it may have been put back together with the shift rod in the wrong orientation relative to its coupler. You may need to drop the lower unit and get the shift rod in the right position, then re-assemble. When in the correct orientation, it should shift easily between F-N-R. I have found that trying to shift gears without the drive shaft rotating makes it tougher to do so. Hope this helps.

best,
PM T2

The shift in the gear case is spring loaded towards forward as a fail-safe measure. It will normally go into reverse a lot harder than forward and will slip into forward from neutral with very little effort, because of the spring pressure. Sometimes you may have to turn the drive shaft or prop shaft a bit to get it to go into reverse, when vise-grip testing, to align the dogs, too.

As for why it won’t work when together, I have seen a few cases where the shift shaft splines are so worn that there is not enough movement at the bottom, to shift all the way. Also, people like to force them in reverse and this can twist the shift shaft. Make sure the reverse lock mechanism is free moving, too….Part of shifting into reverse involves moving the reverse lock hook. In fact, sometimes they won’t shift into reverse if there is no tilt angle pin installed or you have lowered the outboard in neutral and the hook is laying against the pin. Try to always raise and lower the engine in forward gear. (NOT RUNNING, of course.)

When you turn the shift shaft it moves a shift shaft in the gearset. It has a stepped shift shaft that has three different steps in the shaft. This forces the third shift shaft inside the prop shaft to move the shift dog on the splined shaft. There are three positions F, N, R, & on the Merc everything must be timed correctly in order to get all three. ALWAYS install with shifter & lower both in forward position to get it correct. You also have shock absorbers on your leg. That means the shift coupling has a rubber coushing. Be sure the coupling is good & not rotted as well.

I Agree! Need a service manual!