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If you want to oval your thumb hole just a little here's what I do: You will need a piece of ~100 grit sandpaper and a pencil. First rub the high sides of your thumb with the pencil tip side to mark it with graphite. Carefully insert your thumb into the ball thumb hole as you normally would when bowling.
Keeping this in consideration, how tight should your thumb be in a bowling ball?
If the hole is tight halfway down the hole but loose and larger near or at the top of the ball, the hole will not properly fit your thumb and your release will suffer from consistency. The flat portion of your thumb (top of the first knuckle joint to the back of the knuckle) is not as wide as the side to side portion.
Similarly one may ask, how do you raise the thumb hole in a bowling ball?
If you want to oval your thumb hole just a little here's what I do: You will need a piece of ~100 grit sandpaper and a pencil. First rub the high sides of your thumb with the pencil tip side to mark it with graphite. Carefully insert your thumb into the ball thumb hole as you normally would when bowling.
No. If a bowler doesn't use their thumb during a delivery, they can either have a thumb hole or a balance hole but not both. If the ball has a thumb hole but the bowler doesn't use it, then as long as it meets static balance requirements, the ball is allowed.