World Champion ACER Racing 15x21x4mm Ceramic Ball Bearing. This is a 15x21 mm ceramic ball bearing also known as a size 6702 ball bearing. Diamond polished Silicon Nitride (Si3N4) Balls, HSR cage technology and sealed on both sides. Lubricated with lightweight and 100% synthetic ACER Racing SIN Oil instead of thick grease for super fast and super smooth action.