just a quick question... i recently got an mercury outboard and i was wondering if there is supposed to be a seal and plug in the flush hole or is it ok to leave it open, the motor didnt have one.
That plug fits into the side of the waterpump cover. Running without one would allow water to leak out and lower cooling water pressure and volume. Not good. Plug it with something, ideally a threaded plug that fits flush in the waterpump body.
