A hydraulic drive unit having a center section for a hydraulic pump and hydraulic motor mounted in a casing. The center section includes a cylindrical protuberance for locating the center section in the casing, and at least one fastener opening formed in the center section for receiving a fastener engaged to the casing. The drive unit may also include a pair of flat surfaces formed on the center section and adapted to mount with features in the casing, and a bypass shaft extending into the casing and at least partially disposed in the center section.