Dear MXA,
Is it common for rear hubs to break on Yamaha's. I had one break on my old 2003 Yamaha YZ250F and just recently on my 2006 YZ450F. I run Renthal's chain and sprockets, but my rear sprocket bolts always come loose even after using Loc-Tite. Please give me some answers so I don't have to buy any more hubs!
You have had bad luck, but you have already diagnosed your problem. When your let the sprocket bolts get loose, they ovaled out the holes in the hub flanges. This ovalizing makes it easier for the sprocket bolts to get loose again. When the sprocket is loose it bangs back and forth against the hub flange and, voila, it breaks the hub. The unfortunate thing is that once the hubs gets ovalized you are on the road to disaster. The only cure is constant vigilance.
It is important to note that many aftermarket sprockets do not have the same taper in the bolt hole as the stock sprocket bolts. When the taper on the sprocket hole and the sprocket bolt are different, it is hard to keep the bolts tight. We have had good luck by putting the sprocket bolts in a drill body and spinning them for 15 seconds to try to match the tapers. A little grinding polish helps seat the bolts. Apart from these tips, you will need to check the bolts constantly.