We will discuss the neutral operation of the transmission first because the selector valve lever should always be in the neutral position whenever you start the engine.

When the selector valve is in the neutral position, oil flows along one path. No pressure is applied against any of the hydraulic components. Therefore, the forward clutch disk rotates without turning any of the parts inside the transmission. This allows the transmission output shaft to remain stationary.

Forward Operation

When the lever of the selector valve is moved to the forward position, oil is admitted under pressure from the pump to the passage in the transmission and into an orifice in the pinion shaft. Oil then flows through a radial passage in the flywheel to an intersecting horizontal passage leading to the cavity between the flywheel and the forward clutch piston. A piston-type dump valve, located at the outer diameter of the flywheel, controls the buildup and release of pressure against the piston. When the flywheel is rotating, the force exerted by the piston backup spring tends to push the valve out toward the periphery of the flywheel. When the selector control valve is opened and admits oil to operate the forward piston, the pressurized oil overrides the force of the dump valve spring and pushes the valve in toward the center of the flywheel. When the valve is pushed in, it uncovers a port in the flywheel leading to the piston and admits pressurized oil into the cavity at the front of the piston. The pressurized oil moves the piston toward the backplate, locking the clutch plate between the piston and the backplate and causing the clutch plate to rotate with the backplate. Since the pinion shaft is splined through the center of the clutch plate, the shaft rotates in the same direction and at the same speed as the clutch plate. This is the same speed at which the engine flywheel is rotating because the flywheel and the shaft are locked together hydraulically into one assembly. During this operation, oil has also been flowing along the path to provide lubrication.