Shimano makes a 11 12 13 14 15 17 19 21 23 25 cog set. Both the 11 and 12 tooth cogs have serrations for the lock ring. I took this set removed the 11 tooth cog and put it on a freehub made for 7 cogs. The 25 tooth cog is offset from the carrier placing it 1.6 mm closer to the spokes. You will need to use a 1 mm spacer behind the 25 tooth cog placing it .6 mm closer to the spokes than a standard 7 cog set. This places the 12 tooth cog .8mm closer to the dropout. I used a truvative crank made for a 9 cog chain a 10 cog chain and the system worked. This setup lets you build a stronger wheel than a standard 10 cog cassette would allow.