Abstract: The pitch diameter serves as a crucial metric in assessing thread gauge quality. Measurement techniques for the thread pitch diameter predominantly employ a 1D-length measuring machine equipped with both a double-ball stylus and a contour scanner. During the APMP.L-S9-2019 cylindrical thread gauge international comparison, notable discrepancies were observed in the measurement outcomes of the two methods, especially for gauges with an inferior tooth profile. The contour scanning approach determines the pitch diameter by applying least-square fitting to the tooth profile, leading to a positional translation in comparison to the double-ball stylus method's actual contact surface. This causes the pitch diameter line to deviate. Consequently, when the straightness of the tooth profile is compromised, significant differences arise in the measurement results between the two methods. The findings have been corroborated through theoretical analysis coupled with experimental validation. For plug thread gauges, the contour scanning technique tends to yield measurements lower than the double-ball stylus method. Conversely, for ring thread gauges, the contour scanning's results surpass those of the double-ball stylus method. The differential between the two methods approximately equals the cumulative straightness of the tooth profile on both sides. The analytical methodology introduced herein is also relevant when comparing the three-wire and scanning methods.