The truth is it would be FANTASTIC to have a set of rule to follow about proper thumb pitches and spans. The world would be a great place.
However, the mechanics of the hand is simply not the only factors in determining pitches. Timing (using muscle in the swing), using muscle in your thumb (grabbing the ball), thumb hole size and many other factors help to determine the correct thumb pitches for any specific person.
To say the perfect pitches for a 4.25" span with someones hand that has normal flexability, thumb length, strength and moisture is 1/8" reverse and 0 lateral is rediculous for a bowler with late timing and likes to squeeze the ball. He will NEVER get out of the ball unless the thumb hole size is way to large.
The rule is there is no rule. There are starting points but they might not even be close in the end. Not to mention as a beginner bowler improves and swings loosen up the pitches in the thumb changes and the release improves and there are constant changes in everything.
So a definitive answer is just not possible. There is one guy I know who has a 15/16 size thumb who drills a 1 3/8 thumbhole with a span of 4.5" and uses 7/8 reverse. Of course he squeezes the thumb. Yet he averages 230+ for the last 20 years that way. Is that wrong? Who in there right mind would have ever recommended this? How can any set or rules cover this exception?
The perfect grip for anyone is one that the bowler accepts, feels right to them, allows them to release the ball the way they want with the consistancy they want. Forcing anyone to bowl like anyone else by stating rules that can not be written is just wrong.
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