Originally Posted by
Al1943
If you can tighten the cassette with the spacer in place, you need the spacer. Loose cassette cogs will damage the freehub.
With the spacer in place re-adjust both derailleur limit screws and cable tension.
BTW, the highest speed cog is the smallest.
You guys were right! Problem is fixed
Sorry I was wrong, I don't know to adjust a derailleur after all
I was just playing with the "thingy" to adjust the tension. I thought it couldn't release the derailleur more than the minimal position. Releasing the wire did the trick... now the cassette is very stable with the spacer!
Thanks a lot for your help!