The ServoCity Dual-Bearing Timing Belt Idler Pulley (4mm Bore, 18.5mm Diameter, 9.5mm inside Width) features dual 4mm bore bearings and a polycarbonate pulley, designed to adjust tension and trajectory of a timing belt. It offers a smooth surface for both smooth and toothed belt sides, with flanges ensuring belt security. The design allows for varied offsets based on mounting direction, maintaining smooth rotation even under axial load due to the coincident inner races of the bearings.