Khris, For some reason I’ve never heard of any 400s with this problem, but they do use a straight key to secure the CB drive gear to the crankshaft. Really all you need to do to check your bike for a worn key/loose gear is remove the right cover. Not the clutch cover, the whole right cover, this means water pump, brake lever, kickstart, oil delivery pipe and feeder line. So you need to drain oil and coolant first off. It isn’t particularly hard or time consuming, just be careful, methodical and patient or you may end up misplacing some o-rings, tearing gaskets and losing little dowels that Yamaha puts all over their motors. You can safely reuse the cover gasket if you get it off w/out tearing it. There are two hollow dowels in boltholes to line up the cover, one that holds the oil feeder line and an o-ring just below the oil filter cover, and finally there is one big dowel and o-ring for the water pump to head passage. The two bolts on upper right side of round clutch cover also secure the engine cover, so remove all the outside case bolts plus these two clutch cover bolts. I think the manual, for some reason, has you remove the impeller from its shaft before you remove the cover. This isn’t necessary, you can leave the impeller and shaft in the cover, just make sure you allow the shaft to line up with the notch in the counterbalancer shaft, which drives the pump, when you reinstall the cover. You can leave the radiator hoses on and just safety wire or zip tie the pump cover out of the way. Be careful, there are also two little dowels and a big, oblong o-ring here waiting to get lost. Also be careful with the kickstart shaft seal on reinstall. Be gentle or the outer splines on the shaft will bugger the seal. Once you have the cover off just check the CB shaft or its drive gear on the main shaft for play. The CB drive gear should have zero play with the crank. If you can move the CB shaft at all then you need to remove the main gear and replace this key (to do this you then need to remove the clutch and main gear). Hope this helps!