NIST has characterized two large diameter (35.8 mm) piston/cylinder assemblies as primary pressure standards in the range 0.05 MPa to 1.0 MPa with uncertainties approaching the best mercury manometers. The realizations of the artefacts as primary standards are based on the dimensional characterization of the piston and cylinder, and models of the normal and shear forces on the base and flanks of the piston. We have studied two piston/cylinder assemblies, known at the National Institute of Standards and Technology (NIST) as PG 38 and PG 39, using these methods. The piston and cylinder of both assemblies were accurately dimensioned by Physikalisch Technische Bundesanstalt (PTB). All artefacts appeared to be round within ±30 nm and straight within ±100 nm over a substantial fraction of their heights. PG 39 was dimensioned a second time by PTB, three years after the initial measurement, and showed no significant change in dimensions or effective area. Comparisons of the effective area of PG 38 and PG 39 from dimensional measurements, against those obtained with calibration against the NIST ultrasonic interferometer manometer (UIM), are in agreement within the combined standard (k = 1) uncertainty of the dimensional measurements and the UIM. A cross-float comparison of PG 38 versus PG 39 also agreed with the dimensional characterization within their combined standard uncertainties and with the UIM calibrations. The expanded (k = 2) relative uncertainty of the effective area is about 6.0 × 10⁻⁶ for both assemblies.