I don't understand it as I now have 60 games and counting and no cracks. I'm not saying it never will, but it hasn't yet.
You guys are doing something wrong when you drill these balls. If you don't put the pin right up against finger hole, use sharp bits, don't drill so fast as to make the holes hot, bevel the holes, glue the inserts down in the filler part of the ball, don't force thumb slugs in if the hole might be a wee bit tight for the slug, (work it out) don't leave balls in car, don't leave balls in garage, etc., then the balls shouldn't crack.
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Brick