This balancing shaft assembly (A) comprises a balancing shaft (1) rotatable with respect to a housing (H) around a central rotation axis (X-X'), at least one bearing (B) allowing the rotation of the balancing shaft (1) with respect to the housing (H), each bearing (B) comprising an inner ring (2) coupled in rotation to the balancing shaft (1), an outer ring (6) fixed with respect to the housing (H), and rolling elements (8). The at least one bearing (B) is mounted on a bearing area (3) of the balancing shaft (1). The inner ring (2) is divided into a first portion formed by semicylindrical body (11) mounted against the bearing area (3) of the balancing shaft (1), and a second portion formed by a half ring (15) mounted around a semicylindrical portion (302) of the bearing area (3) of the balancing shaft (1), the inner ring (2) having an outer raceway surface (2A) formed by a semicylindrical surface (9) of the semicylindrical body (11) and by a semicylindrical surface (13) of the half ring (15). The half ring (15) has an outer radius (R15) equal to the outer radius (R11) of the half cylindrical body (11).