The effect of processing and machining on the dimensional stability of polychlorotrifluoroethylene (PCTFE) rod stock and oxygen gas cylinder valve seats was investigated. Initial testing focused on two types of extruded rod stock and one type of compression-molded rod stock made from the same lot of Neoflon® CTFE M400H. To accommodate valve seat manufacturer preferences for certain rod stock diameters, two representative diameters were used (4.8 mm (0.1875 in.) and 19.1 mm (0.75 in.)). To encompass a variety of possible sealing configurations, seven different valve seat types with unique geometries or machining histories made from the above types of rod stock were tested. Property changes caused by processing and machining were evaluated using thermomechanical analysis (TMA), specific gravity, differential scanning calorimetry, zero strength time, and intrinsic viscosity. The dimensional stability of valve seats as revealed by TMA tended to be more dispersed and grouped by manufacturer and did not overlap rod stock TMA data, suggesting that machining had an important effect on seat properties. These and other findings are discussed in the context of process history, annealing, percent crystallinity and molecular weight. Structure-property characteristics of emulsion-polymerized Kel-F® 81 and suspension-polymerized Neoflon M400H PCTFE are compared, and the merits of current PCTFE specifications evaluated.