If I ran the engine regularly the growl only gets so bad. I have let one in an OS108 go so long that the prop could be wiggled, the crank was riding against its bore. This took 110 flights over three years for that bearing to develop over .015" radial clearance. The bearing did not appear rusty just dull silver inside and out, clearly corrosion and use wore down the steel. If an NSK 6904 bearing can go that long without it coming apart they must be well made and they are my top choice as a replacement if I can't find a high end stainless one.