Take one of your balls (preferably the one with your favorite thumb) and send it in to BowlingBall.com and have it copied/installed on a new ball (they will also do it for the other ball if you buy the outer part. That way you have the interchangeable thumb and can try it out. If you like it, take it somewhere and have your other balls retrofitted. If not, then it'll be just like a slug that you paid $10 extra for.