This study investigates the influence of operating diametral clearance on the performance of angular contact ball bearings (ACBBs). It examines critical factors affecting diametral clearance, including mounting conditions, external loads, temperature fluctuations, and rotational speeds. A novel model combining quasi-static and fit-up approaches is proposed to analyze the effects of operating diametral clearances on ACBB performance. This model incorporates key elements such as ball-race contact loads, interactions between the shaft and inner ring, interference fits between the housing and outer ring, centrifugal expansion of the rotating shaft and inner ring, and temperature-induced changes. Internal clearance variations are computed using the thick-ring theory. Simulations are conducted to predict ACBB characteristics under various fit-up conditions, including contact load distribution and stiffness, with results validated using commercial software. The study also explores the impact of various operating diametral clearances on ACBB performance under differing fitting conditions, external loads, and rotational speeds.