We recently replaced a 12 lb entry level reactive resin ball that had a narrow (maybe 1/32") crack from a finger that ran about 3" heading away from grip center. Our supplier replaced it based on pictures we sent and didn't request the ball be returned. It was a conventional grip with drilled holes, no slug or finger grips. All holes were well beveled and the pin was well away from the fingers (the crack did not go toward the pin). I drilled the replacement ball from the original drill sheet. The ball that cracked was drilled in November, 2014 and brought to the center with the crack in October.
Since I was not going to be at the bowling center when the customer picked the ball up, I used the original drill bit to check the fit of the thumb hole of the cracked ball. To my surprise the bit went into the hole to about 1/2" below the coverstock and stopped. I would have had to use a hammer to force it in. My conclusion is that the core had swelled, putting enough pressure on the coverstock to cause the crack. The original bits did fit in the finger holes.
I talked to the manufacturer's Technical Director. He confirmed that the different resins used in the coverstock, filler, and core could cure at different rates. He said the coverstock had enough elasticity to absorb minor swelling. He also said that they are constantly working on ways to eliminate cracking. I'm going to return the ball to them for their analysis.
If you drill balls and have some cracked balls in the center, or when you get a cracked ball in, if possible check with the original bits to see if the core has swelled. If you used a slug or sleeve you'll have to drill it out and can't do the check. Please report any swelling back here.
I have deliberately not given the manufacturer's name because I doubt that, if this is a cause of cracking, it's isolated to one company. I also told the TD I was going to post this here. -- JohnP
