Response to Hello John, I apologize, but we don't have a thread size for this part.  These goto filters Hayward SwimClear Cartridge Filter C2020, 2025, 3020, 3025, 4020, 4025, 5020, 5025, 5520, Hayward Star Clear Plus C751, C900, C1200, C1750, C1900Hayward Star Clear C250, C500, C750, C1000, Hayward ASL Cartridge Filter, Hayward Easy Clear A.G. Filter, Hayward Pro Grid Perm-A-Glass DE, Hayward Regen-X FiltersHayward Perflex D.E. FIlters EC-30, EC-40 & EC-40AC

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