Advance HCD600A Marine Gearbox The Advance 600 series marine gearbox has the functions of reverse, forward, clutch, deceleration, and withstand propeller thrust. The HC600A/ HCD600A/ HCT600A gearbox has a vertical off-center primary transmission structure, the HC600A/ HCD600A/ HCT600A gearbox has a vertical off-center secondary transmission structure, and the HCT600A and HCD600A …