Abstract: A machine for balancing large rotatable parts such as marine propellers in the plane of rotation. A swing assembly hangs from a support assembly such as two spaced posts on a stand by means of two or more thin metal sheets which extend from the support assembly to the bottom of the swing assembly. The rotatable part is centered on the top end of a vertical arbor mounted for rotation about a vertical axis in a vertical column on said swing assembly. As the arbor and part are rotated, any imbalances causes a deflection of the swing in a plane perpendicular to the thin metal sheets which act as sensitive leaf springs. Deflection is measured by an indicator adjacent to the swing. Since the metal sheets are very strong in tension, very heavy parts can be tested. These sheets, due to their length and slight thickness, are easily deflected sideways by very small off-center loads. Thus, small imbalances can be detected, measured and corrected.