1.)  Oval thumb hole and shorter span w/ forward thumb pitch helps me from my old days of longer span and reverse.  Find this grip/fit much more versatile in ability to go from forward roll to side roll.

2.)  Magic Carpet

3.)  Vise IT.....I have two.....normal size and another which is over size with one more 1/2 side cut.

Tighter thumb has allowed me to use less muscle in arm swing and also more ball speed without grabbing at the bottom.  When I hear some of the situations on this forum I see a lot of these factors mentioned.  

I'd say look at your ball fit Danes07.  Get another opinion or two.
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Scott

I am not a driller but I have discover the following. I will ask my driller to drill the thumb hole smaller. That will be to drill the bowling ball first at 59 and then at 61 half way down the thumb hole. I personally do the finishing of the thumb hole as oval. It is my thumb and I am the one to really know how the thumb should fit in the thumb hole.

The 59 drilling at the bottom half avoids my thumb not to be too loose and to wiggle. You are tapering the thumb hole. This avoids me to squeeze the ball or knuckle the bowling ball.

Keeping my thumb straight.

I hate drillers that take the sander and go crazy going up and down the thumb hole without asking first how you like your bowling ball thumb hole to be.

Two ideas:

First, I'd check Moon57's suggestion. It can be that the poor thumb exit is just the symptom of a release flaw that has its origins much, much earlier, e. g. a timing problem like fast feet, a bent upper body, lack of slide, etc. This can all result in a release where the ball is much earlier than the rest of the player, and naturally you have to cling to the ball or top it, just get everything in sync again - but with little power and lots of deviation potential.

Secondly, I'd check with which part of your thumb you are actually holding the ball. Is it the thumb "pad" (first digit) or do you hold the ball with your thumb's base, at the thumb hole rim? The latter can force you to apply a Kung Fu death grip, cranking the thumb. In this case, I'd suggest to experiment with tape to make the hole tighter (with a comfortable span that allows you to hold the ball with your flat thumb inside of the hole), or even consider a different hole shape.

Good luck!
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DizzyFugu - Reporting from Germany

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I am just going to throw this out there, you might want to try wearing a wristmaster type of support. I have been battling this kind of thumb issue, and the wristsupport seems to prevent me from squeezing the ball. You will also need to start tighting up you thumb hole. This advise is provided that you already have a properly fitted ball, and it is bad habit problem. I have tried most products on the market, including a thumb straight and the majic carpet. I not saying those don't work, but this is what worked for me.