pump impeller on my 2004 mercury 2 stroke 60 hp. After completing the process (following directions from the mercury manual) specifically: placing shift lever in forward before removing lower unit, upon re-assembly the prop would only go forward. I assumed I had misaligned the spines on the shaft for the transmission and which in the final analysis was correct. But in trouble shooting I learned a few things: First it's amazing how little rotation exists in the movement from forward to reverse in this spine shaft, like 2 teeth. Second the spine shaft on the lower unit had approximately 200° of free rotation and I thought I had broken the cam inside the lower unit. Upon further analysis, and encouragement from wise neighbor, by taking a pliers and rotating this spline on the lower unit past the end of its free travel I was able to shift into neutral and reverse. You can spin the drive shaft while moving the spine on the lower unit. I located the spline and on the lower unit fully counterclockwise and reinstalled the lower unit with the shift lever in forward and it works fine now. I was about ready to bring it to a shop and start drinking! But really wanted to be able to do this on my own just in case and now feel comfortable in the process.