An apparatus for simultaneously treating a plurality of cords for use in power transmission belts. The apparatus has a heating chamber, heated air in the chamber, and a guide system for simultaneously guiding a first cord in a first predetermined path and a second cord in a second predetermined path through the heating chamber. The guide system has at least a first roller with a first axis and first structure for causing the first cord and the second cord to move at least partially around the axis of the first roller progressively axially relative to the first roller as the first cord moves in the first predetermined path and the second cord moves in the second predetermined path through the heating chamber.