DETROIT (Reuters) - Ford Motor Co has told its U.S. dealers to stop test-driving and selling C-Max and Focus models produced during a week this month because of potential problems with the steering gear assembly, a notice sent to dealers on Wednesday shows. A Ford spokeswoman said Thursday none of the affected vehicles were sold to customers and that less than 50 were on U.S. dealer lots. Some C-Max and Focus models built at the Michigan Assembly Plant outside of Detroit from Aug. 8 to Aug. 15 may not have enough ball bearings in the steering gear assembly, the notice to dealers obtained by Reuters said. This could lead to a loss of control, increasing the risk of a crash. The Ford spokeswoman said more details regarding the issue would be made available later today. (Reporting by Bernie Woodall; Editing by Bernadette Baum)