Author's Note: this follows the article: Output Shaft Noise Diagnosis & Repair - John Brown (January 2000)
Driving to work one day the engine suddenly stopped making my FY K100 roll. Output shaft rivet failure was my first thought.
I followed the same preliminary steps in the repair of my output shaft as John Brown, but following the advice of Haynes I removed the engine from the frame. This allowed me to turn the engine on the valve cover, making access to the lower innards much easier.
When the bottom came off, the heads of the rivets that secured the shock absorber to the drive gear fell out, confirming my diagnosis.
The first picture shows the difference in the new versus old driven gears. The second picture shows where to look to see if your rivets are loosening. The crankshaft cover can be moved out of the way without draining the cooling system by flexing the drain hose and swinging the cover on it. You will need to be careful when reassembling to get the crankshaft cover seal into its groove properly to prevent leakage, but I did not find it difficult.
Rotating the back wheel with the transmission in gear, while peeking under the back of the crankshaft should let you see if there is any play between the driven hub and the gear.