Just a suggestion, When I replace the shift shaft O- ring & brass keeper I install on the shift shaft using inert lube like vasoline on the O-ring before installing then drop the assembly into the gearcase & tap the brass keeper into the cavity. This saves the O-ring from forcing over the shift shaft groove & ripping a chunk from the O-ring possably & everything is not running dry, Also with a small brass punch I seat a little preload on the O-ring to know it is seated but not too tight, but has friction & lube. Please use an inert lube to not attack the O-ring also. I’m just extra picky.