I'm back. So my wife and I launched the boat yesterday. Foolishness on my part for not checking if the lever at the helm would go into forward or reverse, but its stuck in neutral. I did start it up in the driveway to verify the impeller was squirting water, but never felt comfortable checking forward and reverse with a spinning prop.
So we launch the boat and she goes floating off with the engine running normally. And then she says, "Hey, this GD thing won't go into reverse!". I thought she was joking but nope, it won't and the wind is taking her around the point. Luckily, she grabs an oar and somehow paddles close to shore where I wade in and grab her. We then got it towed over to my dock as I didn't want to put it back on my neighbor's jury rigged trailer.
So now, I'm trying to get a mobile mechanic to come out so we can assess.
I'm not trying to beat you up but why didn't you check it out on the trailer?
The shifter was in neutral when you reassembled it? I posted previously that
it HAS TO be in forward. Mercruiser sells a tool for holding the propshaft in place while the lower is being installed- if the lower is shifted into forward or reverse and the propshaft rotates, it won't stay in gear, so the propshaft needs to be held against the clutch dog by trying to rotate it in reverse. This is the same as the force that resists the prop's rotation when it's in water.
I'm using GC to mean 'gearcase'.
The upper shift shaft is in the gimbal housing and when the whole drive is reinstalled, the foot on the lower shift shaft and upper need to fit together and can't be forced. However, when the lower gear case is removed by itself, all you'll see is the splined shaft because the foot is mounted in the upper gearcase. This means that when setting up the lower GC to reinstall it, you need to turn the splined shaft to line up with the shift shaft by turning it in the direction that puts the lower GC in forward gear. This is a PITA but it has to be done correctly and it is possible, as you found out, to mate the two if the lower is in neutral or reverse. The upper shaft has a limit to how far it can rotate, the lower has three positions and the shaft doesn't click when it goes into any of them- IMO, they should have made that happen because it would save EVERYONE a lot of time and frustration.
You need to haul the boat out, regardless of which trailer is used. This isn't something you want to attempt in the water.
The .pdf of the Gimbal housing shows the shift shafts at the bottom of the diagram (Parts 19 and 20), the second shows the lower GC with the shift shaft at the right- this is the part with the spline and if you rotate it manually, you'll find that you can only rotate the prop in the 'forward' direction but it will click- this happens when the drive is shifted into Reverse. The next shaft position is Neutral and the prop should rotate smoothly in both directions and in the last position, the prop will only rotate in the reverse direction, which means the GC is shifted into Forward.
I'm also including the diagram for the lower GC, showing the propshaft and shifting mechanism. When you look at the middle group of parts that include the propshaft, you'll see Part #53- this is the shift lever and the splined shift shaft fits into it- if you didn't disassemble this, all you'll need to do is make sure the lower GC is in Forward gear when the shifter is in the Forward position and when you look at the foot, it's aimed toward the bow of the boat.
Note!
The shift cable rotates the upper shift shaft which receives the lower shift shaft and if that was causing the difficulty when you tried to reassemble it, the upper shift shaft (Part #19) may have been forced upward in its bushing and that will need to be corrected. I think this happened when I reassembled mine and I found that the upper/lower shafts don't always mate because the upper can move upward- I haven't removed the lower GC to verify this yet because I need to move the boat out from the back of my garage to be able to go behind it and work on it, but I plan to do this in before the weekend.
My own boat has a shifting problem-
My boat would shift into Forward, but had difficulty going into Reverse and after I raised the drive to go home, I felt that the foot and upper shaft weren't mating, so I left it for later. I'll report on what I find.