I fly Senior Pattern association events where the OS 65AX is pretty much the standard engine. I've heard time and again that that engine eats its rear bearings like crazy, and the factory bearing will not hold up. We push our engines harder than a lot of RC pilots, and we fly them more than most sport flyers do. One guy in our group runs nearly 50 gallons of glow fuel a year through his and buys rear bearings in bulk.
The way it's been explained to me (and I could have it wrong) is that to save weight, OS made the crankcase smaller which necessitated a smaller rear bearing. My Novarossi R60F engine doesn't have this problem, which is one of several reasons I think it's a superior engine for what we do.
The advice I've gotten from the guys who really run these engines is to immediately replace the factory bearings with plain steel ones from Boca. Then when it starts feeling the least bit rough or a glow plug dies, replace them.