This photo (right) shows the 5th gear as it rides on the main shaft of the transmission. The gear rides outside of the cast iron housing, inside of the aluminum housing at the tail end of the transmission. The inside of the gear is splined where it rides on matching splines that are machined into the main shaft. It's secured to the main shaft with a thrust washer and a retaining nut. The nut shown here is an updated nut with a locking tension screw.