Re: 1957 QD-18 LU seal kit needed HELP!
I got one from laings, part SK1 for my 56 15, shows same part for the 57 18hp, but I haven't installed it yet. If above thinks vintage outboard is better I'd go with it.
I can post pics of it if you'd like.
edit: I don't think the bag said sierra on it but it might have. I seem to think it looked like he pieced the kit together himself.