The Flushmate ST100500 Cartridge Wrench is designed to remove or adjusted the Flushmate Cartridge assembly. The Flushmate wrench can be used to remove or adjust the Flushmate cartridge assembly The ST100500 Wrench is designed to work with Flushmate cartridges for 1.6GPF and 1.0 GPF Flushmate models including Series 501A, 501B, 503, and 504. The Flushmate Flush Valve Cartridge is located on top the Flushmate system in the center of the vessel. This is the main valve that opens and closes to deliver the water to the toilet bowl. The ST100500 Wrench can be used to remove or adjust the cartridge. The ST100500 Cartridge Wrench works with the Flushmate Cartridges C-100500, C-100501, C-100502 and C-100101.