Shares of RBC Bearings are up 14% Friday after the industrial manufacturer said it's paying $500 million to buy a unit of Dover Corp., a move that diversifies its product lines and shores up the aerospace business. After the deal, expected to close later this year, RBC Bearings would own Sargent Aerospace & Defense, a unit of Dover that makes parts and provides repair services for airframes, rotorcraft, submarines and land vehicles.