did you hammer the tabs of the lockwasher down flat?
you should replace that washer each time, they are only about $2 from yamaha, ive re-used them in the past and then have trouble getting the nut back off because there is a bump in the metal washer you cant thread the nut past, make sure that washer is beaten flat, use a flathead and a rubber mallet , order a couple spares from yamaha, along with the nut that holds it on, ive had one strip out while riding 100 miles from home, if only i spent the $5 and had one in my toolbox!