Replacing friend's cassette. Found spacer. Is a 10 speed cassette. Don't know if it is 11 speed hub. Do I keep or throw? : r/bikewrench
How To Install 7 Speed Cassette On 8/9/10 Speed Hub
Issue with rear hub: installed an 11-speed cassette. Noticed it was loose after tightening. Took to bike shop. They added 2 spacers to eliminate rattle of the cassette. Now the smallest gear