So the Ape has been dramatically dropping out of 6th gear (so much so that I've been restricted to only using 5th gear). It just griiiiiinds in no gear, even with the clutch in, until I slow down enough to finally find a lower gear. It also found a false neutral a couple of times on the change upward from 3rd to 4th. So out came the gearbox today.

When I had a look, it's missing the circlip that holds the final pinion gear (2nd) on the primary shaft, which meant that the teeth between 6th and 3rd were not held together, and also to a lesser extent between 5th and 4th. Check out the wear, particularly on 6th.

Oh, and you gotta love cassette gearboxes!! Soooo easy to work on. Anyone would think they designed this bike with racing in mind