The Waterway 550-7500 Replacement Filter Locknut Assembly Clearwater II Cartridge Filter is a replacement part for the Clearwater II Cartridge Filter system. The assembly includes a locknut and a threaded retainer ring that securely fastens the filter cartridge into the filter body. The locknut assembly is made of high-quality materials to ensure durability and long-lasting use. It is designed to fit perfectly with the Clearwater II Cartridge Filter system and can easily be installed without the need for special tools or equipment. With the Waterway 550-7500 Replacement Filter Locknut Assembly, you can ensure the efficient and effective operation of your Clearwater II Cartridge Filter system. Top I.D. is : 15-11/16" Bottom I.D. is : 16-7/8"