Fig. 1 (above) shows the unit in the no gears selected or neutral position with the parts rotating under power, shown in blue and the 1st / 2nd gear selector sleeve and thrust bearing assembly shown in yellow with that for 3rd shown in green. The sleeves also rotate but the thrust bearings are kept stationary by the selector forks (not shown). The parts shown in white can be considered stationary since no power is being transmitted to them, although they may rotate when the forward /reverse box (not shown) is in neutral due to oil drag. The power is transmitted to this assembly via the primary shaft (not shown) which is connected to the engine by a flexible coupling/ spider arrangement and acts on the gear second from left shown in blue.
Fig. 2 (above) shows 1st gear selected with parts as before but with the drive transmitted from clutch drum shown in red. When a gear is selected and the clutch lever pulled by the driver, a plunger is moved which in turn is linked via a bell crank and clutch operating rod with clutch pressure springs to the selector fork. The fork straddles the thrust bearing and moves it away from the clutch drum and through various levers and pressure plates/clutch linings, the drive is transmitted to the 1st gear pinion. This pinion is in constant mesh with the layshaft gear cluster through which the speed reduction is achieved and drive is transmitted to the large pinion on the far right. This large pinion is splined to the mainshaft, which passes through the clutch drum and transmits the drive to the forward /reverse box.
Fig. 3 (above) shows 2nd gear selected, this time the selector fork has moved toward the clutch drum and pressure is now applied to the next clutch assembly and as can be seen, the drive is passing through a higher ratio set of gears giving the medium speed range.
Fig. 4 (above) shows 3rd gear selected by the movement of the mechanism described earlier but this time the 3rd gear has its own set of forks acting on an independent thrust bearing/selector shaft (green). As can be seen, the,3rd gear clutch assembly is splined directly to the mainshaft and therefore drives this directly with no reduction in rotational speed.