Advance J300 Marine Gearbox The Advance 300 series marine gearbox has the functions of deceleration, reverse and forward clutch, and withstands propeller thrust. Among them, the 300/HC300/J300/D300A gearbox has a vertical off-center primary transmission structure, and the T300/T300/1 gearbox has a vertical off-center secondary transmission structure. This series of gearboxes …