Originally Posted by
TimothyH
Ask them how long a single speed cog last and whether single speed riders use their one gear too much.
Then get them to change the cassette under warranty.
Then find a different shop.
-Tim-
+1 to all of the above. I get 6000 -7000 miles on a 105 cassette and chain, all in a hilly area, and I use the 16T cog a LOT. Even at that distance the shifting is still fine when I replace them. I don't think Superman could wear out a cassette in 400 miles. As Tim said, you need a different shop.