18.5 x 26 x 4 Ceramic Balls (Si3N4) Stainless Steel Shielded Ball Bearing One Ball Bearing 18.5mm x 26mm x 4mm Double Metal Shielded, radial and deep groove ball bearing,bearing is made of Stainless Steel with Ceramic Balls (Si3N4),bearing is closed with 2 Metal Shields, one Shield from each side to protect the bearing from dust or any possible contamination,bearing is sealed and self lubricated,.Most common application is the Align RC helicopter engine.