AMH99253 Primary CountershaftOne New Aftermarket Replacement Primary Countershaft for John Deere (JD) Combines This aftermarket replacement primary countershaft fits the John Deere 7720 Titan II combine. The primary countershaft is a critical drivetrain component in the combine's separator system, transmitting power from the engine to drive the separator and cleaning systems during harvest. A worn or bent countershaft causes vibration and can lead to separator system failure