I've put an 8-speed cassette on what I thought was an 8/9/10 speed freehub and it's a bit wobbly. As in, it looks like it needs a spacer. The internet reckons it shouldn't, but there you go. If I just put one or more spacers on before the cassette and tighten the lockring, is that OK? Dead soon?
Count how many threads you end up engaging with and without a spacer then report back. As long as your spacer means you still engage sufficient threads it's fine.
How wobbly? If it's only a little bit, the 1mm spacer used to fit most 10-speed cassettes should be enough, and you only lose one turn of lock ring thread engagement compared with your current bottomed out condition.
I've put an 8-speed cassette on what I thought was an 8/9/10 speed freehub and it's a bit wobbly. As in, it looks like it needs a spacer. The internet reckons it shouldn't, but there you go. If I just put one or more spacers on before the cassette and tighten the lockring, is that OK? Dead soon?