For the performance characteristics of a race engine, the gear ratios of engine types 351/352 in the upper three gears are not close enough together. Since engine type 521/522 the upper 3 gears are in closer steps. Gear shafts of 351, 352, 521, 522 are interchangeable.
Primary transmission gradiation RD250: 21:68
Primary transmission gradiation RD350: 23:66
Oil pump transmission RD250 21:22
Oil pump transmission RD350/400: 23:21
To convert a RD 250 engine to the primary transmission ratio of a RD 350 the following parts needs to be changed: