Spray arm came off in wash cycle

The spray arm comes off into 3 parts but is really only supposed to be one. I removed the nut from the old assembly and attempted to place the new assembly on by hand. The new nut seemed to be slightly smaller than the original so it would not attach to the dishwasher. Using a kitchen knife, I removed the new nut from the arm unit and attached the new arm to the original nut and it went back on easily. The connection of the new arm to the new nut was 3 tabs that needed to be depressed with the knife and was a different conection method than the assembly that kept coming off in the wash cycle. The replacement part had a much better design than the original.