Originally Posted by
Rogerogeroge
Keep in mind that the grade of the ball bearing should match the grade of the races. If you go with super hard balls, that may cause wear to the races. And replace all bearings, not just singles, and always replace with bearings from the same batch.
Not that I answered the OP's question, but I thought worth mentioning.
The grade (e.g. Grade 25) refers to how exact roundness and size is. Not how hard the material is. As long as all balls are the "approximate ame size" it is good. Grade 10 would be "rounder and more same size".
Some say Grade 100 is good enough (we have a bicycle only, not a gas turbine etc.), so Grade 25 is good enough and still cheap.
Dirt has 100000 more impact than the slight size differences.