Callous was on side of thumb (outer). I narrowed it down to too tight a thumb hole and turning my hand too early before thumb had released from ball.
Now that the ball has been opened up no more issues. Callous free. Thumb has no soreness or pain. You wouldn't be able to tell my thumb from someone who doesn't bowl.
I had mentioned this to my PSO a few times and he said the thumb should be tight. It was so tight it made a whoosh on every release. And I don't clench the ball because I grew up bowling thumbless so never put thumb in house balls.
When got another ball went to him and told him to open up the sides as I was developing callouses. After that and a few more tweaking of the hole I'm now callous free and my thumb releases fine. Just a few adjustments with tape from natural swelling, etc.
I like my thumb holes loose and my fingers with grips extremely tight. Tight to the point where I almost have to force my fingers in.
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