Quoted from snakesnsparklers:Regarding the higher grade steel balls, it's a waste of money for a pinball machine. Higher grade means a tighter diameter tolerance and pinball machines are not picky as to ball diameter as long as you use a 1-1/16" dia. ball.
Well, I heard that the Stern factory balls are just a gritty mess (on a small scale, I'm sure, but still bad for your PF) and that most people just toss them out right away before even putting them in the machine. Never heard of the "tighter diameter" thing before! 
Lubbock, TX
