This replacement spring is designed for valves inside 35 and 38 Frame pumps. Fits 3521C.44101, 3521DHS.44101, 3535HS, 3531DHS, 3531CHS, 3535.44101, 3531D.44101, 3537HS, 3545HS, 3541DHS, 3541CHS, 3821HS, and 3821KHS. Made with 316 stainless-steel. This is an OEM part from CAT Pumps. Genuine manufacturer parts ensure a proper fit when used with the proper model pump.