This is a replacement set of bearings for the Align 450X/XL/SA/SE and 600/600N line of helicopters. On the 450X/XL/SA/SE line they fit inside the main rotor holders and support the feathing shaft. On the 450X/XL HDX series they are used to support the collective pitch control arm. On the 600/600N series they are used as tail rotor control arm bearings. Package includes four 3x6x2.5mm MR63ZZ bearings.