The 691 is an ultra-small, single-row miniature ball bearing designed for high-precision instruments and compact mechanical assemblies. With a 1 mm bore, 4 mm outer diameter, and 1.6 mm width, this open-type bearing is engineered to provide minimal friction and high-speed stability in extremely confined spaces. Its open configuration allows for low-torque performance and the flexibility to use specific precision oils or greases depending on the application’s RPM requirements.