Standard OE replacement cog for a 7800, 7900, 6700 and 6600 10-speed Shimano cassette with a 12t cog. Inside edge is knurled to grip on the lockring. Treated steel is incredibly durable.
Please note: only replaces 12t cog on CS-7800, 7900, 6700 and CS-6600 cassettes. This cog can't be used to retrofit a cassette with a 11t or 13 first position cog.