Park Tool FR-5C Lockring Remover
Fits Shimano, SRAM, SunRace, SunTour, Chris King and other cassette lockrings.
Also fits Shimano disc brake lockrings.
Packaged carded: FR-5C
Always use a quick release skewer to hold the tool in place and snug to cogs when loosening. For solid axles, use the axle nut.
Tool works well, 3/8" drive or you can use a large wrench. Has held up under impact use on a stripped lockring. Good tool and reasonably priced considering.